Testing and Troubleshooting Digital RF Communications Receiver Designs Application Note 1314

Wireless Test Solutions Table of Contents

Page Page

1 Introduction 15 3. Troubleshooting Receiver Designs 2 1. Digital Radio Communications Systems 15 3.1 Troubleshooting Steps 3 1.1 Digital Radio Transmitter 15 3.2 Signal Impairments and Ways to Detect Them 3 1.2 Digital Radio Receiver 16 3.2.1 I/Q Impairments 3 1.2.1 I/Q Demodulator Receiver 17 3.2.2 Interfering Tone or Spur 4 1.2.2 Sampled IF Receiver 17 3.2.3 Incorrect Symbol Rate 4 1.2.3 Automatic Gain Control (AGC) 18 3.2.4 Baseband Filtering Problems 5 1.3 Filtering in Digital RF Communications Systems 19 3.2.5 IF Filter Tilt or Ripple 19 3.3 Table of Impairments Versus Parameters Affected 6 2. Receiver Performance Verification Measurements 20 4. Summary 6 2.1 General Approach to Making Measurements 7 2.2 Measuring Bit Error Rate (BER) 20 5. Appendix: From Bit Error Rate (BER) to Error Vector Magnitude (EVM) 8 2.3 In-Channel Testing 8 2.3.1 Measuring Sensitivity at a Specified BER 22 6. Symbols and Acronyms 9 2.3.2 Verifying Co-Channel Rejection 23 7. References 9 2.4 Out-of-Channel Testing 9 2.4.1 Verifying Spurious Immunity 10 2.4.2 Verifying Intermodulation Immunity 11 2.4.3 Measuring Adjacent and Alternate Channel Selectivity 14 2.5 Fading Tests 14 2.6 Best Practices in Conducting Receiver Performance Tests Introduction

This application note presents the The digital radio receiver must fundamental measurement principles extract highly variable RF signals involved in testing and troubleshooting in the presence of interference and digital RF communications receivers— transform these signals into close particularly those used in digital RF facsimiles of the original baseband cellular systems. Measurement information. Several tests verify setups are explained for the various receiver performance in the presence receiver tests, and troubleshooting of interfering signals. These tips are given. performance verification tests are categorized as either in-channel or The demand for ubiquitous wireless out-of-channel measurements. communications is challenging the physical limitations of current wire- This application note includes: less communications systems. Wireless systems must operate in a • A block diagram of a digital radio very limited area of the radio spectrum communications system. and not interfere with other systems. • Common receiver designs. The maturing wireless markets are becoming much more competitive, • In-channel tests, including sensitivity and product cycle times are now and co-channel immunity. measured in months instead of years. • Out-of-channel tests, including Consequently, network equipment spurious and intermodulation manufacturers must produce wireless immunity and adjacent and alternate systems that can be quickly deployed channel selectivity. and provide bandwidth-efficient communications. • Best practices in the receiver performance tests. Digital modulation has several advantages over analog modulation. • Troubleshooting techniques for These include bandwidth efficiency, receiver designs. superior immunity to noise, low • An appendix that relates Bit Error power consumption, privacy, and Rate (BER) to Error Vector compatibility with digital data services. Magnitude (EVM). These advantages, coupled with advances in digital signal processing The setups required to perform and in analog-to-digital conversion, the receiver performance tests are have spawned the current migration included in this application note to digital RF communications formats. along with descriptions of potential errors in the measurement process. Digital RF communications systems Troubleshooting techniques applicable use complex techniques to transmit to the design of digital radio receivers and receive digitally modulated signals are also provided. through the radio channel. These complexities challenge designers in the isolation of system problems. Signal impairments can be traced back to a component, device, or subsystem of the digital RF communications system. Successful receiver design depends on the ability to locate sources of error.

1 1. Digital Radio Communications Systems

The digital radio signal experiences Consequently, the measurement Certain parts of the digital radio may many transformations in its migration challenges are similar for both parts be implemented in a Digital Signal from a baseband signal at the trans- of the digital radio system. However, Processor (DSP), an Application- mitter to its replication at the receiver. unique problems exist at various Specific Integrated Circuit (ASIC), A rudimentary block diagram of a locations in the system. For example, or a Digital Down Converter (DDC). digital radio communications system the receiver must detect weak signals The DSP, ASIC, or DDC has different (Figure 1) reveals the transformation in the presence of noise and is there- levels of involvement in the various process the signal undergoes from fore tested with very low level signals. digital radio designs. Sometimes it is origination to reception. The transmitter must not interfere difficult to distinguish those problems with other radio systems and is originating in the digital portion of The system-level diagram in Figure 1 consequently tested for the amount the radio from those originating in displays the symmetry of the digital of interference it produces in the the analog portion. This application radio. To a certain degree, the receiv- nearby frequency channels. note describes how to isolate and er can be considered a reverse imple- clarify sources of error in digital mentation of the transmitter. radio receiver tests and designs.

Figure 1. Block Diagram of a Digital Radio System

Transmitter

Baseband I/Q Filter Modulator IF Filter Upconverter Amplifier I I Input Channel Coding/ Symbol (Data or Voice) Interleaving/ Encoder Processing Q Q

Power Control IF LO RF LO

Channel

Receiver

Preselecting Baseband Filter DownconverterIF Filter Downconverter Filter I I Bit Output Demodulator Decoder (Data or Voice) Q Q Low-Noise Amplifier with Automatic Gain Control RF LO IF LO

2 1.1 Digital Radio Transmitter Of the many different ways to imple- After downconversion to the IF, the ment a digital radio receiver, most signal is separated into two distinct The digital radio transmitter (Figure 1) designs fall into two basic categories: paths. To convert to baseband, accepts a baseband waveform and I/Q demodulation and sampled IF. each path is mixed with an LO whose translates that signal into a waveform frequency equals the IF frequency. that it can effectively transmit The upper-path signal (I) is simply through the channel. Before the 1.2.1 I/Q Demodulator Receiver mixed with the LO and then filtered. transformation from baseband to a I/Q demodulation implemented with In the lower path, a 90° phase shift is Radio Frequency (RF) channel, the analog hardware is a commonly used introduced in the mixing signal. This waveform is digitized to utilize the digital radio receiver design. The lower-path signal (Q) is converted to advantages of digital modulation. function of the analog I/Q demodulator baseband by mixing with the phase- Coding is applied to the signal to (Figure 3) is to recover the baseband I shifted LO signal, and then filtered. more efficiently use the available and Q symbols. This process produces the in-phase bandwidth and to minimize the (I) and out-of-phase (Q) baseband effects of noise and interference that components of the data stream. will be introduced by the channel. For a detailed explanation of I/Q The coded signal is filtered, modulated, modulation, consult (Ref. 2, pg. 23). and changed back to an analog waveform that is converted to the desired frequency of transmission. Finally, Figure 2. Receiver Block Diagram the RF signal is filtered and amplified before it is transmitted from the antenna. A more detailed description Preselecting of digital transmitters can be found in Filter Downconverter IF Filter the companion Agilent Technologies Output Demodulator application note, Testing and (Data or and Decoder Troubleshooting Digital RF Voice) Communications Transmitter Low-Noise Designs (Ref. 1, pg. 23). Amplifier with Automatic Gain Control 1.2 Digital Radio Receiver LO The digital radio receiver (Figure 2) can be implemented several ways, but certain components exist in all receivers. The receiver must extract Figure 3. I/Q Demodulator the RF signal in the presence of potential interference. Consequently, Baseband a preselecting filter is the first compo- Mixer Filter nent of the receiver, and it attenuates out-of-band signals received by the ADC I antenna. A Low-Noise Amplifier (LNA) boosts the desired signal level while minimally adding to the noise Preselecting of the radio signal. A mixer down- Filter Downconverter IF Filter LO converts the RF signal to a lower Intermediate Frequency (IF) by mixing the RF signal with a Local Low-Noise Oscillator (LO) signal. The IF filter 90-Degree Phase Shifter Amplifier φ attenuates unwanted frequency components generated by the mixer Baseband LO Filter and signals from adjacent frequency channels. After the IF filter, the ADC Q variations in receiver design manifest themselves. Mixer

3 Although the I/Q demodulator receiver 1.2.2 Sampled IF Receiver 1.2.3 Automatic Gain Control (AGC) is a popular design, it has potential AGC is used in digital radio receivers To decrease analog hardware problems. Unequal gain in the I and to handle the wide range of signal complexity, the digitally modulated Q paths and/or a relative phase shift levels encountered at the receiving signal can be sampled earlier in the other than 90° (quadrature error) antenna. AGC compresses the signal signal path, which increases the will cause image suppression problems range by reducing the gain of the IF, digital or software complexity of in the baseband mixers. I/Q demodu- and sometimes the RF, stages as the the receiver design. The sampled IF lators inherently produce a spurious signal level increases. A strong RF receiver converts the analog signal to response at DC (that is, in the center signal can overdrive the mixer and a digital data stream earlier than the of the passband) regardless of the cause excessive signal distortion. I/Q demodulator does (Figure 4). input frequency. As a result, I/Q The receiver must also process weak demodulators are most commonly In this receiver, the IF signal is RF signals in the presence of noise. used in single-channel base station digitized. The sampled data stream Therefore, the RF portion of the receivers that have a separate receiver from the ADC is digitally demodulated receiver may incorporate AGC to for each frequency channel, rather into its I and Q components, and the process the full range of signal levels than in multi-channel base station original signal is reconstructed. presented to it. Used in the IF stage, receivers that use a single, wide- AGC can prevent overload and bandwidth receiver for the entire This type of receiver is becoming maintain a reasonably constant band of frequencies. more popular because of advances signal input to the demodulator stage. in ADCs and DSPs. The sampled IF For all applications, the AGC circuitry The I and Q data streams are sampled receiver design requires less analog must maintain allowable levels of by Analog-to-Digital Converters (ADCs). hardware than the I/Q demodulator signal distortion over a broad range This allows filtering and signal type and does not split the analog of power levels. Also, the AGC should corrections to be performed with signal into two paths. The I/Q respond quickly to signal level changes digital signal processing. Baseband demodulation is actually performed as it processes signals over its entire filtering by a DSP, ASIC, or DDC in a DSP, ASIC or DDC. Digital I/Q dynamic range. removes many of the problems demodulation avoids phase and associated with analog filter imple- amplitude imbalance between I and mentations (for example, phase and Q signals. The trade off is more digital group delay problems) and provides signal processing and power-hungry filter characteristics closer to ideal ADCs fast enough to capture all the than those of analog filters. Baseband information in the analog signal filtering, whether it is analog or digital, (two factors that reduce battery life is better behaved than IF filtering. in mobile phones). As with the I/Q demodulator, the sampled IF receiver requires a downconverter that does not degrade the incoming signal.

Figure 4. Sampled IF Receiver

Preselecting Filter Downconverter IF Filter

ADC

Low-Noise Amplifier

4 1.3 Filtering in Digital RF Gaussian filters, such as those used Communications Systems in GSM systems, do not provide the theoretical zero ISI like the Nyquist Distortion-free transmission of the filters do. The Gaussian filter has a digital I and Q signals theoretically Gaussian shape in the time and requires infinite bandwidth. An frequency domains, and it does infinite-bandwidth RF communications not go to zero at the symbol spacing. system would interfere with other This causes some ISI, but each symbol systems and would not provide interacts significantly with only the efficient use of radio spectrum. preceding and succeeding symbols. Filtering narrows the bandwidth of The bandwidth-time product (BT) of RF systems, but it also slows down the Gaussian filter corresponds to the signal transitions. alpha of the Nyquist filter, and typical BT values range from 0.3 to 0.5. Baseband filtering rounds off the Unlike Nyquist filters, Gaussian filters rapid transitions in the transmitted are not split into matched pairs in the data, but this can cause Inter-Symbol transmitter and receiver. They are Interference (ISI). A Nyquist filter, only used in the transmitter. GSM which is a type of raised-cosine filter, receivers typically use Butterworth minimizes ISI by forcing the filter’s filters that have a sharper roll-off than impulse response to zero at the symbol the Gaussian filters. Consequently, points (except at the center of the sensitivity is improved because less filter). Thus, the time response of the out-of-channel noise and interference Nyquist filter (Figure 5) goes through is allowed into the receiver’s pass- zero with a period that exactly band. corresponds to the symbol spacing. Adjacent symbols do not interfere A more thorough examination of with each other at the symbol times filtering is provided in (Ref. 2, pg. 23). because the response equals zero at all symbol times except the desired one.

The sharpness of a raised-cosine Figure 5. Impulse Response of a Nyquist Filter filter is described by its alpha (α) and quantifies the occupied bandwidth of the signal. An ideal (“brick 1 wall”) filter would have an alpha of zero. Typical alpha values range from 0.5 0.35 to 0.5. Filter alphas also affect hi transmitted power. A low alpha 0 value results in low occupied band- Symbol Period width but requires high peak transmit power. Consequently, the filter alpha -10 -5 0510 t must be carefully chosen to achieve a i balance between spectral occupancy and required transmit power. In some systems, a root-raised-cosine filter is implemented at both ends of the digital radio, and the resulting overall filter response is a raised cosine.

5 2. Receiver Performance Verification Measurements

This section contains test setups and 2.1 General Approach to 2.2 Measuring Bit Error Rate procedures for performance tests on Making Measurements (BER) digital radio receivers. Each receiver must meet strict performance criteria The most comprehensive receiver BER is the fundamental measurement defined by the various standards of test is to evaluate the reconstructed used when testing receiver performance the telecommunications industry’s baseband signal that has been parameters such as sensitivity and governing bodies (the ITU, ETSI, processed by the receiver. In this selectivity. It is the percentage of TIA, and others). Design teams test, one piece of test equipment erroneous bits received compared must develop performance criteria stimulates the antenna port of the to the total number of bits received for their receiver, or for a portion of receiver and is considered to be an during an observation period. that receiver, and conduct unique ideal transmitter. Another instrument Virtually all BER test instruments use performance tests to verify correct monitors the demodulated digital bit a Pseudo-Random Binary Sequence implementation and modeling of stream. If desired, impairments can be (PRBS) as the test signal. PRBS signals components in the receiver. More- introduced by inserting interference are usually labeled PNx, where x is over, these performance tests verify in the channel between the source the number of bits being permutated receiver compliance prior to the and the receiver, or by altering in the sequence (for example, design’s submission for type parameters in the source, to determine PN9 = 29–1 or 511 bits). Since an approval. the receiver’s ability to operate prop- entire PNx sequence can be recon- erly under less-than-ideal conditions. structed from any sequence of “x” Performance verification tests are bits, using a PRBS signal eliminates divided into in-channel and out-of- The following tests assume the receiver the need to synchronize the received channel measurements. In-channel is complete. If the digital portion of and transmitted bits. Alternatively, measurements test the receiver’s the receiver is unavailable for testing the entire PRBS is reconstructed in operation within the frequencies (for example, if it’s still under devel- the BER tester (BERT) receiver from occupied by the desired signal. Out- opment), then the analog RF designer the first correct “x” bits received. The of-channel measurements verify that needs to establish performance goals received signal is then compared to the the receiver is not being adversely for the analog portion of the receiver. reconstructed correct bit sequence. affected by (or affecting) other signals Typical performance goals are the For a thorough explanation of BER outside its specific frequency channel. estimated optimum noise figure for testing, see (Ref. 3, pg. 23). Although the performance tests pre- the receiver to pass the performance sented in this application note focus verification tests and the estimated Two popular methods exist for BER on digital RF cellular applications, optimum Signal-to-Noise Ratio (SNR) testing of mobile phones: baseband many of the concepts and tests apply for proper ADC operation (at the BER and loopback BER. The feature to other forms of digital RF digital conversion point). set of the unit-under-test (UUT) communications. dictates which test method to use. For the baseband BER test, the demodulated PRBS signal at the receiver remains at baseband and is compared to the reconstructed PRBS by the BERT (Figure 6). Typically, CDMA mobile phones and sub- assemblies use the baseband BER measurement method.

Conversely, for the loopback BER test the received signal is retransmitted, or looped back to a receiver (Figure 7). In the loopback test, the UUT demodulates the incoming RF signal, decodes it, then re-encodes the data stream (with possible errors), and retransmits the signal, often to the original transceiver. To attain the BER, this received signal is compared

6 to the expected PRBS that is Figure 6. Baseband BER Test Configuration reconstructed by the BERT (Ref. 3, pg. 23). GSM handsets are tested using the loopback method. RF Signal The Agilent E4438C ESG signal generator can be configured to provide the RF signal that carries the PRBS and Agilent E4438C ESG Signal Generator perform the BER measurement.

Data is managed in telecommunications systems by a hierarchical system of bit RF Signal grouping. Speech frames are nearly Source the lowest-level building blocks in this hierarchical system. Not all bits in a speech frame are equally important. Some bits are so important that Encoder Baseband Signal the entire frame is erased if any of Baseband Modulator 00110110110001 them are bad. This leads to a new Pattern parameter for expressing receiver Generator performance—Frame Erasure Rate (FER). It is the percentage of erased frames compared to the total number Bit Error Rate Tester Comparator of frames sent during an observation (Option UN7) period. Frame erasure also leads to a modification of our BER measurement. When frames are erased, only the BER of the remaining frames is measured. This parameter is called residual BER (RBER).

Figure 7. Loopback BER Test Configuration (this test set-up applies only to GSM/EDGE)

RF Signal RF Signal

Agilent E4438C ESG Agilent E4440A PSA Signal Generator Spectrum Analyzer

RF Signal Receiver

PRBS

Encoder Baseband IF Signal Modulator Pattern Generator

Demodulator

Option Decoder 300

Comparator

7 2.3 In-Channel Testing with a very accurate signal set to a may be Continuous Wave (CW), relatively low power level and see if narrowband, or of the same type as The most significant in-channel test the receiver output is acceptable. the desired signal. The ability of the measures the sensitivity of the receiver. Alternatively, the signal level is receiver to remain sensitive to the Sensitivity specifies the minimum adjusted for a specified SNR or other desired signal while subjected to the signal level for a specified percentage performance metric. For analog FM interfering signal is a measure of its of errors in the demodulated informa- receivers, the performance metric is co-channel immunity. tion. As the separation between known as SINAD (12 dB is typical). transmitter and receiver increases, or SINAD is the ratio of signal-plus- as fading occurs in the radio channel, 2.3.1 Measuring Sensitivity at a noise-plus-distortion to the noise- the signal will drop into the noise Specified BER plus-distortion at the same output. floor from the perspective of the Similarly, for digital receivers the receiver. Information will be lost Sensitivity is one of the key specifica- specified performance metric is the when the signal approaches the noise tions for a digital radio receiver and BER or FER (Figure 8). floor. The ability of the receiver to is specified at a particular BER (or FER). Sensitivity is the minimum capture the information in a signal Co-channel immunity testing is similar received signal level that produces as it drops to very low levels is a to sensitivity testing. The level of a specified BER when the signal is function of the receiver’s sensitivity. signal distortion is monitored with an modulated with a bit sequence of The go/no-go method for sensitivity interfering signal present in the same data. testing is to stimulate the receiver RF channel. The interfering signal Because sensitivity is often Figure 8. Understanding SINAD expressed in voltage units, such as µV, the following equation will be The top curve in Figure 8 is the desired audio output of the receiver. As the RF input to the receiver is used to convert to dBm: reduced, this curve falls off. The bottom curve is the residual hum and noise of the receiver. As the RF 2 input is reduced, the AGC of the receiver adds gain, which increases the residual hum and noise. dBm = 10 * log (Vrms /Zo) + 30 SINAD is the difference between these two curves. The level of RF input required to maintain a SINAD of 12 dB is generally defined as the sensitivity of an FM receiver. where Vrms = receiver sensitivity in volts rms Zo = receiver impedance (typically 50Ω).

Desired Audio Signal For example, if a receiver has a sensitivity expressed as 1 µV, the sensitivity can be converted to –107 dBm for a system with a 50Ω impedance.

To perform the sensitivity test, 12 dB SINAD connect a signal source to the antenna port of the receiver with a cable of known loss. Then connect the output of the receiver to the BERT (Figure 9).

Audio Output of Receiver (dB) Residual Hum & Noise If the approximate sensitivity is unknown, the signal level should be set to a nominal level (such as –90 µ RF Input to Receiver ( V) dBm) and decreased until the specified BER occurs. The sensitivity is the power level of the signal minus Figure 9. Sensitivity Measurement Setup the loss in the cable. For example, if the signal generator is transmitting a Signal Generator –106 dBm signal when the specified Modulated RF Signal BER is reached and the cable loss is 4 dB, then the sensitivity is –110 dBm DUT BERT for the receiver. Data Agilent E4438C ESG

8 2.3.2 Verifying Co-Channel Rejection 2.4 Out-of-Channel testing Intermodulation immunity tests for distortion products that are generated Out-of-channel, or blocking, tests Most receivers are required to maintain when more than one tone is present verify correct receiver operation in a specified BER in the presence of an at the input of the receiver. In this the presence of out-of-channel signals interfering signal within the channel. test, two interfering signals are and monitor the receiver’s susceptibility Frequently, this co-channel interfering combined with the desired signal to internally generated spurious signal will be a CW signal. Figure 10 at the input of the receiver. The responses. Three prominent out-of- illustrates the test setup for the co- frequencies of the interfering signals channel tests verify receiver perfor- channel rejection measurement. This are set such that one of the third-order mance: spurious immunity, intermod- setup includes a power combiner that intermodulation products lies within ulation immunity, and adjacent/ has some power loss associated with the passband of the receiver. The alternate channel selectivity. For it. Maximum insertion loss of most power of these interferers is raised certain digital formats, the single- 2-way resistive combiners is near until the sensitivity of the receiver tone blocking test verifies receiver 6 dB when combining two noncoher- is compromised. ent signals such as in this test. For performance with a large signal in all measurements using a power a nearby frequency channel. A large Adjacent channel selectivity measures combiner, the combiner loss should single tone slightly offset from the the ability of the receiver to process be characterized and offset by an carrier frequency could desensitize the desired signal with a strong signal increase in signal power from the a receiver to the desired signal. The in the adjacent channel. Alternate signal generators. single-tone blocking test is straight- channel selectivity is a similar test in forward and will not be covered in which the interfering signal is two RF The frequency of the desired signal, a this application note. channels away from the passband of digitally modulated test signal, is set the receiver. to the center of the passband of the Spurious immunity is the ability receiver. The power of this signal is of the receiver to prevent single, 2.4.1 Verifying Spurious Immunity typically set to a level relative to the unwanted signals from causing an undesired response at the output of measured sensitivity of the receiver Spurious responses, also called spurs, the receiver. Spurious immunity is (for example, 3 dB above). The manifest themselves in radio receivers similar to co-channel immunity, but frequency of the interfering signal in two ways: they are generated the interfering signals occur across a is set within the passband of the internally by the receiver, or they broad range of frequencies instead of receiver. The power level of the result from the interaction of the in-channel. interfering signal is set to a nominal receiver with external signals. level at which the BER of the receiver must not exceed the specified level. The required BER level is usually the Figure 10. Co-Channel Rejection Measurement Setup same level specified for the receiver sensitivity measurement. The differ- Modulated RF Signal (Desired) ence in power levels between the two signals is the interference ratio. Signal Generator Combiner Σ For example, suppose a 931.4375 DUT MHz pager has a sensitivity of –105 dBm with a BER of 3%. The desired signal is set to 931.4375 with a power Agilent E4438C ESG with BERT level of –102 dBm. At this power Demodulated, level the BER is less than 3%. The Signal Generator Decoded Data channel width for the pager is 25 kHz. The interfering signal is set In-band CW or to 931.4380 MHz. The power level Modulated RF Signal of the interfering signal is first set (Interfering) to –105 dBm and gradually increased Agilent E4438C ESG until the BER is again 3%. If a level of –97 dBm is required to return the BER to 3%, then the co-channel rejection is 5 dB.

9 Both types of spurs should be identified. internally generated spurs must be output amplitude of the interfering Replacing the antenna of the receiver identified (as described above) and signal is set at a specific level at which with a load will ensure that the should be below the specified level. the BER of the receiver under test receiver is not picking up any stray To perform the spurious immunity must be less than a specified level signals. Connect the final analog measurement, one signal generator (usually the BER specified in the output of the receiver to a spectrum supplies a modulated test signal in sensitivity test). The amplitude analyzer. Any spur viewed on the the desired RF channel at a level difference between the test signal and spectrum analyzer is internally above the sensitivity of the receiver the interfering signal is the spurious generated by the receiver and may (usually 3 dB above). The second immunity (SI) of the receiver: be a harmonic of the power supply, signal generator supplies an interfering a harmonic of the system clock, or a signal. This interfering signal is SI = Pint – Ptest (dB) spur from an LO. adjusted to several frequencies to Spurs from the signal generator used verify the receiver’s immunity to to provide the interfering signal can Spurious response immunity is a spurs (Figure 11). measure of the receiver’s ability to cause a good receiver to appear bad. prevent single, unwanted signals The interfering signal may be Any spurs created by the interfering from causing an undesired response modulated or unmodulated, depending signal generator should be less than at the output of the receiver. Prior upon the frequency range and the the receiver’s spurious immunity. to making this measurement, the communications standard. The 2.4.2 Verifying Intermodulation Figure 11. Spurious Immunity Measurement Setup Immunity Intermodulation products may be Modulated RF Signal (Desired) generated within the receiver when more than one signal is present at the Signal Generator Combiner input of the receiver. Intermodulation Σ DUT products are caused by receiver nonlinearities. Two-tone intermodulation is a common method of testing a receiver. The test signal is the same Agilent E4438C ESG with BERT Demodulated, signal used in other measurements Signal Generator Decoded Data (for example, spurious immunity). The frequencies of the interfering signals are set such that one of the Out-of-band CW or third-order intermodulation products Modulated RF Signal (f = 2f – f and f = 2f – f ) falls (Interfering) rx1 1 2 rx2 2 1 within the passband of the receiver Agilent E4438C ESG (Figure 12).

The power levels of the interfering Figure 12. Intermodulation Products signals are set equal to each other at a specified level, and the BER of the f2 f1 f2 desired signal is checked. As with other receiver tests, the required BER f1 level is usually the BER at which the Antenna sensitivity is measured.

f f f f 1 2 Preselecting Low-Noise rx1 rx2 Filter Amplifier IF Filter

frx1=2f1– f2 frx2=2f2– f1 LO

10 Whenever two signals are input to The second signal generator inputs For example, the sensitivity of a combiner, the nonlinearities of either the adjacent channel signal, an NADC base station receiver is the signal generators may create offset by one channel spacing, or the specified at –110 dBm with a BER intermodulation products (Figure 13). alternate channel signal, offset by two of 10-3, or 0.1%. The adjacent channel There are several techniques for channel spacings. The out-of-channel specification requires that the BER be reducing signal generator signal is set to a specified level at no worse than 10-3 with the in-channel intermodulation products: which the BER of the test signal is signal set to –107 dBm, a 3 dB below a certain rate (usually the same increase, and the adjacent channel 1) maintain a frequency separation level specified in the sensitivity test). signal set to –94 dBm, or 13 dB above between the interfering signals that the in-channel signal level. This is greater than the bandwidth of the Automatic Level Control (ALC) of the sources; 2) add attenuators to the Figure 13. Intermodulation Immunity Measurement Setup outputs of the signal generators; 3) use hybrid combiners; 4) use isolators; Modulated RF Signal (Desired) and 5) turn off the ALC of the sources. Signal Generator Combiner All these techniques may be applied Σ DUT simultaneously to reduce intermodulation products. Maintaining a large frequency separation is usually the most effective means to combat this Agilent E4438C ESG with BERT Demodulated, problem. For example, if the ALC Decoded Data bandwidth is 1 kHz, the signal Signal Generator separation should be at least 10 kHz. Out-of-band CW or If this cannot be done, adding attenu- Modulated RF Signals ation at the signal generator outputs (Interfering) theoretically reduces the intermodulation products 3 dB for every 1 dB Agilent E4438C ESG of attenuation. Signal Generator

2.4.3 Measuring Adjacent and Alternate Channel Selectivity

Adjacent and alternate channel selectivity measure the receiver’s Agilent E4438C ESG ability to process a desired signal while rejecting a strong signal in an adjacent channel (one channel away) Figure 14. Adjacent and Alternate Channel Selectivity Test Setup or alternate channel (usually two channels away). The selectivity Modulated RF Signal (Desired) tests are very important for commu- Signal Generator Combiner nications receivers in which channel Σ spacings are narrow and adjacent DUT and alternate channel power is hard to control (for example, Specialized Mobile Radio, or SMR). An adjacent Agilent E4438C ESG with BERT and alternate channel selectivity test Demodulated, Decoded Data setup is shown in Figure 14. One signal Signal Generator generator inputs a test signal at the desired channel frequency at a level relative to the sensitivity of the Modulated RF Signal receiver (usually 3 dB above). in Adjacent or Alternate Channel Agilent E4438C ESG (Interfering)

11 implies that the adjacent channel signal where For a receiver with a noise-equivalent Φ cannot increase the receiver noise floor n = signal generator SSB phase bandwidth of 14 kHz, a Pac at the by more than 3 dB. For alternate noise (dBc/Hz) at the channel adjacent channel of 70 dB, a margin channel selectivity, the alternate spacing offset of 10 dB, and a channel spacing of channel signal is set to –65 dBm, Pac = adjacent or alternate channel 25 kHz, the required SSB phase noise or 42 dB above the in-channel signal selectivity specification (dB) is –122 dBc/Hz at a 25 kHz offset. level. Figure 17 displays the specified Be = receiver noise-equivalent This is typical for an analog FM NADC adjacent and alternate channel bandwidth (Hz) receiver. Unlike the FM receiver in selectivity spectrum. Pmar = test margin (dB) this example, most digital communications receivers have adjacent In addition to level accuracy, the Since Pac and Be are fixed by the channel selectivity values less than spectral characteristics of the test specifications or design, the test 15 dB. For a GSM receiver with a and interfering signals are important. margin determines the power that noise-equivalent bandwidth of 200 For many receivers, the single sideband the signal generator phase noise will kHz, a Pac at the adjacent channel of (SSB) phase noise of the signal be allowed to contribute to the IF 9 dB, a margin of 10 dB, and a channel generator used to produce the passband of the receiver. A large test spacing of 200 kHz, the required SSB interfering signal is a critical spectral margin increases the confidence that phase noise is –72 dBc/Hz at a 200 kHz characteristic. If the phase noise the receiver operates properly in the offset. The required SSB phase noise energy inside the passband of the IF presence of SNR degradation due to is driven primarily by Pac. filter is excessive, the receiver may fading in the channel or due to appear to fail the test (Figure 15). imperfections in receiver components. Table 1 lists the values of adjacent For a system using a new technology and alternate channel selectivity for The required signal generator SSB or new operating frequencies, a large various communications systems as phase noise may be calculated from: test margin should be used to well as the required signal generator compensate for uncertainties. SSB phase noise. A 10 dB test margin Φ = P – 10 * log(1/B ) + P n ac e mar was used. Clearly, for the digital RF communications formats, the signal Figure 15. Phase Noise in Adjacent Channel Selectivity generator SSB phase noise is not as important as for analog FM systems.

Channel Spacing For selectivity tests the spectral shape of the signal is the special characteristic that is of primary importance. The digital modulation formats used by GSM, CDMA, NADC, and PDC characteristically leak a small amount of power into the adjacent channels. Figures 16–18 plot IF Rejection Curve amplitude versus frequency for the Level (dBm) selectivity values specified in Table 1. The impact of the spectral shape on the adjacent and alternate channels of the receiver is evident. To properly test your digital radio receiver, the Adjacent Channel Power (ACP) of SSB Phase Noise your signal generator must be below the required system specification Frequency plus the desired test margin.

12 Table 1. Maximum Tolerable SSB Phase Noise

System Channel Approximate Adjacent Maximum SSB Alternate Maximum SSB Type Spacing Receiver Noise Channel Phase Noise Channel Phase Noise Bandwidth Selectivity @ Offset Selectivity @ Offset Analog FM 25 kHz 14 kHz 70 dB –122 dBc/Hz @ 25 kHz GSM 200 kHz 200 kHz 9 dB –72 dBc/Hz @ 200 kHz 41 dB –104 dBc/Hz @ 400 kHz NADC 30 kHz 35 kHz 13 dB –68 dBc/Hz @ 30 kHz 42 dB –97 dBc/Hz @ 60 kHz PDC 25 kHz 33 kHz 1 dB –56 dBc/Hz @ 25 kHz 42 dB –97 dBc/Hz @ 50 kHz

Figure 16. GSM Adjacent and Alternate Channel Selectivity Spectrum Figure 17. NADC Adjacent and Alternate Channel Selectivity Spectrum

–44 –65

) 41 dB 42 dB dBm (

–76 –94 Amplitude (dBm) Amplitude 9 dB 13 dB –85 –107

fc +200 +400 fc +30 +60 Offset from Nominal Center Frequency (kHz) Offset from Nominal Center Frequency (kHz)

Figure 18. PDC Adjacent and Alternate Channel Selectivity Spectrum

–58

42 dB

1 dB Amplitude (dBm) –99 –100

fc +25 +50 Offset from Nominal Center Frequency (kHz)

13 2.5 Fading Tests 2.6 Best Practices in Conducting When measuring the adjacent channel selectivity performance of an analog A unique challenge for the receiver is Receiver Performance Tests radio receiver, the phase noise of the to overcome the random effects of the By following certain guidelines in out-of-channel test signal is extremely radio channel. In a cellular environ- conducting receiver performance important. Conversely, when making ment, a radio signal may take a number verification tests, you can be sure out-of-channel tests on digital radio of paths en route from the transmitter that your test results are valid. receivers, the phase noise of the test to the receiver. These multipath signals Performing in-channel and out-of- signal is much less important. The may add up constructively (in phase) channel receiver tests within the power in the modulation sidebands or destructively (out of phase) at the confines of a shielded room greatly of the test signal greatly exceeds the receiver as a function of the distance reduces interference from outside power contribution from the phase each signal travels. The effect of this sources. The shielded, or screen noise sidebands. The portion of the phenomenon can be fading of the room provides isolation from RF test signal that spills over into the received signal strength, which can signals that could potentially interfere adjacent channel has the greatest greatly stress signal reception. Fast with the receiver. Also, impedance impact on the out-of-channel testing fading distorts the shape of the base- mismatches between the signal of digital radio receivers. Because band pulse. This distortion is linear generator and the receiver create of this, ACP is the most important and creates ISI. Adaptive equalization reflections that degrade measurement specification for out-of-channel test reduces ISI by removing linear accuracy. The test equipment used to signals. distortion induced by the channel. conduct receiver tests should be care- Slow fading results in a loss of SNR. fully chosen to reduce measurement Error-correction coding and receive uncertainties and increase confidence diversity are used to overcome the in proper receiver operation. effects of slow fading. When making a sensitivity measure- Fading tests can be performed by ment, the level accuracy of the signal routing the test signal through a radio- generator is extremely important. channel emulator before the signal is The measurement system will intro- processed by the receiver. This device duce some amount of error, and the provides several paths for the signal amplitude level accuracy of the signal to travel in the simulated RF channel generator is the main source of this before being recombined at the error. In addition to level accuracy, receiver. The receiver must be able the signal generator must also have to process fading signals with an accurate modulation. Distortion in acceptable BER. The fading measure- the signal modulation will degrade ment setup (Figure 19) is similar to the the sensitivity of the receiver being sensitivity measurement setup with tested. the exception of the channel simulator.

Figure 19. Fading Measurement Setup

Modulated RF Signal

Signal Generator Electrolit, Spirent, Others Channel Simulator

DUT

Agilent E4438C ESG with BERT Demodulated, Faded RF Signal Decoded Data

14 3. Troubleshooting Receiver Designs

Digital RF communications systems 2. Co-channel Immunity. Check for A noise figure measurement on the require complex digital radio compression occurring in the RF front end, or any analog component transmitters and receivers. Complex analog components or check for an or subsystem of the receiver, is a designs challenge engineers in the algorithm implementation problem two-port measurement (from input isolation of system problems. Most in the digital realm. to output). For more information physical impairments can be traced on noise figure measurements, see 3. Spurious Immunity. Look for any back to a component, device, or sub- (Ref. 6, pg. 23). The TOI measurement interfering tone (see section 3.2.2). system. Successful receiver design is also a two-port measurement see If no interfering tone is found, often depends upon the ability to (Ref. 7, pg. 23). ADC measurements perform a Fast Fourier Transform find the source of error. This section process the digital output of the ADC (FFT) on the data from the ADC to suggests some basic techniques for and are unaffected by probe placement. convert to the frequency domain. troubleshooting a receiver that does Then check for spurs generated by not pass a certain test. Also, a table 3.2 Signal Impairments and the ADC. that links measurement characteristics Ways to Detect Them to possible causes of error in the 4. Intermodulation Immunity. Measure Certain signal impairments appear different sections of the receiver is the third-order intercept (TOI) of in specific measurements. In these provided. the RF front end. If it meets the measurements, variations from the expected value, measure the TOI expected results can help locate 3.1 Troubleshooting Steps and gain of each analog stage. problems in different parts of the If the receiver under test fails any 5. Selectivity. Look at the shape of receiver. The following sections of the performance tests, you should the IF filter (see section 3.2.5), and explain some common impairments attempt to isolate the source of the check for excessive LO phase noise and how to recognize them through error in the receiver. The following is or sidebands. their effects on the different measure- a suggested troubleshooting procedure ments. With the exception of the IF to follow if your receiver does not Specific guidelines should be followed filter measurement, Agilent 89400 or meet the expected performance when connecting to the receiver during 89600 series vector signal analyzers criteria. troubleshooting. When connecting to (VSA) are used to troubleshoot analog nodes of the receiver, the test receiver designs in this application Test Failed: probe alters signal characteristics to note. The IF filter measurement is a certain degree, which increases performed with the Agilent 8753E 1. Sensitivity. Measure the BER versus uncertainty in the test results. In a vector network analyzer (VNA). the input power. If the BER is high conventional analog receiver there at high input powers, check for I/Q are many accessible test points, such impairments (see section 3.2.1), as the outputs of the LNA, the LO, excessive group delay in analog the mixers, and the various filters. components, or phase noise from an Accessibility of components in the LO. If the BER is high at low input digital radio receiver depends on the powers, measure the noise figure level of circuit integration. Many of of the analog front end (from the the components of receiver subsystems antenna port to the ADC). If the are embedded in Integrated Circuits noise figure is higher than expected, (ICs). For receivers containing ICs, measure the noise figure and gain tests are normally conducted at the (or loss) of each stage of the receiver. subsystem levels of the receiver. To If no noise figure problems are test embedded components, strategic detected, the gain of the front end test points must be designed into may be low, there could be a detection the IC. algorithm problem in the digital portion of the receiver, or a spur may be desensitizing the receiver (see section 3.2.2).

15 3.2.1 I/Q Impairments imbalances are detectable by viewing slightly different conversion losses in the constellation diagram of the symbol the I and Q mixers or by different filter Constellation diagrams are useful time and comparing with the ideal losses in the I and Q signal paths of in displaying the characteristics of grid of the constellation. These ideal an I/Q demodulator. Even subtle signal impairments related to I and Q. grids indicate where the symbol imbalances are often visually detected Matching problems due to component states should occur. by zooming in (magnifying the scale) differences between the I side and Q on the constellation and using the side of a receiver can cause gain I/Q gain imbalance results in a markers. Without the ideal grids it imbalance or quadrature errors. distorted measured constellation would be difficult to detect small These differences may be attributed relative to the reference (Figure 20). imbalances. to mixers, filters, or ADCs. Subtle This imbalance may be caused by

Figure 20. I/Q Gain Imbalance (excess I gain and reduced Q gain relative to the ideal constellation locations) Figure 21. I/Q Quadrature Error

Figure 23. A Sinusoidal Spur Indicated by the Circle Around Figure 22. I/Q Offset One of the Constellation Points

16 I/Q quadrature errors result in a tipped The radius of the circle is proportional processor clock, an intermodulation or skewed constellation (Figure 21). to the amplitude of the interfering product, or an internally generated Quadrature errors are caused by a signal, but this display format contains spur. This interfering tone could phase shift other than 90° between no information about the interfering cause the receiver to fail many of the I and Q paths. Different group frequency, which may be the key to the performance verification tests. delays in the baseband I and Q filters identifying the cause. also create quadrature errors. This 3.2.3 Incorrect Symbol Rate distortion of the constellation The presence of spurs on a modulated increases the possibility of errors signal may be difficult to determine The symbol clock of a digital radio in the interpretation of the received on a constellation display or through dictates the sampling rate of the base- symbols and will increase the Error spectrum analysis. An alternative band I and Q waveforms required to Vector Magnitude (EVM). parameter can be used to check signal accurately interpret the symbols and quality: EVM. A description of EVM recover the digital data at the receiver. I/Q offsets are shifts in the origin of and how it relates to the BER can be In the transmitter, the symbol rate the I/Q constellation and can occur found in the Appendix. The magnitude dictates the creation of the baseband when DC offsets are introduced by of the error vector versus time graph I and Q waveforms to properly put the rounding errors in the DSP or by LO may hint that the error observed is valid states in the correct locations, feedthrough in the transmitter sinusoidal in nature, but what is really ensuring proper encoding of the digital (Figure 22). needed is a method to determine the data. It is imperative that the trans- frequency of the spur. mitter and receiver have the same 3.2.2 Interfering Tone or Spur The error vector spectrum can indicate symbol rate to be compatible. An interfering signal can cause the the frequency of spurious signals that An internal clock generator deter- amplitude and phase of the transmitted cannot be observed on traditional mines the symbol rate of a system. signal to be different each time the spectrum analyzers or on a constella- This generator must be set correctly. signal passes through the same state. tion display. In Figure 24, a spur offset Symbol rate errors often occur when This will result in a spread at the from the carrier by approximately 47 the wrong crystal frequency is used symbol locations in the constellation kHz is detected at the output of the (for example, if two numbers have diagram (Figure 23). Random smearing IF filter. This spur was most likely been accidentally switched in the fre- of the points indicates noise, but a caused by an in-band CW signal quency specification). If no problem circling of the symbols around the undetectable by traditional spectrum exists with the crystal, the receiver constellation states indicates there analysis (Figure 25). This in-band CW is having a synchronization problem. may be a spur or interfering tone. interferer could be a harmonic of the Either the receiver is not properly

Figure 24. Error Vector Spectrum Reveals Spur Figure 25. Signal Spectrum Conceals Spur

17 recovering the carrier frequency, or defines the shape of the filter in the To verify baseband filter performance, the receiver is not achieving symbol frequency domain. A low alpha creates examine the vector constellation lock. To recover the proper carrier a sharp filter shape in the frequency diagram for excessive overshoot of frequency, the receiver must lock domain, but also creates high over- the signal trajectory between symbol onto the phase of the carrier. To shoot in the time domain, which can states. The magnitude of the error accurately extract the symbols from be recognized on a vector diagram. vector versus time would be a good the carrier, the receiver must also It is important to verify that the indicator of roll-off factor discrepancies. determine when symbol transitions receiver has the appropriate base- If the wrong roll-off factor is used, the occur. A timing recovery loop provides band frequency response and time magnitude of the error vector will be the mechanism for the receiver to characteristics for the specified alpha. high between symbol points and low achieve the necessary symbol lock. at the symbol points (Figure 26). When the receiver does not achieve In cases where baseband filtering is proper phase lock and/or proper shared between the transmitter and The correct roll-off factor can be symbol lock, symbol rate errors occur. the receiver, the filters must be found by using different roll-off If you suspect an incorrect symbol compatible and correctly implemented factors in the VSA while viewing the rate and no problem exists with the in each. The type of filter and the error vector time display. When the crystal, verify proper operation of the corresponding roll-off factor (alpha) correct value is used, the magnitude carrier and timing recovery circuitry are the key parameters that must be of the error vector between symbol of the receiver. considered. For raised-cosine filters, decision points will be approximately an error in the selection of alpha may equal to the magnitude of the error result in undesirable amplitude over- vector at the decision points (Figure 27). 3.2.4 Baseband Filtering Problems shoot in the signal. It may also result Furthermore, equalization can be Baseband filtering must be correctly in ISI. Incorrect filtering due to a applied to decrease the errors caused implemented to provide the desired wrong roll-off factor may affect the by baseband filtering problems. baseband frequency response and to amount of interference from an avoid ISI as well as overshoot of the adjacent channel signal. This could baseband signal in time. The alpha cause an otherwise good receiver to parameter in a raised-cosine filter fail many of the performance verification tests.

Figure 26. Vector Diagram and Magnitude of the Error Vector Figure 27. Vector Diagram and Magnitude of the Error Vector Versus Time for Wrong Roll-off Factor Versus Time for Correct Roll-off Factor

18 3.2.5 IF Filter Tilt or Ripple 3.3 Table of Impairments isolate the source of the error. EVM Versus Parameters Affected is a powerful signal analysis tool The IF filter attenuates out-of-channel that can be scrutinized to pinpoint interference. Errors in the design of Table 2 shows the physical impairments sources of interference in receiver this filter can affect the overall signal. encountered with digitally demodulated tests. Frequency response and group IF filter problems include filter tilt or signals, and the parameters that these delay measurements prove effective ripple in the frequency response and impairments affect. in the detection of filtering problems. variations in group delay. Ideally, Phase error analysis can detect The key to troubleshooting is to the filter should be flat across the sources of unwanted phase noise. frequency band of interest, and its identify the impairments that could group delay should be constant be causing signal degradation. Each Strategic use of these analysis tools across the same frequency band. of the different impairments uniquely will enhance your ability to track Filter tilt or ripple in the frequency affects the quality of a digitally down sources of error in your digital response causes linear distortion in demodulated signal. As the table radio receiver designs. The ability to the signal. Improper matching of any indicates, the I/Q constellation is quickly locate design problems can component between the antenna and typically affected by physical impair- greatly reduce product development the IF filter also causes tilt or ripple. ments. Although the constellation and test verification times, and facilitate For example, mismatch between the diagram is a good indicator of problems, the type approval of receiver designs. preselecting filter and the LNA causes further analysis may be necessary to reflections that result in distortion of the overall frequency response of the receiver. Table 2. Impairments Versus Parameters Affected Physical Impairments Parameters Affected Filter tilt or ripple causes distortion I/Q Gain Imbalance I/Q Constellation (Figure 20) on the demodulated baseband signal. I/Q Quadrature Errors I/Q Constellation (Figure 21), Average EVM, Magnitude of the Error This distortion is discernible in the Vector versus Time, Error Vector Spectrum constellation diagram. Also, the I/Q Offsets I/Q Constellation (Figure 22) magnitude of the error vector will be Interfering Tone or Spur I/Q Constellation (Figure 23), Average EVM, Error Vector Spectrum higher than expected at the symbol (Figure 24) points as well as during symbol tran- Incorrect Symbol Rate I/Q Constellation, Phase Error sitions. Since the IF filter is the main Baseband Filtering Problems I/Q Constellation, Average EVM, Magnitude of the Error Vector versus contributor to the frequency response Time (Figures 26 and 27) of the receiver, IF filter shape distortion IF Filter Tilt or Ripple I/Q Constellation, Magnitude of the Error Vector versus Time, is observed and analyzed by performing Frequency Response (Figure 28), Group Delay a frequency response measurement on the filter alone, as shown in Figure 28.

Figure 28. Undesired Tilt and Ripple in the IF Filter

REF CH1 S21 LOG 1 dB/ –7 dB 1:–10.172 dB 190.050 000 MHz

PRm

Cor 1

Center 190.000 000 MHz Span 10.000 000 MHz

19 4. Summary 5. Appendix: From Bit Error Rate (BER) to Error Vector Magnitude (EVM)

Digital RF communications receivers BER is the best measurement to verify has been stripped away. EVM is the are challenging to design, test, and receiver performance, but BER testing root-mean-square (rms) value of the troubleshoot. Two digital radio is not always possible in the subsystems error vector over time at the instants receiver designs were discussed in this of a digital radio receiver. Also, BER of the symbol clock transitions. By application note: I/Q demodulator can indicate a problem exists, but it convention, EVM is usually normalized and sampled IF. Receivers must may not help identify the source of to the outermost symbol magnitude meet strict conformance standards. the problem. An alternative to BER at the symbol times and expressed Common in-channel and out-of- testing is to examine the quality of a as a percentage: channel tests verify that receiver demodulated signal. The most widely designs meet these standards. To used modulation quality metric in EVM = (rms error vector / outermost reduce measurement errors, best digital RF communications systems symbol magnitude) x 100% practices should be followed, with an is the EVM. EVM provides a way to The error vector information of awareness of measurement caveats. quantify the errors in digital demodu- the trajectory between the points A basic troubleshooting procedure lation and is sensitive to any signal (viewable in the magnitude of the helps to isolate design problems. impairment that affects the magnitude error vector versus time display of Application of these testing and and phase trajectory of a demodulated the Agilent 89441A VSA) helps you troubleshooting techniques can signal. troubleshoot baseband filtering reduce product cycle times and problems in your receiver design (see increase confidence in proper As shown in Figure 29, the error vector section 3.2.4). Also, the spectrum of operation after the receiver is is the vector difference between the the error vector can help you locate manufactured and put into use. reference signal and the measured signal. The error vector is a complex sources of interference (see section quantity that contains a magnitude 3.2.2). The magnitude error and and phase component. Expressed phase error between the two vectors another way, the error vector is the provide a way to view unwanted residual noise and distortion remaining phase and amplitude modulation after an ideal version of the signal that may occur in your receiver.

Figure 29. EVM and Related Quantities

Magnitude Error Magnitude of Error Vector

Q Error Vector

θ Phase of Error Vector Measured Signal

Phase Error φ Ideal Signal (Reference)

20 EVM may also be normalized to the easily measured figure-of-merit that square root of the average symbol can be used to monitor design changes, power. In this way, EVM can be locate design problems and, when related to the SNR: baselined against a BER measurement, indicate the likelihood that a design SNR = –20 * log (EVM / 100%) will meet the required specifications. Hence, the connection of BER to EVM The importance of the above equation is through the SNR, the more general is that it relates EVM to BER through indicator of likely signal quality the SNR. (Figure 31). Many textbooks have standard Measurements of EVM and related curves that relate BER to SNR, as in quantities can provide powerful Figure 30 (Ref. 8, pg. 23). Generally, insight into the performance of a these curves assume that the noise is digital radio receiver. When properly Additive White Gaussian Noise (AWGN) applied, these signal quality measure- with a finite peak-to-average ratio, or ments can pinpoint sources of error crest factor. The assumptions made by identifying the exact type of in generating textbook plots of BER degradation in a signal. For more versus SNR will not necessarily apply detail on using EVM measurements to a particular receiver. The noise in to analyze and troubleshoot vector- a receiver under test, for example, modulated signals see (Ref. 4 and may not be AWGN but may instead Ref. 5, pg. 23). have a strong spectral component. In addition, the steep slope of BER curves makes BER estimations from measured SNR (or EVM) more prone to error. However, EVM provides an

Figure 30. Probability of Error Versus SNR Figure 31. SNR Versus EVM for Crest Factor of 1.4

10–3 Peak-to-Average Ratio of 1.4 30 10–4 16-PSK 16-APK 10–5 or 16 QAM 8-PSK –6 P(e) 10 8-APK 28 –7 Class I 10 QPR 4-PSK (QAM) 10–8

BPSK SNR (dB) 10–9 26 10–10 6 810121416 18 20 22 24 26 SNR (dB)

24 2.5 2.7 2.9 3.1 3.3 3.5

21 6. Symbols and Acronyms

α Alpha (roll-off factor) of a IC Integrated Circuit Nyquist filter IF Intermediate Frequency ACP Adjacent Channel Power ISI Inter-Symbol Interference ADC Analog-to-Digital Converter ITU International AGC Automatic Gain Control Telecommunications Union

ALC Automatic Level Control LNA Low-Noise Amplifier

ASIC Application-Specific LO Local Oscillator Integrated Circuit NADC North American Digital AWGN Additive White Gaussian Cellular Noise PDC Pacific Digital Cellular BER Bit Error Rate PHS Personal Handyphone BERT Bit Error Rate Tester System

BT Bandwidth-Time product PRBS Pseudo-Random Binary (roll-off factor) of a Gaussian Sequence filter Q Quadrature-phase CDMA Code Division Multiple Access RBER Residual Bit Error Rate

CW Continuous Wave RF Radio Frequency

DDC Digital Down Converter SMR Specialized Mobile Radio

DSP Digital Signal Processor SAW Surface Acoustic Wave

DUT Device Under Test SNR Signal-to-Noise Ratio

ETSI European TDMA Time Division Multiple Telecommunications Access Standard Institute TIA Telecommunications EVM Error Vector Magnitude Industry Association

FER Frame Erasure Rate TOI Third-Order Intercept

FFT Fast Fourier Transform UUT Unit Under Test

GSM Global System for Mobile VNA Vector Network Analyzer Communications VSA Vector Signal Analyzer I In-phase

22 7. References

1 Testing and Troubleshooting 9 Theodore S. Rappaport, Wireless Digital RF Communications Communications: Principles Transmitter Designs, and Practices, Prentice Hall Agilent Application Note 1313, 1996: Upper Saddle River, New literature # 5968-3578E. Jersey.

2 Digital Modulation in 10 Bernard Sklar, Rayleigh Fading Communications Systems— Channels in Mobile Digital An Introduction, Communication Systems Part I: Agilent Application Note 1298, Characterization, IEEE literature # 5965-7160E. Communications Magazine, July 1997, Vol. 35 No. 7. 3 Measuring Bit Error Rate using the Agilent ESG-D Series RF 11 Robert H. Walden, Performance Signal Generators Option UN7, Trends for Analog-to-Digital literature # 5966-4098E. Converters, IEEE Communications Magazine, 4 Using Vector Modulation February 1999, Vol. 37 No. 2. Analysis in the Integration, Troubleshooting, and Design of Digital RF Communications Systems, Agilent Product Note 89400-8, literature # 5091-8687E.

5 Ten Steps to a Perfect Digital Demodulation Measurement, Agilent Product Note 89400-14A, literature # 5966-0444E.

6 Fundamentals of RF and Microwave Noise Figure Measurements, Agilent Application Note 57-1, literature # 5952-8255E.

7 Measuring Third-Order Intermodulation, N dB Bandwidth, and Percent AM with Built-in Functions, Agilent Product Note 8590-8, literature # 5091-4052E.

8 K. Feher, Digital Communications, Prentice Hall 1981: Englewood Cliffs, New Jersey.

23 www.agilent.com

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