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Testing and Troubleshooting Digital RF Communications Receiver Designs Application Note 1314

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Testing and Troubleshooting Digital RF Communications Receiver Designs Application Note 1314 Testing and Troubleshooting Digital RF Communications Receiver Designs Application Note 1314 I Q 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 communi- cations 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 wave- form 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
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