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Better Measurements Using RF Signal Generators Application Note 1390 Signal Sources Provide Precise, HINT 1

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Better Measurements Using RF Signal Generators Application Note 1390 Signal Sources Provide Precise, HINT 1 9 Hints for Making Better Measurements Using RF Signal Generators Application Note 1390 Signal sources provide precise, HINT 1. HINT 6. highly stable test signals for a Improve Source’s Effective Improve FM Deviation Accuracy variety of component and system Harmonic Distortion test applications. Signal generators Use the Bessel Null method to set add precision modulation capabili- Use a low pass filter at the output of your signal generator’s frequency ties, and are used to simulate system your source to decrease its harmonic deviation. signals for receiver performance distortion. testing. HINT 7. HINT 2. Extend the Amplitude Range This guide helps you improve the Increase Power Level Accuracy accuracy of your measurements that Use an amplifier or an attenuator to involve using RF signal sources. You Use a power meter to increase the increase or decrease respectively, may increase the accuracy of your accuracy of the signal level at your the amplitude range of your signal data by using more than one of the device under test (DUT). source. hints in your test setup. HINT 3. HINT 8. Improve Frequency Accuracy Optimizing ACLR for CDMA Receiver Testing Select the appropriate frequency reference to improve absolute or Use the attenuator hold function to relative frequency accuracy. optimize for ACLR. HINT 4. HINT 9. Improve Source Match Select the Optimum Phase Noise Profile Use a fixed attenuator to reduce the mismatch error. Choose the appropriate phase noise profile to optimize in-channel or HINT 5. out-of-channel measurements. Combine Source Outputs for TOI Measurements Use a proper setup to isolate sources,improve match. Control Frac-N ALC Burst Output Modulator Modulator Attenuator Phase Detector Source Output φ Σ X2 VCO Multiplier Function ALC Generator d/dt Detector ALC Burst Mod Driver Driver Divide Function by x Generator Ext. AM ALC Hold From Ext Pulse Reference Section External FM/PM input Typical signal generator block diagram 2 HINT 1. Improve source’s effective harmonic distortion Accurate harmonic distortion Note: You can calculate the percent measurements require a spectrally distortion for a particular harmonic, pure signal source and a spectrum mth harmonic as, analyzer. The harmonic distortion of (∆dB) the signal source and the dynamic 20 %dm = 100 x 10 range of the spectrum analyzer limit the quality of the measurement. Or you can calculate total harmonic However, the signal source is often distortion: calculate the distortion the limiting factor, with harmonic for each harmonic as above and find distortion performance on the order the root sum of the squares, of 30 dB below the fundamental. figure 1 shows a typical harmonic %THD= √∑(%d )2 distortion measurement. The m harmonic distortion of a signal is often specified by stating the amplitude of the largest harmonic in dB relative to the fundamental. Use a low pass filter to improve Fundamental the source's effective harmonic distortion, as shown in figure 2. Choose the cutoff frequency of the dB Relative harmonic distortion low pass filter such that the Largest fundamental frequency is passed harmonic largely intact, while the harmonics are attenuated significantly. You can verify the performance of the source/filter combination directly with the spectrum analyzer. freq If the loss through the filter at the fundamental frequency is significant, the loss should be accounted for Figure 1. when setting the source output level. Use the spectrum analyzer to check the fundamental level at the output Spectrum of the filter, or for better level Analyzer accuracy see Hint 2. 8563A SPECTRUM ANALYZER 9 kHz - 26.5 GHz Device Low pass Under FREQUENCY RF MOD Filter Test WCDMAT Waveform: Recon Filter: 2.5 MHz ON ON ON Sample Clk: Arb Ref: Int DUT Figure 2. The harmonic distortion of a signal source improved by installing a low pass filter at the source’s output. 3 HINT 2. Increase power level accuracy In your test setup, you are likely to Note: the accuracy of the power use passive devices such as cables, meter measurement depends on the filters or switches between your calibration factors of the sensor; be source and the DUT. The accuracy sure to enter the calibration factors of the signal level at the DUT is into the power meter prior to effected by the use of these calibration. components. In some applications, for example receiver sensitivity mea- Once you have completed the surements, the accuracy of the input calibration of your power meter, signal level is critical. To have the set the power meter's measuring desired power applied to the DUT, frequency to the signal frequency. perform the following test prior to Connect the sensor in place of your making your measurements. The DUT as indicated in figure 3 and setup consists of your signal genera- measure the power level. If there is a tor, power meter with a power sen- difference between the power sor and the cables or switches that meter's reading and the indicated are necessary in the measurement as level on the source, use your source's shown in figure 3. amplitude offset feature to make the necessary adjustments. Match the Calibrate the power meter to the displayed power level of your source power sensor for an accurate power to the power meter's reading. Once measurement. It is assumed that you you adjust the amplitude at a partic- are familiar with the calibration and ular frequency, then the source will zeroing of the power meter in use. automatically display the correct value for different amplitudes at that same frequency. Since the accuracy of the power meter is very high (uncertainty in the tenths of a dB range), you can have confidence that the power level is accurate. Power Meter FREQUENCY RF MOD WCDMAT Waveform: Recon Filter: 2.5 MHz ON ON ON Sample Clk: Arb Ref: Int Test Port Power Sensor Cables, Switches, Etc. DUT Figure 3. Setup for improving level accuracy of the signal. 4 HINT 3. Improve frequency accuracy For certain measurements, the When the absolute frequency of the absolute frequency of the stimulus signal is important, increase the signal is most important, but other frequency accuracy of your source measurements require accurate by finding the most accurate relative frequency spacing between external-frequency reference avail- multiple signals. For instance, to able. Choose the instrument in your create multi-tone inputs with known setup with the most accurate time frequencies more than one signal base and connect all the other generator is often used. The frequen- equipment to this reference. cy accuracy of each source relies on its internal frequency standard. It is Some instrument manufacturers very possible for these standards to offer high-stability ovenized be slightly off in frequency, thereby reference oscillators as an option. causing relative frequency errors in These frequency and time standards the measurement. are extremely accurate, but can be expensive. For example, assume you are trying to set a 1 kHz separation between You can always improve frequency two signals centered at 200MHz, and accuracy by using a house standard your sources have ±1 x 10-6/year (a high-accuracy frequency reference aging rate. Your sources frequency distributed throughout your facility). error in this case is 200 MHz x 1 x Connect your signal generators and 10-6 = ±200 Hz. The separation could all your other equipment to this be anywhere from 600 Hz to 1400 Hz, reference. A distribution amplifier (see figure 4). To increase the may be needed to maintain proper accuracy, connect the time bases of levels and impedance matching. the two sources together. Take the reference signal output of one source, usually located on the back panel of the box, and connect it to the reference signal input of the other source. Now the uncertainty Amp 1400 Hz of the separation is (dB) 1 KHz x 10-6 or 0.001 Hz. 600 Hz 1 KHz freq Nominal (Hz) Figure 4. Shaded area illustrates the relative frequency error range of the example. 5 HINT 4. Improve source match Source match is important because When a load is not well matched, many test devices present bad there is a reflection from the load, matches. Mismatch between the which travels back toward the source and the load impedance source. Instead of being completely changes the effective signal input absorbed at the source, some of it is level to the device under test. re-reflected back toward the load. Complicating the picture, the test This re-reflected wave adds device is seldom connected directly constructively or destructively at the to the source. There are cables and load, depending on the phase of the often other components, such as signal. From a measurement point of adapters and filters between the view, concentrate on the maximum source and the load. If you are and minimum power transfer, which using adapters to accommodate the represents the maximum and connector type of the test device, minimum error that can be incurred filters to eliminate source harmon- due to impedance matching ics, and so forth, be aware that these problems. components degrade the source match as seen by the test device. The following example will help This mismatch may be reduced in illustrate the difference inserting several ways. The simplest way is to the attenuator will make in the insert a fixed attenuator with good measurement. match at the input of the test device. This improves the effective source match by twice the value of the attenuator in dB. Example Given: Source SWR = 1.9 Device Under Test with SWR = 1.5 Find:Mismatch error • Before inserting an attenuator SWR-1 1.9-1 0.9 ρ = = = = 1 S SWR+1 1.9+1 2.9 0.31 DUT ρ = 0.5 = D 2.5 0.2 mismatch error = 20 log [1+ρS ρD] = 20 log [1+(0.31)(0.2)] max mismatch error ± 0.52 dB 0.52 dB • After inserting a 10 dB attenuator with r = 0.32 2 mismatch error = 20 log [1+ρS ρD(atten) ] Attenuator DUT = 20 log [1+(0.31)(0.2)(0.32)2] 0.055 dB max mismatch error ± 0.055 dB 6 HINT 5.
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