Using Fixed and Adaptive Equalization Together
Often the adaptive equalizer is used in conjunction with digital demodulation to compensate for linear errors in the measurement system. The frequency response of the channel for which the equalizer is compensating can be viewed. This response is the composite of all linear distortion between the modulator and the measurement hardware, not just of the channel. To see only the channel effects, do the following:
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Connect the modulator directly to the VSA.
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Set up the digital demodulator appropriately for the signal.
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Choose a trace and set it to show the . (It is blank until the next step is finished.)
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Turn on the adaptive equalizer and let it run until the measurements show the best accuracy (the lowest EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format.). Make sure that enough points per symbol are chosen to make the cover at least the measurement span.
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Select the trace and copy it to register D2. The trace may also be saved in a file and later recall it into D2.
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In the tab of the User Correction dialog box, select and Register
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Reset the adaptive equalizer and rerun the measurement, again allowing the equalizer to adapt. Now the trace should remain essentially flat.
If the measurement hardware is now moved to a different point in the signal path and let the equalizer adapt, the trace will show the response of the channel between the modulator and the new measurement point. For example, if the above procedure is used at the input of an up converter, then move the measurement hardware to the output of the converter and reset the measurement center frequency appropriately, the will show the frequency-shifted response of the converter (after the adaptive equalizer settles).
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