Delay Measurements

A customer asked in email about making delay measurements for measuring phase delay, and the question is general enough that I’ll post the response here:

The QA401 reports delay as part of all measurements in which the GEN1 is solely enabled. This is highlighted in the plot below.

To explore the delay across the audio band, you’ll first want to ensure you’re on the higher sample rate of 192K in order to minimize the impact of the DAC/ADC filters. This is done via the Settings->Options->General tab.

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Next, we want to use the roller wheel on the mouse to adjust the frequency of the generator so that we can easily sweep across the band to study the reported delay. Control-Click on the GEN1 button, set the frequency knob sensitivity (1 octave is good for exploring) and then uncheck the “Use Hz” box. This will force the system to use a log step.

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Now you can click the FREQ1 knob and use your mouse wheel to change the generator frequency and see how the reported delay changes. For example, below you can read at 7.5 kHz the measure delay is 10.3 uS, very close to the 10.1 uS measured above at 1 kHz.

If we want to see this delay in the time domain, the click on the TIME and OUTPUT buttons in the DISPLAY OPTIONS section. That will let you see the measurement burst:

Use your mouse to zoom in on a single cycle:

Here you can see the output in yellow, and the input is helpfully shown in a dim yellow. The delay from the input is the reported 10.3 uS.

To zoom back out, just hit the DEFAULT button in the AXIS SETTINGS group. To go back to the freq domain, hit FREQ and INPUT.