Question Regarding Automated AMP Frequency Response

[Wax_Masterson]

I have recently received my QA403 and have started running some basic tests in order to get familiar with the unit. I have a pair of 6” RG174 loopback cables connected to the + inputs/outputs with the – inputs terminated into 75 Ohms.

When I run the automated AMP Frequency response test, I notice a sharp rolloff before reaching 20K. I have run the test at the following settings:

• 64K FFT @ 48Ksps
• 128K FFT @ 96K Ksps
• 256K FFT @ 192 Ksps

I’m not sure if I have something setup incorrectly or if there is a problem with the QA403. Here is a graph of the three runs:

Also, when starting the analyzer, if I change FULL SCALE INPUT (dBV) to any value less than 24, the analyzer displays a yellow message “Connect the QA403, See File-> Device to change” in the lower left corner and appears to lock-up, with the only means of resetting being to disconnect the USB cable and reconnect it.

[VAR]

What does the response look like if just running a “standard” frequency response using the ExpChirp method over the same freq range?

[Wax_Masterson]

I believe this is what you are looking for but if not, let me know.

[Giuseppe]

AMP Frequency response test

Hi Ivan, Your plot is showing 192Ksps it looks like. I took the QA403 on my desk and ran 3 sweeps, starting with 64K FFT for 48Ksps sample rate. And 96K I used a 128K FFT, and at 192K I used a 256K. For the sweeps, you can see the 48K has a bit of peak (0.01 dB at 20 kHz) and then drops off sharply. That is purely the digital filter inside the ADC. For the 96 and 192K sample rates, you can see a bit of what looks like a first order filter rolloff before the sharp drop. That is an RC in the fr…

You can see how to do the correct test recommended by Matt

[VAR]

Looks pretty darn flat to me. I think you could use much less attenuation than 42dB- probably like 18dB with a 0dBV input. My guess is that here is something off in your setting for the AmpFreqResp plug in as far as your start and stop frequencies and step size…?

[Giuseppe]

Try this setup and you will see that the frequency amplitude response will be correct.

[Wax_Masterson]

@Wax_Masterson

automated AMP Frequency response test

Giuseppe and VAR . . . thanks to both of you for the timely replies and excellent recommendations. Both problems have now been resolved. The early rolloff problem was resolved by changing the THD stop frequency to 24kHz per Giuseppe’s recommendation. The Automated AMP Frequency response test now plots correctly, and is identical to Guiseppe’s plot.

With regard to the QA403 disconnect problem (last paragraph of first post), this problem was resolved by removing the USB connection from my Dell docking station port and connecting it to an externally powered USB hub (also connected to the Dell docking station). If I was to wager a guess, it would be that the QA403 was drawing too much current from the docking station and it was disconnecting the QA403, thus requiring a USB disconnect/reconnect to work again.