AFG-2005 noise generation bug

09.10.2016 13:49

The noise output function in the GW Instek AFG-2005 arbitrary function generator appears to have a bug. If you set up the generator to output a sine wave at 10 kHz, and then switch to noise output using the FUNC key, you get output like this:

Noise output in frequency domain.

Noise output in time domain.

The red trace shows the spectrum of the signal from the signal generator using the FFT function of the oscilloscope. The yellow trace shows the signal in the time domain.

The noise spectrum looks flat and starts to roll off somewhere beyond 10 MHz. This is what you would expect for an instrument that is supposed to have a 20 MHz DAC. However, if you set the output to a sine wave at 1 kHz before switching to noise output, the signal looks significantly different:

Noise output in frequency domain, low sampling rate.

Noise output in time domain, low sampling rate.

These two later oscilloscope screenshots have been made using the same settings as the pair above.

This is obviously an error on the part of the signal generator. The setting for the sine wave output shouldn't affect the noise output. It looks like the DAC is now only set to around 4 MHz sampling rate. Probably it has been switched to a lower sampling rate for the low-frequency sine wave output and the code forgot to switch it back for the noise function.

This behavior is perfectly reproducible. If you switch back to sine wave output, increase the frequency to 10 kHz or more and switch to noise output, the DAC sampling rate is increased again. Similarly, if you set a 100 Hz sine wave, the DAC sampling rate is set to 400 kHz. As far as I can see there is no mention of this in the manual and you cannot set the sampling rate manually. The FREQ button is disabled in Noise mode and there is no indication on the front panel about which sampling rate is currently used.

I've been writing about the AFG-2005 before. It's an useful instrument, but things like this make it absolutely necessary to always have an oscilloscope at hand to verify that the generator is actually doing what you think it should be doing.

Posted by Tomaž | Categories: Life

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