I would be interested in using the QA403 for near DC signal measurements, down to around 1 Hz for tone arm resonance measurements. I don’t need an enormous precision, just enough bandwidth to go down to 1 Hz.
Would it be doable without any DC modifications ?
If a DC modification is needed (input capacitor suppression), does the ADC converter allow DC measurements ? Or is it blocked by Delta-Sigma converter filters ?
Last, if the input capacitors are removed to allow DC, what is the internal reference voltage of the ADCs ? Would the input digital zero be at 0V, or 2.5V, or something else ?
Hi @Olivier, if you remove the caps the input voltage is zero-referenced, which means referenced to the BNC shells. So, if you bypass the caps and short the inputs with a shorting block, the subsequent circuitry will condition everything to make the ADC happy.
But the input caps are series 4.7uF followed by 100k shunt, which should give you a corner around 0.4 Hz. I think if you set the Measurement STart to 1 Hz and the Measurement Stop to 10 Hz, and used a larger FFT (64k or so) should be able to make RMS measurements that would reveal much of what you might be looking for.
Seems like the original low end bandwidth is ok for this use without any modifications. A moving coil cartridge signal is around 1 mV RMS, should be OK with the +42 dB input gain setting. Thanks to the enormous signal / noise ratio of the QA403 i don’t think that a preamp is needed.
As a side note, an FFT with 64k samples would give a very slow display at 48 kHz Fs. Would be nice to have a decimation setting and a slidding FFT window, and use a smaller FFT window.
This would give a faster and more complete display of this kind of very low frequency signal.
Hi Olivier, yes, your understanding is correct BUT the 42 dB input is the full scale input. So, +42 is least sensitive, and 0 dBV is most sensitive. Use the 0 dBV and you should easily be able to see noise differences around 1 Hz. I don’t think you’ll need an amp. When you add the RMS measurement, that will report to the total “energy” between the frequencies you specify. Right click on the RMS button and you’ll see Measurement Start Frequency and Measurement Stop Frequency. Change that from 20 to 20k to 1 to 10 Hz. And then, add an RMS Volts measurement. And when you run, you’ll see the traces in that region. Pick a 64K FFT to ensure you are getting below 1 Hz. In the plot below, you can see the region from 1 Hz to 20 Hz (right click on XLOG to set your desired limits). If your moving coil was playing a signal at 1 mV, that’d be a -60 dBV amplitude. And so, you have about 60 dB of margin to measure differences in response around 1 Hz.
And when you want to go back to default settings, just do File->New Settings and that will restore everything to defaults (FFT size, measurement start/stop, etc).
PS. This topic seems broadly interesting and you are in some new territory here. If time permits, please share plots as I think this would be interesting to see what you learn.
