By using ADC path (in XR-USB-AUDIO-2.0-4C) I am trying to capture an analog signal and perform some DSP analysis on the signal.
The input analog signal is centred around 48KHz and has a BW of 2KHz i.e, signal is from 47 to 49KHz. To take advantage of bandpass sampling theorem I am using sampling frequency of 88.2KHz (lower than the Nyquist rate i.e Fs>= 98KHz). However I donot see any signal on the captured data.I have 2 questions:
1) Is there some kind of Analog-anti-aliasing filter at the front end of ADC to filter frequencies below 44.1KHz (when Fs=88.2KHz). If not, is there any other reason for not receiving the signal while using bandpass sampling technique.
2) If there is a way to avoid this issue.
Bandpass sampling using XR-USB-AUDIO-2.0-4C
You can't do the kind of bandpass sampling you want to do with XR-USB-AUDIO-2.0-4C. It contains a sigma-delta oversampling ADC (2x CS4270) with digital low pass filter. http://www.cirrus.com/en/pubs/proDatash ... 270_F1.pdf
Refer to figure 40 for the bad news that your 48kHz signal will be down by 50dB or thereabouts when sampled at 88.2kHz.
If you want to obtain the 48kHz signal with this kit you would be better off sampling at 176.4kHz or 192kHz so long as you don't mind the extra bandwidth.
You could probably achieve what you were looking for by changing the XMOS firmware so the codec runs at 176.4kHz but you downsample by two (i.e. just toss out every 2nd sample) before putting it over the USB -- if you need to achieve a lower sample rate for some reason. This would take some mucking about but it might not be too hard. Or you could just do the filtering in the PC software.
I suppose if you wanted an even lower USB rate you could downsample 192kHz by 3 to get an effective 64kHz sampling, that would give you a "standard-ish" rate and alias the 48kHz to 16kHz.
Refer to figure 40 for the bad news that your 48kHz signal will be down by 50dB or thereabouts when sampled at 88.2kHz.
If you want to obtain the 48kHz signal with this kit you would be better off sampling at 176.4kHz or 192kHz so long as you don't mind the extra bandwidth.
You could probably achieve what you were looking for by changing the XMOS firmware so the codec runs at 176.4kHz but you downsample by two (i.e. just toss out every 2nd sample) before putting it over the USB -- if you need to achieve a lower sample rate for some reason. This would take some mucking about but it might not be too hard. Or you could just do the filtering in the PC software.
I suppose if you wanted an even lower USB rate you could downsample 192kHz by 3 to get an effective 64kHz sampling, that would give you a "standard-ish" rate and alias the 48kHz to 16kHz.
Thanks for the insight.