I want to extend the bandwidth of a PCIe or PCI soundcard to accommodate more spectrum for my SDR radio. I'll start with one that has a great noise floor spec. and no ripple in the passband. All I've found so far is the M-Audio Audiophile 192 and it's an old design. I'm guessing that they all have an anti-aliasing filter. I'm wondering if it's possible to bypass that and work on the amplifiers to make the passband flat to 100kHz?
Audiophile 192 features high-definition 192kHz sampling rate.
How to search for simple answers? like NO.
Search Engine: M-Audio Audiophile 192 specifications
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The decimation filter is integral part of the ADC chip. Therefore "no"
Even if you could change the cutoff frequency of the antialiasing filter, you cannot change the bandwidth of the card.
It samples at 192kHz. It therefore has a bandwidth of 96khz. You cannot get more bandwidth through it without changing the sampling rate.
If you check the datasheet that Tony Stewart provided, you will see that the built in anti-aliasing filter allows you to use up to around 89kHz out of the theoretically available 96kHz when using the 192kHz sampling rate. That's pretty good, and I don't think you will do better with anything you implement yourself.
If you want to sample a signal that has a frequency higher than the sampling rate, you can run your signal through a bandpass filter and then down convert it so that its frequency range is within the bandwidth of the ADC.
Say you want to sample a signal containing frequencies from 150kHz to 200kHz.
You pass it through a bandpass filter that removes everything below 140kHz and 210kHz. Now, you mix it with 140kHz, and run that through a low pass filter at 96kHz. The result is a signal with 70kHz bandwidth, with your signal of interest between 10kHz and 60kHz. You can then sample that with your ADC at 196kHz. You do have to keep the down conversion in mind when doing FFTs and things if you need to calculate frequencies from the sampled data.
I'm not going to try to write an explanation of the physical limitations of sampling, and why you only get a bandwidth of half the sampling rate. There's an explanation of the Nyquist theory on Wikipedia. I suggest you read that. It has a clear explanation, and links to other parts of sampling theory.