Hi,
I am developing a SDR receiver as my bachelor thesis.
It is my first hardware project, so I got stuck on basics, probably.
Here is expected block scheme:
I will use a SDR concept, which contains an analog I/Q demodulator (AD8333) for mixing input frequency to lower.
Then will be signal sampled by A/D converter on 1,5 MSPS (ADS7851). So, Nyquist frequency is 750 kHz. But after mixing there are certainly a lot of strong higher frequencies.
I know, it is neccessary to use anti-aliasing filter. But what is sufficient value of attenuation on 750kHz?
I tried compute passive Butteworth filter 5. order - cutoff freq. 550kHz and attenuation on 750kHz is only -13dB. Because application is SDR receiver, it is important to reach cutoff frequency as high as possible for maximum bandwith.
There is a simple RC circuit described in ADS7851 datasheet:
I think, it will have not sufficient attenuation on 750kHz.
My theory:
Output signal range of VGA: 4V p-p
Voltage resolution of ADC: (4 × VREF) / (2^N) = (4*2,5V) / 2^14 = 0,0006V
So, if there is an signal of frequency higher then 750kHz, with amplitude 4V (max), it should be attenuated below 0,0006V in ideal case.
10 * log(4/0,0006) = 38dB
Is it right?
Thank you!