Thoughts on a multichannel SDR using ADRF6850

What. I'd like to use a direct-conversion ZIF chip like the ADRF6850 to build a multi-channel tuner that will pick up, say, two [known] channels, one at 156.3 Mhz that's 50 Khz wide, and one at 855 Mhz that's 25 Khz wide. Demodulation will be done in a Software-Defined Radio such as GNURadio.

How. Rather than have one broadband tuner that picks up and downconverts this wide spectrum, I'd like to have two tuners - Tuner1 is tuned to the 156MHz channel and Tuner2 is tuned to the 855MHz one. I take the baseband output of Tuner1, upconvert to 25kHz IF with a 50kHz lowpass filter, upconvert Tuner 2's output to 62.5kHz IF with a 25kHz bandpass filter centered at 62.5kHz. Then I merely sum the two IFs, and then pass it through an ADC before feeding it to GNURadio where I have the channel selection and demodulation pieces. The advantage will be that the sampling rate can be kept much lower than if I used a wideband tuner to pick up everything in the 150-900 MHz range.


156.3MHz → direct conversion with ADRF → ZIF → upconvert I, Q to 25kHz IF using modulator → 50kHz lowpass → summer1

855MHz → direct conversion with ADRF → ZIF → upconvert I, Q to 62.5kHz IF using modulator → bandpass at 62.5k → summer1

Questions: (1) Any problems with the idea itself? (2) Any issues with using the ADRF6850 chip with this idea or any better choices?



Message was edited by: Ramkumar Ramaswamy. Added two lines depicting the signal flow for clarity.

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