Everybody wants to use SDR technology for spread-spectrum, digital data, or other up to speed modulation processes. I'd like to find one that will do 118-137 MHz on 25 kHz channel spacing. I can't find any reference to such a device or a written word about this approach.
Anybody got a reference to get me going? Best I could find so far is a chip that will let me do 200 kHz. wide channels and that just ain't gonna work.
Thanks,
Jim
Keep in mind the 200 kHz number you're reading is likely the maximum bandwidth the device will digitize at one time, in that frequency range, not what the software "listening" to that stream of data down the USB cable (or on some chipsets, raw serial data that bad to be converted to USB format first by another chipset) is going to limit itself to.
The software in the PC (or micro PC like the RaspPi) defines the limits of which chunk of that 200 kHz you're attempting to "receive" and what modulation type will be "found" inside. "Channel spacing" is done in software.
(As is telling the dongle to jump to a new 200 kHz "block".)
As you can probably guess, designing a quality bandpass or even software switchable/tunable set of filters to place in front of an SDR A/D helps in specific overload scenarios from nearby RF sources (both in physical distance and frequency proximity).
Think of them not as radios but as broad banded receivers like you'd see in a spectrum analyzer, that digitize the whole swath of what they can, and the software will then "pick out" what it wants, digitally.
I wouldn't expect the thing to not lose its mind if you fired up the kilowatt VHF amp near it, while trying to receive something in VHF. For that you'll still need a physical filter.
This is one of the reasons that the rigs like the FlexRadio are fairly spendy. Thy have higher quality (faster and more broad banded) digitization happening *after* a set of software switchable filters for multiple bands, to keep the unwanted stuff out of the A/D segment of the radio, altogether.
Does that help?
