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SDR - Software Defined Radio / Re: Fobos SDR
« on: September 02, 2024, 2018 UTC »
That video starts to touch on the issues with the radio, but doesn’t actually get to what’s causing them. The mirroring he sees is the worst bit. It’s like the entire spectrum is doubled and folded back on itself. Say, for example, the sampling rate is set to 8 MHz. The software will set the center frequency to 4 MHz. You can’t change it, so don’t try. Say you have a mediumwave signal on 800 kHz. You will also hear it at 8000 kHz - 800 kHz, or 7200 kHz. That’s the source of the mediumwave signals showing up in the 40 meter ham band that he mentions. Rig Expert claims that this is sampling theorem aliasing issues, which is bullshit; aliasing happens at the edges of the sample, not throughout the entire spectrum. I think what’s happening is that the radio always sends the samples from both HF1 and HF2, one on the I channel and the other on the Q channel, and then presumes that the client software will distinguish which one they want to use. No other radio works this way, so almost no client software has the capability to do this. The exception is HDSDR, which actually does let you choose between decoding the I or Q channels. Did you know it could do that? Me neither. It’s still pretty broken when you do that, though. Bjarne Mjelde has a post on his blog at arcticdx.blogspot.com that digs into this further. There are images showing up from outside your selected bandwidth as well. He uses the example of Vatican Radio showing up on 405 kHz. Odd. It was also visible at its mirror image, 8000 - 405, or 7595 kHz. Well that makes sense given the mirroring issue. Except VR doesn’t broadcast on 7595. The signal is actually coming from 15595 kHz, and if you subtract 8000 from that, you get 7595. Oh, and the reason you can’t change the center frequency is because the software hard codes the center frequency to the sampling frequency / 2. I looked at the code, and it’s there. Above 25 MHz, specifically FM broadcast, I didn’t find any of these issues, but needed a Kitz Tech preamp in order to hear stations that my Elad FDM-S3 hears. With the preamp, they’re very similar. For MW and HF, though, the radio is unusable, and Rig Expert doesn’t seem to understand what the problem is.
