There are four things I would see with using an older PC with only USB 2.0 ports for SDR:
- Not enough power, you will need to plug a special Y shaped cable into two USB ports to get the required power (one connection is for data plus power and the second connector is for additional power). Since each USB 2.0 High Speed port is in theory rated for 5V (+0.25V/−0.60V) @ 500 mA, in theory two USB 2.0 HS ports could provide 4.4W in total if there is zero voltage drop across the cable (FYI: There is always a voltage drop across the cable, shorter cable = less of a loss).
- USB 2.0 HS has limited throughput, over 10x less than USB 3.0 so depending on how the USB stack was implemented (I’ve not tested this with a device or read through the source code) I would guess, for a single channel only, that somewhere from 7MHz(USB isochronous) to possibly 10 MHz(USB bulk) would be the maximum bandwidth possible (at least on paper, reality is probably less).Two channels half that, four channels quarter that. But since the ADC and DAC in the LimeSDR have minimum sample rates, I suspect you would be limited to one channel only.
- A PC that only has USB 2.0 HS ports probably has extremely limited processing power to handle very much bandwidth anyhow. So depending on the processing you may be restricted to processing 2 MHz to 5 MHz of bandwidth or less.I usually look up the “Average CPU Mark” for the CPU in the machine @ https://www.cpubenchmark.net . My rule of thumb is that if the CPU has a rating of 3500, then it should be OKish for most basic DSP with 20MHz of bandwidth or less. If you were only using 5 MHz then a quarter of that rating, but processing power required really does depend on the DSP being carried out. And smart application of DSP (e.g. decimate as much as you can and low pass filter in the very first block) can reduce CPU requirements.
- An older PC would also have much lower RAM bandwidth, which could also limit performance.