I love the product and have backed the early bird LimeSDR. Now I have a few questions.
I am a bit confused about the sample rate quote on the crowdsupply page.
61.44MSPS (Limited by USB 3.0 data rate)
The LMS7002M v2.8 datasheet quotes RF bandwidth at 160MHz analog/60MHz digital. I’m not sure I understand the drop from 160MHz to 60MHz. Perhaps a divide by 2 makes sense because we need an I and a Q for each sample. I notice the v2.0 datasheet only quoted 108MHz RF/54MHz IQ bandwidth. Although that quote is for a reduced bandwidth, at least the factor of 2 makes sense there, which again I think is because of IQ sampling.
This question leads into a few other questions.
Is the LimeSDR really limited by USB3.0 data rate, considering that one channel can only produce 60MSPS? Or is it that each channel can produce/consume 60MSPS and having multiple simultaneous channels could cause an application to bump into the USB3 data rate ceiling?
Does the maximum sample rate change for the PCI express option?
For PCIe2.0 x1 using the theoretical maximum transfer rate of 500 MB/s, I come up with maximum bandwidth of any SDR “sample pusher” to be about 166 MSPS (12-bits/sample for both I and Q). PCIe3.0 x1 bumps that to 984 MB/s, or 328 MSPS. I’m not sure I’d ever want or be able to consume or produce that much data across a serial bus, due to other limitations like bus bandwidth, latency, and host CPU processing… but curiosity gets the better of me. I think real world applications will of course implement as much logic in the FPGA as possible to avoid the need for raw IQ data across any bus option.
Lots of questions! Thanks for your time, awesome SDR.