Hello everyone. I am been lurking on here for days and doing reading and watching videos on LimeSDR for a few weeks now. I think I finally have enough information to ask few a questions understand the answers I get. I am thinking of getting a LimeSDR for designing a FMCW radar capable of ranging small drones at least upto 300 meters. I have read about FMCW radar and have some basic theoretical understanding. I have also read similar project done by Luigi LimeSDR CW radar. Now I have few questions that am not sure about and need some insights on.
First
I am thinking of choosing of 3GHz (I have read legality of transmitting such a signal in my country) as my starting frequency and have a bandwidth of 100Mhz that is, use a sawtooth to generate a chirp ramping from 3 Ghz - 3.1Ghz with a ramp time of 5ms. Is this possible with LimeSDR?
if yes?
I know the stated bandwidth of LimeSDR is 61.44Mhz this will limit my radar bandwidth and reduce my range resolution. Now I know that LimeSDR has 2 receive channel, would I be able to use
RX1_2/RX2_2 = High band specific > 1.5GHz
RX1_3/RX2_3 = Broadband 100kHz to 3.8GHz
to listen to half of the transmitted 100Mhz band each and then combine them to increase my bandwidth?
I have a lot of other questions I would like to ask but I like to get these answers first. Thank you!!
So the dedicated TX PLL can be tuned to any frequency from 30MHz to 3.8MHz. And the dedicated RX PLL can be independently tuned anywhere from 30MHz to 3.8MHz. But the two TX channels share that one TX PLL and the two RX channels share their one independent RX PLL (ref: Transmitter side specs )
So both TX channels are tuned to the exact same analogue frequency. And both RX channels are tuned exact same analogue frequency (which can be independent of the TX channels).
The two TX channels (and two RX channels) receive (or provide) their data to the digital TSP (Transceiver Signal Processor) part of the LMS7002M chip. The TSP has four independent NCO’s (Numerically Controlled Oscillators), one for each TX channel and one for each RX channel. And ultimately what that means is that both TX channels must be within the same 61.44MHz of bandwidth and independently of that both RX channels must be within 61.44MHz of bandwidth.
The end result is that with default hardware you can not combine two RX channels for more than 61.44MHz of bandwidth. And you can not use two TX channels for more than 61.44MHz of bandwidth.
The above makes me wonder if you used an external 40MHz reference oscillator on a LimeSDR-USB (instead of the 30.72MHz internal oscillator) could you access 80MHz of bandwidth (40MHz from one TSP RX/TX digital channel and 40MHz from the second TSP RX/TX digital channel, so that each is less than the 60MHz digital interface maximum). Or might the FPGA be limiting the maximum to less, or is possibly something else ?
