Coherent receiver with LimeSDR

As far as we will soon get our hands on LimeSDR, I consider and plan testing some applications and would like to confirm them with design team and community.
Is it possible to use one LimeSDR board to make coherent two-channel receiver? I understand that we get multiple receiver paths and at least theoretically it is possible, but there is some discussion thread below which states that these RX are optimized for some bands. Can anybody confirm or deny?

Look at the board and the possible antenna inputs, this should help you answer the question yourself :slight_smile: .

I could be wrong - I often am - but I see no way to create a proper two-channel phase-coherent receiver.

@SimonG4ELI I’m having a problem seeing why not ?
Can you explain what you are seeing that I am missing ?

“phase coherent” - two wave sources are perfectly coherent if they have a constant phase difference and the same frequency.

There are 6 RX inputs, after filtering and impedance matching networks, (RXINL,RXINH,RXINW and RXINL,RXINH,RXINW - 3 RX for the A channel and 3 RX for the B channel).

To me the two analogue paths looks fine

  • as long as the same RX input is used for each (e.g. RXINL for channel A and channel B or RXINH for A and B or RXINW for A and B)
    The same clock can be used for both RX channels so everything should be in sync.
    And once past the ADC into the digital domain there could be some misalignment due to different processing times within the FPGA, but this would be a constant. And constant phase difference is still fine for phase coherent.

I’m trying to see what you are, but I must be missing it can you point in the direction that you are looking ?

Basically … what you want to ask … is
can two receiver inputs have the same matching network and,
can two receiver inputs be tuned to the exact same frequency …

I think the answer is yes.

Having 2 Rx inputs with the same matching networks should help in keeping the phase relationship of both receivers … you will still need to adjust in software any phase error … but once thats done im sure that the phase will be maintained within some frequency span – that you will need to determine.

So you will either need to get LimeSDR to change a production board for you … or do the modification yourself. ---- though since iv not actually looked at ALL of the matching networks for all the Rx ports, there might be a pair that are the same to be used. Looking at the diagram above, their should be two ports with the same matching networks.

Now a phase error might be introduced from the RxLo chain … but again, once thats adjusted for in software … should not drift.

Just to be straight, im not a member of the Lime team or community. Im just a ham with a fairly good grasp of the quadrature radio scheme.

Good luck in your project …

Hi, all!
Thank you for your suggestions!
I tend to agree with k7noa and mzs. At least almost identical 2 RX channels should be possible.
Can anybody from design team confirm or deny this fact?


Yes LimeSDR have same matching networks pairs for both channels. This you can also see on the schematics. I mean LNAH on channel A has the same matching as on the channel B. And it is all the same with both LNALs and LNAWs.
And yes on the GUI (LimeSuite7) tab ‘RxTSP’ you can adjust phase separately each channel on block ‘Phase corr’.
Hope that helps.