Working NBFM transceiver on GNU radio


Would you mind post the flow graph file.

Couple of them here too:

Haven’t tried them yet.

Hi @JMG I see you are using osmocom Sink & Source blocks to connect to the Lime SDR. I can not find any osmocom Sink & Source blocks in my own instance of GRC. Do you perhaps have any kind of idea of what I am missing? Did you have to build your GR instance from source or use one of the Myriad developed snaps? Thanks! Steve

Here is the grc file of the NBFM transceiver:

Realising a transceiver does not simply mean combining a receiver and a transmitter. Some additional aspects apply.

Latency of switching RX to TX and vice versa has to be optmized. The throttle block significantly improves switching time.

Handling the “center spike” by the blocks ‘Signal Source’ and ‘Multiply’ is much faster than the alternative of applying the ‘Frequency Xlating FIR filter’, which will otherwise give the same functionality.

The center spike has been shifted to a fixed position at the very edge of the spectrum at 144.00MHz.

I did not find an easy way to completely turn off the transmitter during receive. The settings

  • Osmocom sink: RFgain=0, IFgain=0, BBgain=0
  • Low pass filter: Gain=0

will drop the power level down to an uncritical level below -60dBm.

73 de John


The program will not work under the new version of PothosSDR-2017.05.16-vc14-x65.exe any more. LimeSDR does not connect to GNU-Radio.

LimeSDR does not connect to GNU-Radio.

Not sure what this means. Does switching back to the previous installer make any difference? PothosSDR-2017.05.02-vc14-x64.exe

I’m just curious because I dont think there were limesuite changes between these two installers.
Also curious to know if the device can be seen by some of the utilities:

Hello Josh,

thank you for taking a look at the problem.

Yes, switching back to a previous installer solves the problem. The device can be seen by Lime-Suite.

Booting gives a “Python.exe error” message and it stops after logging these messages:

I have added a 8W power amplifier to LimeSDR. The PA is an old design with an integrated harmonic filter and an RF-operated pin-diode antenna switch, so it can be operated using a single antenna for receive and transmit. This makes it a fully operational transceiver for the 2m band.

The program runs under the Windows installation of “PothosSDR-2017.05.02”.

The system has been modified to operate as an experimental “range extender”. It receives an OOK modulated ISM signal in the 860MHz band and retransmits it in the 144MHz band as a FSK modulated high power signal.


For ease of operation, fine tuning of the receiver frequency has been added.

1 Like

Did anybody manage to run the program under a more recent version of PothosSDR than PothosSDR-2017.05.02?

Anybody know why the FM Receiver is multiplied by a -1.75Mhz Cosine wave? Other examples I’ve seen that decode local broadcast fm, work fine without this block.

On the transmitter side, instead of going to a sink device, I have it connected directly to the LP Filter / Receiver block, however, despite the audio tone sounding correct, the FFTs shows no 19hz pilot signal, nor the 2khz source. Could it be an issue with the filtering, or a mismatch of sample rates?

@Garmus may be able to advise.

It avoids zero IF and its center spike.