Different SXR and SXT Frequency

Projects LimeSDR
1

Hello everyone,
today i wrote a MATLAB program where i used a command called ‘LMS_SetLOFrequency’:

[TX_Centre_Frequency_Set_Status] = calllib(‘libLimeSuite’, ‘LMS_SetLOFrequency’, dev.device,1,0,Fosc);
if TX_Centre_Frequency_Set_Status ~= 0
Error();
end

[RX_Centre_Frequency_Set_Status] = calllib(‘libLimeSuite’, ‘LMS_SetLOFrequency’, dev.device,0,0,Fosc);
if RX_Centre_Frequency_Set_Status ~= 0
Error();
end

This should set the LO in bot TX and RX chains to Fosc (=19.5 MHz) right?

Then i went to check using Lime Suite GUI. By pressing chip–>GUI, I get:
In the SXT tab, Frequency = 100 MHz
In the SXR tab, Frequency = 30 MHz

So why this difference?

Can someone shed light?

2

When setting both Tx and Rx to the same center frequency, chip is configured to be in TDD mode and uses only one PLL to run them, so in your case only the 30 MHz one is used. PLLs can lock only down to 30 MHz, trying to set lower frequencies will set PLL to 30MHz and enable digital mixers to upconvert/downconvert signal to desired frequency.

3

@ricardas So even though SXR is at 30 MHz, and SXT at 100 MHz, the delivered LO frequency is indeed 19.5 MHz…is that what you are saying?

4

Yes, at least it should be that way

5

@ricardas Alright, thanks