Application for using the 2 RX channels simultaneously

Projects LimeSDR
122

Hi IgnasJ,

I’m using LimeSDR v1.4 to receive two channels coherently, using “phase alignment” branch.
I would like to ask if the automatic channel alignment is merged in Master, and if I Can use it without loosing coherence.

Thanks

123

Hi,
phase alignment is merged into master.

124

Thanks @IgnasJ.

One more question.
I keep experiencing the 180º phase jump (for example, in the 10% of the cases).

Is there a way to detect the phase jump at the StartStream, without using an external calibration signal?
It would solve a lot of problems for me.

Thanks

125

To detect the 180º phase jump, you need to feed the same signal into both Rx channels but there is no way to do it using LimeSDR board only without external components (using leakage does not seem to be reliable). I think that to detect it reliably, you need to connect either external signal or signal from one of Tx outputs via splitter to both Rx channels and then check phase offset between channels.

126

Hi @IgnasJ , in order to use the phase alignment branch do I have to activate anything?
How can I know if this feature is active?

Thanks In advance
Mauro

127

Hi,
We are developing array processing algorithms and we would like to use LimeSDR-USB as a hardware platform.
I tried to check the time allignment and the phase coherence between Rx0 and Rx1.
For this purpose the following experiment was performed:
During one run a sequence of chirp waveforms [Fc-Fdev, Fc+Fdev] was sent from Tx0.
Then, based on the received signals from Rx0 and Rx1, the phase difference between channels for different frequencies was evaluated.
The experiment was performed several times (several runs) to collect statistics about the behaviour of evaluated parameters.
In the end, I have obtained the following results

checkRxPhaseAlign_2019-12-29_164754_hist_Fc%3D900M%2CFs%3D4M%2CFdev%3D2M%2CT%3D2%2CNstat%3D600
checkRxPhaseAlign_2019-12-29_164754_hist_Fc%3D900M%2CFs%3D4M%2CFdev%3D2M%2CT%3D2%2CNstat%3D6006000×3000 591 KB

This is collection of histograms of phase differences vs frequency. As I understand, the results shows the presence of the time offset between ADCs,
checkRxPhaseAlign_2019-12-29_164754_offset_Fc%3D900M%2CFs%3D4M%2CFdev%3D2M%2CT%3D2%2CNstat%3D600
checkRxPhaseAlign_2019-12-29_164754_offset_Fc%3D900M%2CFs%3D4M%2CFdev%3D2M%2CT%3D2%2CNstat%3D6006000×3000 254 KB

histogram of time offsets in terms of fraction numbers of the sampling interval,
checkRxPhaseAlign_2019-12-30_104330_drift_Fc%3D900M%2CFs%3D1M%2CFdev%3D500k%2CT%3D60%2CNstat%3D100
checkRxPhaseAlign_2019-12-30_104330_drift_Fc%3D900M%2CFs%3D1M%2CFdev%3D500k%2CT%3D60%2CNstat%3D1007200×3600 279 KB

histogram of drift of phase between channels with time for one run - no questions.

System configuration for the experiment:

  • T = 2s or 60s - Total time
  • Fc = 900MHz - Carrier frequency
  • Fs = 2MHz - Sampling rate
  • Fdev = 500kHz - Deviation frequency of the chirp
  • Nstat = 100 or 600 - Number of runs

I used Matlab to write the test program. You can find it by the link:
https://github.com/RakhDamir/LimeSDR-Matlab/tree/master/_testbenches
Library was compiled using source code from the LimeSuite master branch from 05.12.19.

I would like to ask, if there way to fix this time offset for Rx channels?
Thank you in advance for your help!

Best regards,
Damir