DC Offsets and amplitude fluctuation in TX signal

Hi,

I am using fmcomms2 project (2019r1 and No-OS). I am performing an external loop back test (TX and RX connected with coaxial cable). During the test i am streaming 1 MHz of complex sinusoid and changing the LO frequency(TX and RX synthesizer) using fast lock method. 

When i look into the demodulated signal i see fluctuations in received signal amplitude and dc offsets whenever i change the LO frequency as shown in image below,

The demodulated signal is mathching in frequency (1 MHz).May i know the reason why is it so and how can i avoid it.

The LO frequency change is only 10 MHz at each step and carrier frequency is 100MHz.

Thank you.



added extra details
[edited by: Amaresh at 5:57 PM (GMT -4) on 2 Nov 2021]
  • This can happen because of the VCO pulling issue in which if the TX and RX LO are configured to the same frequency, then there can be spurs creeping into the output. 

    You can check the TX and RX path separately by feeding an external signal generator into the RX input and then check if you are still seeing the variation in amplitude.

  • Hi Srimoyi,

    You can check the TX and RX path separately by feeding an external signal generator into the RX input and then check if you are still seeing the variation in amplitude.

    I will have a check on it and see.

    This can happen because of the VCO pulling issue in which if the TX and RX LO are configured to the same frequency, then there can be spurs creeping into the output

    Will this issue be there even when i don't loop back TX and RX? In that case how can i avoid it ?

    Thank you.

  • Will this issue be there even when i don't loop back TX and RX? In that case how can i avoid it ?

    Yes, if the RX and TXLO is the same, then you will see spurs at the output even if you are not looping back.

    The effect of TX on RX depends on the way the RX LO phase(additive or difference) is affecting the TX PLL