Phase jumps while transmitting DC signal


I’ve run into this sutuation.

While transmitting DC signal (I=const, Q=const) from TSG or digital interface, received signal (as measured by LimeSDR and external spectrum analyzer) have phase jumps, like in phase modulated signal.

Here is phase jump:

After I’ve changed TBB gain, this process started to alternate. For example, 1 second with multiple jumps, another second without jumps.
After running for a while, jumps completely gone.

Jumps after changing gain:

No jumps, after running for some minutes.

At first, it looked like some DAC bit is getting out-of-clock at digital interface.
But even using TxTSG problems remains.
Happened mutiple times, with different sample rates, different gains and at different frequencies (2 - 3 GHz band).

I’ve seen something about this sort of thing before - could it be the DC bias correction kicking in at inopportune times?

There’s a way to tell the board to stop the DC correction algorithm via the API; you might give that a try.

What are Tx/Rx PLL frequencies? If they are very close together it could create problems

They’re same.

RX=2.9 GHz
TX=2.9 GHz

(shifted from 2.4 GHz to eliminate Wi-Fi interference)

That’s the problem. Unless you are running in TDD mode, having both PLLs with the same frequency can produce effects such as this.

Look here for the specific instructions to lock the SXR and SXT: