Phase jumps while transmitting DC signal

Projects LimeSDR
1

Hi!

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:

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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:

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No jumps, after running for some minutes.

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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).

2

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.

3

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

4

They’re same.

RX=2.9 GHz
TX=2.9 GHz

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

5

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

6

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