Record and Playback performance goes down at high frequency

[youngmin]

Hi,

We're testing record and playback using USRP-2920( LV 2013sp1, NI-USRP 14)

To test it, we downloaded record and playback example code.

Signal source is a function generator outputting 6 MHz bandwidth 8 VSB modulation signal.

USRP Rx parameters are center = variant / gain = 0 / IQ rate = 25MHz and Tx parameters are center = same as Rx center / gain = 0 / IQ rate = 25MHz

The problem is... in low carrier frequency like 70MHz, quality of playback signal is great.

But in high carrier freqency like 2.2GHz, SNR and signal power dropped significantly i.e. they tend to drop as carrier frequency goes high.

And  the strange thing is as carrier frequency is increased, a small peak on the center is increased as well.

Please check the attatched file to see the tendancy.

So.. my question is why is this tendancy occures?(we tested another USRP and the result was same as before)

and what should we do to reduce this effect?

Regards,

Youngmin

[vvkakade]

Hi,

I suggest a couple of things:

1. NI USRP 2920 is a non-calibrated device. So setting a gain value of 0 at different frequencies could mean a different actual gain. So try increasing the Rx recording gain (on the USRP) in cases where you observe low SNR. Similarly Tx is not calibrated.
2. Is the signal recording done with cables or over wireless medium ? You can try to eliminate any spurious signals if you use SMA cables while recording.
3. I think the power at the center of the spectrum is the DC offset. You may refer to the DC Offset/LO leakage section in the best practices section here http://www.ni.com/white-paper/13881/en/

Thanks.