5831 mmWave 2nd harmonic distortion issue

[birth]

Problem:

In PXI 5831 mmWave system, the 2nd harmonic distortion of IF is about -36dBc, which is not good enough for application. 

Not sure if this performance is expected. 

Environment:

System PN: 149021D-07L

IF frequency = 5.2GHz

Output power = -10 dBm

Current action:

• Change pre-filter gain to -20dB to decrease 2^nd HD generated by RF chain.
• Result is shown below

Question:

If the action is acceptable?  if not, any recommendation ? 

thanks. 

[Andy_Hinde]

Here is a link to the NI 5831 Specs:

http://www.ni.com/pdf/manuals/377805b.pdf

There are no NI 5831 Rx harmonic specs for IF or mmW ports.

I do see NI 5831 Tx harmonic specs for IF and mmW (non-switched) ports.

I am assuming you are asking about Tx harmonics from generating out of a NI 5831 IF port. Based on the above specs you appear to be lower (-36 dBc) than the 5 GHz IF Tx harmonic spec (-21 dBc), but you are generating at -10 dBm, with the spec footnote stating the conditions werw with a 0 dBm tone offset 100 MHz from the IF upconverter center frequency (LO2). If you generate at 0 dBm you should see the 2nd harmonic also rise, and rise more than 10 dB.

However, I am assuming you are also performing this measurement in loopback with the other NI 5831 IF port acting as receiver. This means you are looking at a combination of the Tx and Rx linearity together and since there are no Rx harmonic specs, you cannot know if you are measuring Tx or Rx.

To validate your NI 5831 performance against the spec you first need to recreate the conditions in the footnote to be able to compare performance. You should generate a complex arbitrary waveform with a 100 MHz offset from DC at 0 dBm. This waveform should be generated with NI-RFSG set to peak power mode (not average power mode) set to 0 dBm.

Then you have to make measurements in such a way that you eliminate Rx linearity as a contributor. You can use a separate 5-10 GHz analyzer with better linearity. You can also use a 5 GHz high pass filter at the Tx output to remove the 5 GHz fundamental before inputting to the analyzer (which in this case could be the 5831 IF Rx).

[Andy_Hinde]

Are you using pre-filter gain on RFSG (Tx) of RFSA (Rx)?