strange NF measurement result

[pavel_adameyko]

Hello, friends.
I've measured NF of LNA recently and got very strange NF plot.

What are the reason of strange oscillations of NF coefficient ???
When i monitor output spectrum, LNA doesn't seem to oscillate.

Thanks in advance.

 

[BigBoss]

Have you checked the oscillation until fmax of the transistors ??

 

[tony_lth]

The LNA match bandwidth is normally narrow, if your bandwidth is (2-1)/1.5GHz=67%, it seems too wide. What is your objective bandwidth?

 

[pavel_adameyko]

Hello
Thanks for your replies.
I've monitored output spectrum up to 26.5 GHz (max frequency of spectrum analyzer) with span of 1GHz - so the output noise floor was near -55 dBm - i've noticed nothing strange.
Actually my working bandwidth 1575 - 1605 MHz. At these frequencies NF oscillations are highly attenuated (as you can see from the picture), but present a little. So i'd like to know the reason.

 

[tony_lth]

so that's OK, I think nth is wrong.

 

[BigBoss]

I guess NF calibration is wrong or there are something strange on the cables...
NF can not drop down to -1 dB..mathematically impossible.

 

[pavel_adameyko]

Nf calibration has been done before measurement and after calibration i carry out one measurement with noise source connected directly to spectrum analyzer (as during calibration) - so i get 0 dB gain and 0 dB NF, as it should be. So i don't think this is due to wrong calibration. The similar phenomenon i get when i measure NF of microstrip line on PCB - although gain remains constant, for example -2 dB, NF hesitates, sometimes becoming less 0 dB which is physically impossible.
I guess this is connected with reflection waves in case of unmatched load.

 

[tony_lth]

It should be no problem except you use very bad performance cables or connector, because your freq is too low, less than 2GHz.

 

[rfboardengineer]

If the device has poor match and the cables to connect are are 1 meter with a dielectric of 2 you could get 100MHz ripple like you see. Put a 3dB pad at the end of each cable (device side) you should see NF+3 and less ripple.