开云体育

I am wondering if the nanoVNA could be used to measure the noise figure for a preamp. If it could, I am assuming it would use S21 results in some manner.

Hi Bruce.

I see no one replied so I will make some noise! Pun intended. What FUN.

Apparently some folks have considered placing the VNA into a spectrum analyzer (SA) mode. Hence, with some care you should be able to read noise power. If you precede the VNA with sufficient gain and a very low noise preamp, say below 1 dB, then the converted VNA as a SA could be used to measure the cold and hot state of a noise source. You can build a pretty decent noise source or find a used hp with an ENR of 15 dB. Then you can use the Y factor to find the NF of the DUT. This is a fun project.

GL, Regards, Alan

HOWEVER:
The frequency steps on the NanoVNA are quite large depending on the settings.
Now, with that said, maybe Rune can create an SA NanoVNA-Saver offshoot application that would perform multi-segmented scans with very small frequency steps?
Erik had replied to my suggestions in an earlier post regarding using this device as an SA.Any comments on Alan's idea?
Regards,Larry

From my limited understanding of the Y factor noise measurement you only need to measure power ratio in a not so critical frequency band. Correct?
If yes. The CH1 input can function as a power meter with a not so we'll defined bandpass filter
Sensitivity is limited by the noise factor of the SA or about 5 and leakage from the SI5351
In console mode you can switch to measuring the CH1 power without the DSP filter so you use the adc anti alias filter as band filter. It's basically working as a direct conversion to DC receiver.

so - this is something that Rune might be able to add to his 'Saver application?

Could always try! But it's not first on the wishlist. I'll keep this thread
linked on the backlog :-)

--
Rune / 5Q5R

Correct.

The measurement conducted is a SPOT NOISE FIGURE measurement. The VNA or a SA may be set into the CW or fixed frequency receive mode. If you desire a swept NF measurement, than perhaps steps may be a small concern but still, that would be easy to average. Yes, conducting NF measure at BAD or bright frequencies is not easy. Try conducting a NF in the middle of the FM radio band. You need to be in a screen room. However, for the most part most of the spectrum is pretty quiet.

The LNA in front of the SA is critical. You need to have a low noise figure to eliminate the SA from contributing to the noise figure value, so called 2nd stage contribution affect and you must have sufficient gain to over come the intrinsic noise power of the SA. You must see a definitive noise power shift on the SA between firing (HOT source) and going COLD (noise source OFF) to determine the Y factor value.

Bandwidth is not an issue as long as there is sufficient noise power visible on the SA. Recall, noise power is directly linked to BW as a 10 log BW value. Our assumption here is that the BW of the DUT is larger than the BW of the measurement system. Measurement of NF of narrow band devices is difficult simply because there is insufficient noise power!

Alan