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Author Topic: Noise Figure question  (Read 1272 times)
N4UE
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Posts: 708




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« on: February 28, 2017, 01:59:50 PM »

Hi. I don't have a HP 8970B Noise Figure unit. But, I was wondering if the following is possible....
I have a Spectrum Analyzer / Tracking Generator and I recently repaired an older Parks 432 MHz preamp.
I understand that the best NF is usually found at less than maximum gain.
So, is it possible to find a good compromise at less than max gain by adjusting up/down the curve?

Maybe this topic should be in Elmers?

thanks
ron
N4UE
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W4OP
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« Reply #1 on: February 28, 2017, 02:10:21 PM »

The relationship between gain and NF is complex and varies from device to device. You are correct that best NF is usually achieved by purposely mismatching the input of the LNA. Simply reducing the gain tells you nothing- it could increase or decrease the NF. The best bet is a stable signal generator set at  a very low level- just discernable- and adjust the LNA for best S/N by ear.

Dale W4OP
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N4UE
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« Reply #2 on: February 28, 2017, 04:07:47 PM »

Hi Dale, thanks for the reply. I have a 8656B and a 8640B (that's under repair). Also, a lot of good attenuators.

Thanks again

ron
N4UE
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W1VT
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« Reply #3 on: February 28, 2017, 04:38:58 PM »

One trick to tuning by ear is to use a wide band FM detector with a weak signal. 
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