I have recently got a HP 461A linear amplifier (from the 60's) which works from 1Khz to 150Mhz.
I have checked that:
- it's input gives a SWR <1.1 across all spectrum (i.e., the input impedance is very close to the nominal 50ohm).
- The 40db Gain is very flat across the spectrum, with +/- 1db deviation at most.
Now I am trying to calculate it's noise values (Factor, Figure, Noise Density, etc), and I want to understand all calculations mostly for educational purposes.
I have a HP 8591E spectrum analyzer so I did the following:
- Connected a 50ohm terminator at the input of the amplifier
- Connected the output of the amplifier to my SA.
- Configured the SA to take into consideration the 40db external amp.
- Used the "Noise Marker" functionality to measure the noise density power in dBm/Hz.
The result is a reading of about -160dBm/Hz (at the moment, summer, room temperature is 300K) See: https://www.dropbox.com/s/udp72vmsg0yqlfh/dbm1.png
I am still a bit surprised by my measurements. They seem too good to be true for this >50years old unit. Doing some calculations,
- -160dBm/Hz is equivalent to 1e-19 Watts/Hz(https://www.wolframalpha.com/input/?i=-160+%3D+10*log_10%28p%2F0.001%29)
- From this I can (using the formula from https://en.wikipedia.org/wiki/Noise_temperature) calculate the Noise Temperature T=24.1 (https://www.wolframalpha.com/input/?i=1e-19+%3D+t*300*k%2C+k%3D1.381*10%5E%28%E2%88%9223%29)
- From this (again using the equation in https://www.wolframalpha.com/input/?i=-160+%3D+10*log_10%28p%2F0.001%29) I obtain a Noise Factor of 1.08 (https://www.wolframalpha.com/input/?i=%28300%2B24.1%29%2F300).
- Taking 10*log_10 of the Noise Factor, I obtain a Noise Figure of 0.33.
All these numbers seem quite impressive. Are they really possible? Granted this HP 461A amplifier did cost >300 USD in 1964 (more than 2k of todays USD, accordingly with https://www.usinflationcalculator.com/)... but is it possible that it is really so good, even compared to today's amplifiers?
Regarding noise, it's original Manual/Datasheet only says "less than 40uV of equivalent wideband (150Mhz) input noise".
Perhaps, rather than focusing on the specific measurements of my amplifier, I'd just like to know if my calculations are correct/sensible. So:
GENERAL QUESTION: If I read in a datasheet that a LNA has noise figure of 0.33, how much should I expect (from purely mathematical calculations) when I plug a 50ohm resistor at the amplifier's input, and measure the noise power density (in dBm/Hz) of its output with a modern spectrum analyzer?
NOTE: I have also asked this question on the EEVBlog forum (see: https://www.eevblog.com/forum/rf-microwave/help-understanding-noise-figure-of-amplifier/msg3134954/#msg3134954 )
