HP 8970B Noise figure meter

[gouzou]

Hi all

Can I use the HP 8970B Noise Figure Meter to measure the NF of an amplifier at 2-3MHz?

 

[albbg]

The low frequency limit of the instrument is 10 MHz

 

[gouzou]

could i use an external mixer to upconvert the signal?

 

[albbg]

Yes I think it's possible. The measurement has to be calibrated using the mixer alone, before. You need a low phase noise and high stability LO

 

[gouzou]

I may want to use my spectrum analyzer for the job.
The noise source operates with a pulsed input only, or can i apply a constant 28V DC to produce the hot noise?

 

[gouzou]

Ok, here's my thought.
Suppose I want to measure the NF at 2 MHz. I can use an external mixer with say 100MHZ LO and get an SSB measurement, say the USB (102MHz).
The measurement bandwidth of the 8970B is fixed at 4MHz (don't know if it's higher), so the instrument will measure noise power from 100MHz-104MHz. Provided that there's no ideal mixer and a LO leakage will always exist, the measured power will be higher, creating a false measurement.
I don't know if this situation can be overcome by the calibration. I mean, does the calibration+measurement cancel out the lo leakage contribution?

 

[jiripolivka]

Ok, here's my thought.
Suppose I want to measure the NF at 2 MHz. I can use an external mixer with say 100MHZ LO and get an SSB measurement, say the USB (102MHz).
The measurement bandwidth of the 8970B is fixed at 4MHz (don't know if it's higher), so the instrument will measure noise power from 100MHz-104MHz. Provided that there's no ideal mixer and a LO leakage will always exist, the measured power will be higher, creating a false measurement.
I don't know if this situation can be overcome by the calibration. I mean, does the calibration+measurement cancel out the lo leakage contribution?

All above methods can be used.
For the low frequency I also used a spectrum analyzer. It was an older HP model, with channel memory for integrated maximum recording, and 1, 2, 5 and 10 dB per vertical division.
I used two noise sources, the ambient and LN2 (77K) temperature (a cooled 50-Ohm termination) and the Y-factor method worked well. You can use boiling water as 390K "hot" input, or, the HP or Agilent calibrated noise source for the test. Usually, however, as the 28V noise source comes as an accessory to the HP8970B noise meter, it as certainly it has not been calibrated at 2...3 MHz.

 

[gouzou]

Yes, you're right on that.
So I see I have two options
1) Use the spectrum with custom-made cold/hot sources if I want to use the Y-factor method (there are others, like the twice power method and the single 50 Ohm measurement method)
2) Use an external mixer to upconvert the signal to a range measurable by the 8970B. In this case, my thought about the LO leakage is valid? See post #6.