Looking for advice measure noise figure/gain using HP 8970A, fixed LO, SMA mixer - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 17, 2025, 05:59:17 pm
content: I have a need measure noise figure/gain of a microwave amplifier at 10.4 GHz (single point frequency.)I have HP 8970A noise figure meter, suitable noise source (nominal ENR 15 dB, ENR table entry close to 10.4 GHz), SMA packaged mixer (known LO drive and conversion loss specs) and fixed frequency LO source (example 10.2 GHz with output level matching LO input drive requirement of mixer.)I don't have GPIB programmable LO source (i.e., HP 8350B etc.) nor any external GPIB system controller. I don't need to sweep across the passband of the amplifier (just a single frequency cal/test is intended.)The apparent 8970 operating mode for this test normally would be "mode 1.1" (frequency conversion in measurement chain with fairly low IF and double sideband response expected.)The problem is this - as far as I can tell the HP 8970 (A or B mechanics vary, but outline is same) expects to have a GPIB controlled external LO so that it (the 8970 acting as a temporary master on the bus) can set LO frequency (RF-IF) /level (dBm suitable for mixer) to drive the external mixer.Under 8970's envisioned operation, it would set the LO source to the required frequency/level, measure (after calibration) the hot/cold noise levels (via the external mixer) and calculate the resulting noise figure/gain at the (10.4 GHz) test frequency, compensating for the presence of mixer response and image - double sideband - but with the 8970 itself tuned to the appropriate IF (example, 100 MHz).I think there are two routes to solve this (lack of programmable LO source driven by the 8970 GPIB), but I don't have a complete picture of either route:1. Ignore measurement mode 1.1 (i.e., use default measurement mode 1.0), set LO so that the IF (frequency the 8970A will work with) is fairly low (example, 10.3 GHz LO and 10.4 GHz target measurement frequency means the 8970A would measure at 100 MHz), then set start/stop frequency to 100 MHz, set ENR table entry at 100 MHz to the table entry of the source closest to 10.4 GHz, then calibrate, then insert DUT, measure noise rigure/gain, and do (something???) to compensate for the double sidedband hig/cold noise level measurements seen by the analyzer2. Use measurement mode 1.1, set the IF to 100 MHz (as in above example), then do (something???) to let the 8970 proceed without using GPIB to set the LO frequency/level, and proceed on as if the configuration followed the mode 1.1 examples given in the user manual.3. Other method?Thanks for any advice, and of course, any requests for clarifying information!Dave

author: RoV
author_rank: Regular Contributor
timestamp: August 17, 2025, 09:02:36 pm
content: I don't have an 8970, so I don't have specific answers for mode 1.1.However, in the default mode and with the frequencies you mentioned, the mixer will add noise contributions at 10.2 and 10.4 GHz. During calibration, being the source flat, you'll have a doubled noise power both with the source ON and OFF and you'll correctly measure DSB NF by inputting the charted ~15 dB ENR. When measuring the amplifier, it will happen the same only if it is equally flat, e.g. behaves the same at 10.2 and 10.4 GHz. If this is true, also amplifier noise power will be doubled and you'll get the right answer (DSB NF = SSB NF). If it is not true, you need a filter before the mixer. This would probably force you to choose an higher IF.Keep in mind that also mismatch between the amplifier and the mixer (or filter) will affect NF if the amplifier is not a multistage high gain.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 17, 2025, 09:17:45 pm
content: Hello RoV,This is very helpful!For purpose of learning this problem, please assume amplifier NF response is flat - 10.2 and 10.4 GHz (for example) produce identical amount of RF energy, so it would satisfy the assumption leading to NF(SSB) = NF(DSB).I wish to understand the basic flow of computations when doing converted/dsb noise figure measurements. Can you help me understand what happens in DSB case?The IF passband of the HP 8970 is ~4 MHz (so when measuring at input port tuned to 100 MHz, total power of 98-102 MHz is measured with noise source on (hot) or off (cold), then applied to "Y-factor" calculation.In example (LO = 10.3 GHz, IF = 100 MHz) noise source power will be converted from 10.4 GHz and 10.2 GHz, and sum will appear at 100 MHz, so in the 98-102 MHz, energy from both mixer products are added, in both cases (noise source on = HOT, noise source off = COLD). Is extra calculation step required due to the added noise power of both mixer products appearing at 8970 input port?Thank you again!Dave

author: RoV
author_rank: Regular Contributor
timestamp: August 18, 2025, 07:11:45 am
content: Hi,I suggest you to look here:https://www.analog.com/en/resources/technical-articles/system-noisefigure-analysis-for-modern-radio-receivers.htmlThere are also several good application notes from hp (possibly renamed Agilent/Keysight)In the special case of flat response of both amplifier and mixer over the two frequencies fLO+/-fIF, no special computation is required for DSB NF.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: August 18, 2025, 12:08:06 pm
content: Thank you for helpful references, I will search for now.Dave