Measure noise figure of something with really low noise...
  • UnknownEditor
    4
    Say you had a really nice LNA with 1 dB noise figure expected but you were measuring 1.5 dB. What are the sources of errors? Reflection coefficient of the LNA (which is not good)? Could some low-level bias line oscillation ruin the data? Which is better, PNA with NF option or spectrum analyzer using Y-factor? Maybe that microscope light should have been switched off?

    Or maybe the LNA is not as good as advertised? Obviously, figuring this out should involve measuring a sample greater than one, but that's where we are for now...

    Steve
    UnknownEditor Options
  • rfdave
    0
    You've probably figured this out by now, as you're a smart fella, but at one point, Keysight has a Noise figure uncertainty calculator on their website that would estimate how much error there was in a measurement, given the VSWR's and the ENR performance. That might give you some direction.
    rfdave Options
  • rfdave
    0
    You've probably figured this out by now, as you're a smart fella, but at one point, Keysight has a Noise figure uncertainty calculator on their website that would estimate how much error there was in a measurement, given the VSWR's and the ENR performance. That might give you some direction.
    rfdave Options
  • epos
    0
    Both PNA and Y-factor measurement options are sensitive to oscillations, and external sources making its way into the signal chain. Y-factor measurement depends on VSWR, and for sub-dB measurements, the ENR of the source is affecting the measurements, and can vary with calibration year per year. AFAIK accuracy with the newest USB-C sources from Keysight is in the range of +/- 0.2 dB NF with a known DUT (I have had the luxury of using a PNA and Y-factor equipped NFA side-by-side). PNA measurements are can be difficult as compression will affect the measurement.
    epos Options
  • UnknownEditor
    4
    Did some experiments with putting an isolator in front of the LNA and trying to subtract out its loss. It definitely improved the bumpiness on the Y-factor measurement.

    Oscillations are a possibility, when we started taking data the NF was like 10 dB but the amp was singing like crazy. Capacitors and resistors seems to have fixed that.

    I will look at the VSWR uncertainty calculation...

    I think part of the larger picture is that just maybe the noise figure shown in a MMIC data sheet was measured on-wafer, and the user has to suffer some loss mounting it to a circuit board. For yucks we might but a Qorvo CMP2626 LNA eval board

    https://store.qorvo.com/products/detail/qpa2626eval-qorvo/597407/

    But it does not appear to have a "through line" to calibrate out the board losses.
    UnknownEditor Options
  • rfdave
    0
    I came across this
    https://www.highfrequencyelectronics.com/Jan10/HFE0110_Marsan_Part1.pdf from the 2010 High Frequency Electronics on measuring sub 1dB NF parts. I haven't scrubbed through it, but it might be applicable. there's a P2 as well.
    rfdave Options
  • UnknownEditor
    4
    That's a good article. Next time I get stuck measuring NF I will be better prepared. Thanks for all your help!
    UnknownEditor Options
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