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Network Analyzer Cal question

Discussion in 'Electronic Design' started by sdy, Nov 24, 2012.

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  1. sdy

    sdy Guest

    I want to measure the SWR of a device. There is a Narda switch between the NA port and the device. If I perform the cal with the standards placed after switch, will that remove the SWR of the switch from the measurement?
     
  2. josephkk

    josephkk Guest

    I do not think it will help as stated. That approach is too simplistic.

    ?-)
     
  3. Gootee

    Gootee Guest

    How could it not?
     
  4. It comes down to whether the Narda switch and the cables/adapters used
    to connect it are of a higher expected quality than whatever you are
    trying to measure. Are they? At HF, it would not usually make sense
    to worry about de-embedding microwave-grade interconnects. At higher
    frequencies closer to the accessories' rated limits, it can be a
    bigger deal.

    By definition, the measurement's reference plane is defined by where
    you placed the standards when you did the calibration. In general,
    the farther these components depart from an ideal 50-ohm transmission
    line at the frequencies of interest, the less likely that you can move
    the measurement's reference plane to the downstream side simply by
    calibrating after the additional components. At some point, you will
    need to de-embed the accessory hardware by characterizing it
    separately, in order to remove its influence from the final
    measurement data. It's hard to say exactly when/where this is
    necessary.

    For instance, when you take a wideband sweep at a remote reference
    plane, you are relying on your analyzer to interpolate its error-
    correction terms between discrete calibrated points. Often, cables
    and accessories will exhibit a lot of VSWR ripple that the calibration
    process will 'undersample.' Calibration alone can't take the
    accessories out in this case, but a good deembedding algorithm can.
    If you are making a narrowband measurement, or a measurement at a
    single frequency, you will probably be fine with calibration alone.

    Joel Dunsmore, who has worked on VNA stuff at Agilent since the 8753As
    in the mid-1980s, just came out with a nifty book on the subject:
    http://www.amazon.com/Handbook-Microwave-Component-Measurements-Techniques/dp/1119979552

    But there are still no "Yes, you can do that," or "No, don't do that,"
    answers to be had, in the general case.

    -- john
     
  5. gootee

    gootee Guest

    Thanks, John. That is interesting and enlightening.

    We have some late-model 4-port Agilent network analyzers, in the labs in mybuilding at work. Given how incredibly expensive they were, one might think that THEY should tell US if what we're trying to do is valid or not!
     
  6. josephkk

    josephkk Guest


    Thank you so very much for expressing clearly what my lab experience from
    back when had taught me.

    ?-)
     
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