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VNA S21 has step in the zero user calibration vs frequency

By VK3NH •  2016-12-31 12:36:41 •  627 clicks
I had noted this issue before when checking the KC901V insertion loss measured in field strength mode using my HP reference stepped attenuator set pictured below. Note all connections here are made SHF rated semi rigid hard line so cable and connector quality should not be an issue.

The KC901V is in VNA S21 mode, sweeping the full 7GHz range with 1dB/div range.

The first result you can see side on in the first picture is after a basic user calibration with the both stepped attenuators set at 0dB. Note there is a small step up from -0.4 dB to 0dB just at the start. The blue hold trace is the calibration result and the yellow is after a 1dB step. I assume the ripple at the higher end is a result of reflections because as stated in the specifications Ports 1 and 2 are not a perfect 50 ohm match.

This result is for a 5dB step.

The next result is a user calibration with the attenuator set at 3dB. The second sweep is with the attenuation set to 5dB to give a 2dB change with respect to the user calibrated reference point. Note that frequency at which the 0.4dB step is has changed. The 2db result is somewhat smoother at higher frequencies now that that of the first display picture.

The last picture compares the hold user cal result (for 3db setting) with a new 0dB user calibration.

Many tests later with this and other fixed reference attenuators I have found that the user calibration step disappears above 7GHz with more than 7dB insertion loss.

I am interested to know what causes this and if it can be changed or compensated for.

regards Noel

frequency  connector  reference  strength  before 

9 Reply | until2018-05-26 09:54Add a reply
    BG8NPK  2017-03-01#1 -Reply

    @VK3NH it may caused by a software issues in factory calibration for your instrument. we have fix this last year so other KC901V dose not have this issue.

    please wait for our software update and you can do a calibration after update the firmware.

    VK3NH  2017-02-23#2 -Reply



    Have you got feedback for me on this or any of the many comments / Themes that have been made since?

    regards Noel

    EA1DDO  2017-01-08#3 -Reply

    @VK3NH Hi Noel,
    I´ve only got a 30dB attenuator.
    A variable one will be a good addition for extra measurements... I need to keep an eye on E-Bay.
    At same time, I am looking for a small test cables to fit in my KC901´s case. Actually I have some Sucoflex (the blue one, good up to 18 GHz), but are too big for this case. I think something made with smaller cable, for example LMR200 or RG400.
    73, Maximo

    VK3NH  2017-01-08#4 -Reply


    Thanks for doing that. That is right except that you have used a 2dB/div scale instead of 1dB/div. If you had a 3dB fixed attenuator to add in line you could have shown the step effect at the start moved across the screen.

    My result with a direct lead connection between ports 1

    EA1DDO  2017-01-08#5 -Reply

    EA1DDO  2017-01-08#6 -Reply

    Hi Noel, Andy, I have just completed this test.
    If I understood correctly, it was just to set S21 mode, pigtail cable between both ports, then user cal.
    Let me know if that is correct.
    Here is my result, very close to Andy´s picture.
    Regards, Maximo

    VK3NH  2017-01-06#7 -Reply



    Thanks for looking into this matter.

    regards Noel

    BG8NPK  2017-01-06#8 -Reply

    We will check this measurement results.

    Andy  2017-01-06#9 -Reply

    Hi VK3NH,

    On my 901V, I also see a step in S21 after user calibration, but not quite 0.4 dB. Perhaps more like 0.2 dB.


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