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Purchased a HP 8711A some time ago to learn how to make better measurements.

I've tried doing an OSL calibration with a 75ohms Agilent cal kit. When reconnecting the short after succesful calibration, I would expect to have a return loss of 0dB across the whole calibrated frequency span.

However, what I consistently get is something like this: S after OSL Calibration

I know it's only .5dB, and this might be the limits of the hardware, but what I don't understand is that I can replicate this exact behaviour: after every calibration, RL of the short starts around 0dB in the beginning of the frequency span, and ends around .5dB.

If these are measurement errors, I'd expect the outcome to change between different calibrations.

Or am I wrong somewhere?

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  • $\begingroup$ just to give this perspective: -0.5dB[power] is about -12%, in -0.5dB[amp] about -6%; so this isn't really much variation $\endgroup$ – Marcus Müller Nov 8 '16 at 18:28
  • $\begingroup$ @MarcusMüller I see what you mean (even though I think 12% should not be that impossible with this kind of hardware), but still leaves my main question: if the calibration is done, shouldn't the short truly be around the 0dB line? $\endgroup$ – Dieter Vansteenwegen ON4DD Nov 8 '16 at 21:18
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    $\begingroup$ well, a calibration is never perfect – it's purpose is to minimize the measurement error, but how well that works is another question. I must admit that I don't know the error boundaries to be expected post-calibration on your device, however. So this might be within what is to be expected, or an indication for fault. $\endgroup$ – Marcus Müller Nov 8 '16 at 22:38
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Seems like I was indeed wrong. The displayed trace displayed was Data/Memory (hence the /M in the top of the screenshot). After redoing the calibration and selecting Data for display, the unit is within +/- .04dB in the whole range. My mistake... Thanks to Default95401 on the Yahoo HP/Agilent test equipment group for pointing that out...

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