S-Meter Calibration

WA2ZKDWA2ZKD
edited April 2019 in Installation, Antennas and Interference
The admin config tab has a place to calibrate the S-meter. What are people using there with various antennas?

Comments

  • G8JNJG8JNJ
    I checked Kernow with a signal generator before shipping it to site and ended up with these settings:-

    S-meter calibration (dB) -16

    Waterfall calibration (dB) -15

    However in most cases I don't think you can rely upon the off-air readings as everyone's antenna (and distribution systems) vary so much.

    Regards,

    Martin - G8JNJ
  • WA2ZKDWA2ZKD
    so you applied no adjustment for antenna then?
  • G8JNJG8JNJ
    No, I just checked the KiWi to make sure it was reasonably close to the dBm calibration.

    I'm not sure how you would calibrate for an antenna.

    Antennas used for measurement purposes generally have a calibration curve showing the Antenna Factor (AF) vs. Frequency.

    The gain depends upon a number of factors including antenna pattern, height above ground, polarisation and frequency, so unless your measuring receiver can accept a calibration file to compensate for these variables you can't get a direct readout.

    Most folks have to read the level from a calibrated receiver and then correct the values using a lookup table, chart or some other external software.

    Regards,

    Martin - G8JNJ
  • WA2ZKDWA2ZKD
    I am thinking about the often flat gain of the LNA in an antenna, like a loop
  • G8JNJG8JNJ
    If you have a genuine flat gain amplifier ahead of the KiWi you could modify the calibration.

    But you really need to calibrate both antenna and amplifier together.

    The loop response is not flat, but over a small frequency range you may be able to get close, if you know the field strength of a particular signal.

    Regards,

    Martin - G8JNJ
  • ka7oeika7oei
    As the operator of the Northern Utah WebSDR, I've always made it a point to make sure that the S-meter reading reflects the signal at the main antenna port for any receiver, +/-1dB in the middle of the passband. Because the most heavily-used receivers are rather narrowband (e.g. <192 kHz) this isn't too difficult.

    The Kiwi is different: Between the normal roll-off at the high end (due to sampling, the LPF, etc.) and the "limited attenuation" high-pass filter (e.g. attenuation goes from about 0 to 12dB between 12 and 7 MHz) to minimized overload from low-frequency HF signals, I've had to give up on making the Kiwi's S-meter read anything useful.

    I suppose that a useful addition would be to have a number of S-meter and waterfall calibration points with frequencies, interpolation being done between. For example, "5000,-8; 7000,-9;10000,-12;12000,-17" (e.g. frequency in kHz, calibration value) to establish an approximated curve: 16 of those would likely be "good enough".

    Certainly an "edge case" but it would be useful, nonetheless.

    73,
    Clint
    KA7OEI
    WA2ZKD
  • n6gnn6gn
    1V p-p OV (+4 dBm in 50 ohms) at the ADC - {20 dB (LTC6401 preamp) + 2 (LPF)} = -14 dBm overload, referenced at the SMA connector. This is what I/ve measured and seems consistent across multiple receivers.

    I haven't measured very many spot frequencies over the Kiwi's 0-30 MHz range but on 5 different Kiwi's, putting "-15" into the S-meter calibration seems to cause the S meter to agree very well with calibrated signal generators at every point I have measured with every Kiwi I've measured. I don't think I've noticed anything like the 12 dB variation Clint mentions.

    Adding to this, the S-meter in the kiwi seems to detect and report rms noise pretty well. If one uses the indicated bandwidth along with the measured no-signal noise floor and compares this with the results of a Y-factor noise figure measurement from a calibrated noise source the results seems to support the S-meter being calibrated with the above value - at least within the +-1 dB or so accuracy of the reporting. Not all noise detectors do this well.

    I'll try to make a more thorough measurement with more points and post the results, when I'm next able to. I've been needing an automated way of measuring known signals, noise and interm-odulation distortion levels anyway.
  • n6gnn6gn
    And now John has added an S meter option to kiwirecorder.

    This all keeps getting more fun all the time !!
  • n6gnn6gn
    Last night I made a measurement of the KiwiSDR LPF in situ using an HP active probe. Since this is a voltage probe, it's accuracy depends upon knowing the impedances at the reference (SMA input pin of a KiwiSDR) and the measurement point, LPF filter output measured at preamp input. Nominally both of these points should be 50 ohm but there is some error due to this assumption. Here is a plot of that measurement made with an HP8714C VNA:
    This measurement indicates that I was a bit low with my previous estimate (in this thread, above) of the filter loss, by as much as 2 dB. A filter I fabricated with the same values but larger (1210) higher Q inductors understandably had less insertion loss.
    As part of generating a nominal calibration table for Kiwis I'm now trying to understand the contributors to the amplitude errors a KiwiSDR makes vs. frequency. Clearly this LPF unflatness is one of them but I don't think it entirely explains what we measure.

    At one time, somewhere on this forum that I can no longer discover, John (I think) mentioned that the algorithm in DSP added some correction to the reported amplitude. This may have been to correct a sin(x)/x artifact of the sampling process - or it may have been something else. I'm foggy on this.

    Can John or someone else point me to something that describes that issue so that it can be added to the effects that make a Kiwi's measurement less than flat vs. frequency over the .01-30 MHz range? Understanding this may help in generating a correction table which will be useful in making absolute noise (antenna temperature) and signal (e-field) measurements.

    Thanks,
    Glenn n6gn
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