On Fri, Dec 6, 2024 at 09:39 AM, Cal (AD8Q) wrote:
I recall builders here reporting they've made VNA connections to LPFs (QMX / QMX+ ?) and made sweeps
I have done this LPF scan with my nanovna.? It is quite simple with the QMX+, if you happened to install your diodes with the cathode side up.? With the QMX you need to tack some wires on the diode pads, so it is a bit harder.?
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Maybe best shown with a photo; see attached.
- You inject the S11 port signal into the filter at the cathode of the diode.
- You should really do this via a dc blocking capacitor, like .01uF or so.? But in my observations, it makes almost no difference in the filter shape, so I don't do it.? If you don't use the blocking capacitor, however, it will give you false readings on LPFs not tuned to: because somehow the nanovna's direct connection at least partially enables the filter it is attached to, by referencing it to the nanovna's ground.
- You receive the S21 port signal from the antenna connector.
- You need to turn on the QMX, and tune to the band you are testing, so it will 'turn on' the filter via the switching diodes.? BUT BE CAREFUL: disconnect the key and/or otherwise disable transmit, so you don't accidentally bump something and transmit into your nanovna - that would definitely break it.
- You see the filter shape.? Note that there is usually (on mine) a little narrow notch that appears within the band you are tuned to - but this is some artifact, because the actual frequency you are tuned to is at a point bumping up out of that notch back up to the passband level.
One more note: it seems that you should be able to do this scan with the qmx 'off', and connect the S21 port to the other diode's cathode at the output of the filter.? This does give a reasonably good view of the passband design and the cutoff frequency, but the deep cutoff notch and the stop-band are not seen for some reason, at least not on the lower bands.? I don't know exactly why.? But it is better anyway to do the scan with the QMX on and filter selected, because you get a better view of parasitics that happen in the full real filter path.? Oh, and as always, I calibrated the nanovna at then ends of my test cables to give an accurate result.
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I attached a couple of photos of my setup, and one of my 40m/30m LPF scan.
I hope this is helpful.? Questions are welcome.
Stan KC7XE
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