[regenfreak]

Originally Posted by G0HZU_JMR

I've had a go at making a first stab at the 100MHz resonator jig. I think the metal sheet I've used is 0.5mm thick so it was really difficult to solder to. I had to use a 4mm bit in my Weller WSP80 iron and a flux pen.

I've just tested the Mi 2388 with the resonator jig and the results agree with the old VNA measurement quite well. This isn't a very well controlled experiment as the attenuator is a continuous rotary type and the 40dB setting is set by eye. Therefore, I really should do an s11 and a jig measurement side by side with exactly the same rotary attenuation selected.

However, with no probe connected the insertion loss was 2.48dB at 101.6MHz. This was exaclty as predicted based on the predicted Q of the resonator inductor. With the Mi2388 touching the top of the resonator (via the tiny peep hole) the BPF response dipped to 93.17MHz with an insertion loss of 5.00dB now showing on the VNA.

The best way to reverse engineer the loading effect of the probe is to see which combination of Rp and Cp gives the same pulling/loading result and this delivered values of about Cp =1.55pF and Rp =83kΩ.

If I look at the Rp of the Mi 2388 down at 93MHz on my old VNA s11 measurement it is about 83kΩ. So this result agrees very closely.

I used an ATC 800B cap for the 5.6pF resonator cap and PPI (1111N series) for the 1pF coupling caps. I also fitted a 10dB attenuator at port 1 and port 2 of the jig before calibrating it. This improves the port match. I didn't bother with a full 2 port cal using the ecal module. A through cal with the 10dB attenuators should be OK for stuff like this.

I think it will be difficult to check the cal using high value SMD chip resistors up at 100MHz. Usually, my VNA tests show the Rp of a 100kΩ resistor falls with increasing frequency. I may have to try various thin and thick film types to try and find a resistor that behaves well up at VHF.

My test jig looks really crude and ugly, and it is only a temporary lashup. It's OK to prove a concept but should really be built into a proper and stable jig.

See below. This was made very quickly and crudely (and it shows ) You can see that the idea is to use the probe on the back of the jig where it's possible to probe a tiny test point that pokes through the peep hole.

Obviously, I'd recommend that you make something more robust and more presentable than this...

Thanks. I am very surprised by the frequency drift of a massive 8.43MHz! I dont really understand.

Before I go to bed, I have spent 10 minutes to put together a "quick and dirty" test jig that I used previously. It is used for finding the unloaded Q of a VHF LC tank. The coil and cap specs are based on your suggestion. I used 1mm silvered plate wire and a 1-8pF piston ceramic trimmer. The wires are capacitive coupling "cat whiskers" so that there is at least -30db insertion losses both ports. The QL approaches asymptotically to Qu in a graph if the insertion loss of the external coupling to the resonator is greater than -30db. I have used my cheapo NWT200 to find unload Q which reads 357 which is somewhat lower than your simulated value of 400. I didnt bother with calibration and normalization with the NWT200 as the software is buggy and flaky, so ignore the absolute value of the vertical axis in db.