[regenfreak]

Thanks. I am at work now.

I chose the resonator components such that a small change in Cp across the resonator would show a large shift in frequency.

I see. You have picked a high-Q LC tank to exaggerate the probe-loading effect. For a typical FM tuner, say L = 70nH , C = 36pF, f = 100.258mHz. A poorman's input capacitance of deltaC = 0.7PF, the frequency becomes 99.297MHz. The frequency shift is about 0.961MHz. Just a correction of last night post, when I probed the first RF air variable capacitor of the homebrew 6-gang FM tuner, the frequency shift was just below 1MHz compared with the calibrated frequency counter. The tuner was properly aligned, including the tracking of the oscillator and all 6 gangs. The frequency display automatically subtracts 10.7mHz in the display. So it is consistent with the calculation.

I'm off to bed now but I tried a quick experiment where I measured ten 220kΩ resistors (SMD 0805) in parallel. I stacked them in four groups to keep the footprint size down.

I then compared this to a single 22kΩ 0805 chip resistor on the VNA up to 150MHz.

The result was that the Rp of the single 22kΩ resistor only fell to 20.6kΩ by 93MHz. At the same test frequency, the ten 220kΩ in parallel had fallen to just 15.5kΩ.

If you put 10 in parallel, the parasitic capacitance would be multiplied by 10. So it is not surprising that the Rp for the ten 22K resistors would be lower than a single 220K.

can put the 1pF and 5.6pF ATC/PPI caps in the post to you FOC if that helps? I have a few spare ones. These SMD caps are physically very rugged, and you don't have to worry about the end caps cracking off unless they are really stressed. They have ultra-low ESR up at VHF.

Thanks that's very kind of you. It is difficult to get hold of 1pF ATC100 caps on ebay. I can send you a PM later. In the meantime, I will try to use through-hole 1pF caps