Relaxation oscillation in LC resonance/ Toko testers

[Radio1950]

Hi Regenfreak

If anyone is remotely interested in building up a coupled FET variable oscillator for HF tests, I recommend they have a look at the circuit used in the MFJ259 variants.
Circuits available online with web search.
This design, similar to the QST one, also uses your "two capacitors in series" trick to reduce the range of the variable capacitor.
And it has RF leveling.

Good luck with your probe.

[regenfreak]

Good luck with your probe.

Thanks. It is still work in progress. I am still trying to understand how it works.
It is basically a capacitive voltage divider, a JFET impedance convertor followed by a bipolar tranny voltage amplifier. Sounds simple but it is not if one really try to understand at a deeper level. Thats how i run into a rabbit hole sometimes.

[Radio1950]

You probably have seen the info in this?
https://www.vintage-radio.net/forum/...ighlight=match

I might build a "PoorMans" myself.
Cigar tube, de rigeur.

[regenfreak]

You probably have seen the info in this?
https://www.vintage-radio.net/forum/...ighlight=match

I might build a "PoorMans" myself.
Cigar tube, de rigeur.

I have that cheap Ukrainian RF probe for a long time which has the same design as the Poor man's probe.

I have got my DIY Q-probe working on breadboard. It can measure Q automatically all the way to 10.7MHz no problem.. I dont need to worry about relaxation oscillation. I use coupling cap rated 500V when means I can probe around the IF transformers installed in powered up valve radios.