New HV and HF probe design

[Bob Reite]

What kind of 10 meg resistors did you use? Maybe they are more inductive than you thought.

[Doug Coulter]

Gosh, I hope not. I used carbon film 1/4w 5% standard DigiKey issue.
That 6th harmonic thing I haven't actually tracked down, but it appears to be happening at a particular frequency around 20 mhz.
After thinking about it, I ran those tests again with the FFT on and found that "whatever" harmonic was going though that frequency got a heck of a boost, it must be a really nasty peak there....
Which indeed might be added inductance. Do they do the spiral thing on those tiny guys? As this goes on I might well try to find something better - maybe carbon composition (does anyone still sell them)?

The only carbon comp R's I've seen for sale lately are specials, rather low values (10k ish) and rated for high peak drop voltages (hard to arc internally or fry unless rating is exceeded long enough for bulk heating?).

We'll see where I wind up. For now this is good for all my high turns ratio ferrite core type stuff. We're buying a VNA in case we want that down the road, but it might need some mods to do measurements more than its built in 1 mW....

VNA link

You'd think if ions could be made to be present (they can in this lashup) that this might reveal at least a little about their motions even at this low level...and a lot of this should kinda scale (it will be nonlinear, but in a moderately predictable way - and I just need to get on the right page here).

[Bob Reite]

I bought myself a http://PocketVNA.com Works pretty good. The carbon film resistors should be good up to VHF but I thought they had a spiral. I remember the good old days of making dummy loads with 2 watt carbon composition resistors I think the self inductance of film resistors is something like 2 nano henrys, which with 2 pF would be resonant around 2 GHz. I'll have to measure a 50 ohm 1/4 watt carbon film resistor with my VNA and see what it's like. The PocketVNA will go from 500 KHz to 4 GHz

[Doug Coulter]

Looks like a nice VNA, but around 2x the price of the one we hopefully get working.
These R's are really tiny - it'd be a real tight spiral, and as you say, in the GHz just from that and the lead length. So the around 20 MHz is a little confusing - even the 3 9" rods in the zig zag shouldn't be doing that (I'd guess a higher number?). At any rate, it looks like a really high Q resonance. I can see a bunch of harmonics, no doubt normal from that generator (40V pk-pk output broadband!), but all except one are "way down" in dB. As I sweep the base frequency, pushing one of them through this resonance puts it way up in amplitude. Saw it kinda by accident as I'd just been flipping the band-switch and not sweeping each band...
Once I figured out what was going on, it's easy to reproduce...