I built one (15 R's and C's) stage for testing. I got a couple weird results, but at anything higher than a few hundred kHz it looks great - Looks like my PCB capacitors are a little large in value - much more fringing and other parastics than I expected, but...with a 200k load (instead of one meg) the load capacity to make it flat at HF is between that of a 1x and a 10x scope probe (and don't believe their specs on that - this ain't Kansas (Tektronix) anymore, Dorothy. We're in the 10pf+ range at 10x...
Anyway, the artwork is in the postscript files attached if anyone wants it - I just print these on transparencies with my laser (using the same one flipped over to be cheap) and expose pre-sensitized board and go from there.
I actually wound up writing a program to make the PCB file after getting a bit frustrated with the latest dosemu making the mouse pointer's DPIs not match....turns out the format is pretty easy to reverse engineer.
- PROBE.zip
- The PCB art and stuff
- (6.97 KiB) Downloaded 617 times
- 15 sections of eventual 45 probe test sample
I pulled out some real vintage heavy hitter test gear to check this stuff:
- GR FTW - but my hernia....
- Messy bench. The probe section is to the right of the GR in quartz pipe.
The gain isn't perfectly flat...varies from a 760:1 to around 600:1 from 1 mhz to 10mhz (I can probably fix that). There is obviously some effect from nearby things with ground or other signals on them...I'm going to try to make this work inside some kind of shielding - all this is non trivial. But it looks pretty good so far.
- scope input and output
At the lower F's with pulses (say .5uS), it shows a crazy big-fast overshoot, followed by a much smaller but DC-accurate pulse shape (and then undershoot on the back side of the pulse). I'll get a scope trace when I next do the required lifting (that oscillator must weigh 60 lbs), but it's weird in that at higher frequency sines - it's just "right" and real close to the calculated theoretical value. There's nothing in the setup with the kind of parastic L or C that would have a big effect down there at low F....a mystery at this point.
But, enough progress that I'm debating whether to change the PCB art a little before making 3 of these, or just making 2 more. These have 15 10 meg stages with very roughly 2pf in parallel with each, PCB spacings are ~ 100 mils so 1kv or so per stage should be no problem whatever, especially if I use glyptal or oil.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.