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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Happy new year, fellow middle-aged drunks! The single best thing you can do to improve the probe, is put a tiny capacitor in parallel with R3, and looking at your scope's calibration output, adjust the capacitance for the best looking square wave. Once you've done that it should perform much the same as a commercial probe (or a $8 eBay special at any rate )
Using ordinary coax instead of the fancy resistance wire stuff just means that this capacitor will have to be bigger, making the probe's input capacitance higher. But that probably isn't a big deal for you. There are some other subtle high frequency effects that you probably don't care about either.
That's assuming you used a gas gap and not a TVS, by the way. TVS have disastrously high capacitance. I remember trying to protect an IGBT gate with one, and it was harder to drive than the IGBT.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Watt The Fork wrote ...
That's assuming you used a gas gap and not a TVS, by the way. TVS have disastrously high capacitance. I remember trying to protect an IGBT gate with one, and it was harder to drive than the IGBT.
i may be having the same problem right now !!! dam 18 V TVS, im switching over to 18 volt zeners for gate drive protection. i think each one has 70pF...
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes I am sure. It compensates for the capacitance of the coax. The coax inductance doesn't matter because it's 50 ohm coax, and you're using it at a 500k ohm impedance level.
I've used coax in low voltage, high current circuits where all that mattered was the inductance, but this isn't one of them.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Never mind, I get it now. I assumed it was the coax since I had tested the voltage divider before making the circuit, but that was with a single R3. Now I'm using 6 1Meg resistors to get 4.5Meg and that increases the inductance quite a bit. Got it tuned OK now, considering I was using the .5V signal from the scope it's quite acceptable.
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Building a simple 10X probe
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