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Originally Posted by G0HZU_JMR
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nemo_07: Such probes are designed for looking at logic or other circuits with moderate or low impedances, not for probing Hi-Q LC tanks. Beside detuning, some of them will introduce measurable damping at 100MHz.
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regenfreak: How you do know? Have you tried? This guy probed the IF output for the residual output of FM front end tuners with multi-stage tuned bandpass filters. He used a Tektronix 500MHz P6202A with input capacitance of 2pF:
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I had a play on a simulator and the BF998 MOSFET probe has the potential to have an effective input Rp that is a couple or orders of magnitude higher than the P6201 at 100MHz. This is partly due to the capacitive divider at the input. However, this can only happen if the MOSFET probe has a snubber at g1 to offset the negative resistance the BF998 will generate here across VHF and well into UHF.
Achieving low input Cp for a useable probe is always going to be a challenge and a lot depends on how small and short (and fragile?) you can make the tip section. This will minimise any capacitance to free space. |
It has been 18 years since the Poorman's probe article published. I hope someone can design a snubber circuit to fix the negative resistance issue.
I have attached the schematic for P6202A. The trimmer C12, R13 and C13 form a low impedance path to the ground at 100MHz. I think it is optimized for signal waveform integrity really. I didnt pay much for my P6202A, but the shipping and import together cost as much as the probe itself, just like everything else sold in ebay from the USA.
Most expensive active probes have replaceable tips. For me, I dont mind if the poorman's probe tips are fragile. I just replace them at a little cost.