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cheap tek 500MHz passive probes on ebay. can they be used on any scope?
tggzzz:
--- Quote from: rf-loop on September 17, 2016, 09:07:55 am ---But do not forget in RF circuits that you connect 8pF capacitor in circuit.
Yes better than 20pF or 16pF but still 8pF is LOT of in many cases.
@500MHz it have 39.8 ohm reactance, not 10Mohm what is DC resistance.
--- End quote ---
Very true.
It is also worth pointing out to beginners the effect the ground lead will have; a 6" ground lead will be about 140nH, and that will resonate with the tip capacitance at ~100MHz.
I've managed to reduce that inductance by copying techniques used in HP probes. FFI see https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
EDIT: updated results at higher frequencies at https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/
JPortici:
:clap:
love me dat blue phosphor
tggzzz:
--- Quote from: JPortici on September 17, 2016, 05:29:33 pm --- :clap:
love me dat blue phosphor
--- End quote ---
Tek 485s are sweet little scopes in more ways than one!
There's a working one for sale on fleabay at the moment (no affiliation): http://www.ebay.co.uk/itm/Tektronix-485-two-channel-350MHz-analogue-oscilloscope-/282171555562
20 hours left :)
JPortici:
too bad my money is constantly feeding my 7k addiction!
David Hess:
When you get somewhere above 200 MHz with high impedance passive probes, the high frequency transient response of the probe is matched to the oscilloscope input so not all 500 MHz probes will deliver optimal response with all 500 MHz inputs. You can see this in the old Tektronix probe recommendation guides where specific probes are recommended for specific oscilloscopes and "A" or "B" revision probes will be recommended for later oscilloscopes of similar or equal bandwidth.
High bandwidth general purpose probes have 2 or 3 user accessible high frequency adjustments besides the compensation adjustment so with a suitable signal source, they can be adjusted to work with any high bandwidth oscilloscope input.
Most people do not have a suitable signal source so not only will they not be able to adjust such a probe, they also will not be able to verify that a more specific probe is not working correctly and in most cases, the differences are not going to matter because the effect of the probe's input impedance will affect the signal that much anyway. The probe and oscilloscope specifications depend on an unrealistic 25 ohm source impedance but the designers had to pick something as an objective reference.
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