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DC coupled 2.7 GHz Active Probe Project - Now Available!
joeqsmith:
M2 is no load, C2 is the loaded waveguide. PP061 resistive probe blue. Rise time is roughly 500ps for the probe, or around 700MHz. Edge without the probe attached measures about 200ps.
With a better scope and lower transitions the probe doesn't look so impressive. :-DD
joeqsmith:
But if you think that's bad here is that same homemade resistive probe. :-DD That's s mighty big loop, lots of inductance. Not too surprised it peaks up. I would expect your probe would do a much better job with this 200ps edge.
Scope is an old WM8500A 5GHz BW.
joeqsmith:
Using a different driver with my Tektronix P6202A probe. It makes a decent filter.
MathWizard:
Cool project, I'd buy this probe down the road.
How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.
1 of the next things I need to buy, is cables, terminators, and splitters, for using between my scopes and signal gen. And I want to try making 1 of those passive probes that's just coax and a 1k resistor, and has very low capacitance. Dave showed 1 in a video on probes. I can't remember their downside, besides low series resistance. But even mucking about with BJT amplifiers lately, I realize how big 15pF of 10x probes can be.
joeqsmith:
--- Quote from: MathWizard on August 09, 2023, 09:56:33 am ---Cool project, I'd buy this probe down the road.
How much worse would it be if you used all the same chips, but used all standard size through hole parts ? What if you didn't pay attention to spacing or layout ? I'm just trying to get a feel for how sensitive these circuits really are. If 2GHz is ~15cm waves, thats getting down to circuit size.
1 of the next things I need to buy, is cables, terminators, and splitters, for using between my scopes and signal gen. And I want to try making 1 of those passive probes that's just coax and a 1k resistor, and has very low capacitance. Dave showed 1 in a video on probes. I can't remember their downside, besides low series resistance. But even mucking about with BJT amplifiers lately, I realize how big 15pF of 10x probes can be.
--- End quote ---
MathWizard, for a 20:1 ratio, you want to use a 950 ohm resistor, not a 1k. The scope's input is 50, so the circuit you are probing sees 1000ohms (assuming a perfect world).
The following was taken from this paper:
https://people.ece.ubc.ca/robertor/Links_files/Files/TEK-Understanding-Scope-BW-tr-Fidelity.pdf
Analog bandwidth is a measurement specification that simply defines the frequency at which the measured amplitude of a sinewave is 3 dB lower than the actual sinewave amplitude (see IEEE-1057). Figure 1 shows an idealized amplitude roll off error as a sinewave signal approaches the specified bandwidth frequency of a measurement device having a first order or single pole Gaussian response. At the rated bandwidth, the measurement error approaches 30%! If you want to make a measurement on a sinewave that has only 3% error, you would only want to measure sinewaves much lower in frequency than the rated bandwidth of the oscilloscope, about 0.3 times the rated instrument bandwidth. Because most signals are more complex than sinewaves, it is a general rule of thumb is use a measurement device, like an oscilloscope, that has 5 times the bandwidth of the signal you intend to measure (explained later and shown in Figure 5).
***
In case it wasn't clear, the following was also taken from the same paper:
In fact, every probe manufacturer assumes that at the maximum specified bandwidth, the probe’s frequency response is down 3 dB.
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