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.
But if you think that's bad here is that same homemade resistive probe. 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.
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.
But if you think that's bad here is that same homemade resistive probe. 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.Yeah, that's a lot of ringing! Very neat how little it's loaded the original signal though.
I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance.
I've been working on an active probe design for around a year. The goal started off as creating a DC-coupled active probe to support a photon counting sensor I am also working on, but it was a very fun project and I spent so much time on it that now the plan is to sell it. Could I have some feedback on the probe/performance, and on the contents of the datasheet, before I start buying the first batch of parts... which will be quite expensive.
I have another post that I'm putting together where I'll go into the development process a bit more.
I'm making two versions, a 1GHz version, and a 2GHz version.
The datasheet is here: https://www.lasmux.com/wp-content/uploads/2023/07/LD-ASP-1G_2G.pdf
Quick specs:
Bandwidth: DC-1GHz, DC-2GHz
Input capacitance (measured at 1GHz): 0.7pF
Attenuation: 20x
DC input resistance: 1Mohm
1GHz version frequency response (linear and log axis):
2GHz version frequency response (linear and log axis):
Tip input impedance of both probes, depending on which ground lead is fitted:
The resistive ground lead can be used to stop a resonance developing on the ground connection, which reduced the input impedance above 1.5GHz. I talk about this a bit more in the datasheet.
In terms of step response for the system, I've 'only' got a 500MHz oscilloscope, so can't properly test the rising edge speed unfortunately. This is the probe measuring a 50 ohm terminated 100MHz signal, with a <100ps rise time. This greatly exceeds the bandwidth of the scope so there's some ringing. The trace looks basically identical if I measure the signal directly by the oscilloscope.
Currently I'm aiming for around £150 for the 1GHz version, and £185 for the 2GHz version.
I have thought about trying to add a blade for the ground rather than the wire to help reduce the inductance.
Like this HPAK N2878A?
I used some copper spring material to make an "L" shape for the blade. The tab was formed into a cylinder and soldered to the coax.
Again, M2 is the unloaded waveguide (probe not attached). C2 is with the probe loading the waveguide. C3 is the measured signal from the blade probe. Of course there are some errors with gain and I can compensate for the phase. A bit too much gain in the plot but works fairly well compared with the others.
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The blob used to mechanically stabilize the resistor is Devcon 5 minute epoxy. It doesn't appear to hurt the performance of the probe at least at these frequencies.
Making similar measurement with an old LeCroy 7200. The only option for saving data is with a floppy drive which I can read with my USB floppy. For plotting, I use CERN's viewer.
Shown looking at the 10X blade probe on channel B2. Most of the error is due to the how the scope works. It has a step generator built-in and can loopback that signal to the display. It's is not the same as probing at the end of the waveguide like I have been showing. I need one more channel for that.
The scope doesn't have a lot of storage and I don't remember it having a way to adjust the trigger independently for each channel. Even at 1nS/div, I can't align the two waveforms. Consider we are using 30+ year old scope (the 7200 was released in 1989), results seem fair.
On another topic, I was considering a differential version of the probe. I've been playing around in LTSPICE, and I could probably get over 1GHz of bandwidth, with the same 0.7pF tip capacitance, +/- 30V input dynamic range, but I suspect my CMRR wouldn't be great without a bit of work on matching the two inputs closely.
I've seen that probe from Andrew Zonenberg before. He has some really nice other projects also. I think the main issue with that probe is the tip capacitance is actually quite high for a resistive probe, and the bandwidth stops at 2GHz. Not sure why it was necessary to use those Vishay FC0402 resistors. Under 2GHz I don't think a normal 0402 resistor would have been an issue, and they are miles cheaper.
My original application was my actively quenched APD as a photon counting module, where I wanted to measure across a 1k ballast resistor restricting current through an APD. A passive probe had way too much capacitance and would have slowed down the response of the signal, a resistive probe would have made the ballast resistor ineffective and probably damaged the APD. An active probe was the only way that I could think of.