Products > Test Equipment
DC coupled 2.7 GHz Active Probe Project - Now Available!
lasmux:
--- Quote from: joeqsmith on August 11, 2023, 02:14:05 pm ---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.
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This is really nice. I love the short L ground blade design. The probe response matches the source waveform really well! Even the ripple is very close.
--- Quote from: joeqsmith on August 12, 2023, 03:03:17 pm ---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.
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Can you save waveforms on the screen? This is how I sometimes do it. I save and display a reference waveform, and then use the delay on the trigger to overlay the live view of another signal.
joeqsmith:
I believe it has the freedom to do what you suggest, if the scope was 100% working. It throws the BRAM (Battery RAM) fault on start as the internal battery is dead. I'm not sure if that is the problem but when I select the internal memory, the scope resets. The scope shows the memory is garbage and selecting clear will also cause a reset. I am using a version of firmware that is known to have problems but it supports the plug-ins that I am using. It's possible that the memory problem could be tied to firmware. 30 years old, maybe even a hardware failure. This thing is an old VME chassis. :-DD
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This was a kickstarter resistive probe. While a try to reduce the inductance, if you read their comments, "the Vishay FC0402 resistors I use in the attenuator are rated for 30V each. I have a series string of 100-75-75-75-75-50 ohms so with 50V at the tip you shouldn't be over 30 on any single resistor." Hard to believe they would get any level of performance out of it.
Note how they terminated the test board so we can't see the loading effects.
https://www.kickstarter.com/projects/azonenberg/akl-pt1-2-ghz-passive-oscilloscope-probe
joeqsmith:
While we wait for your probes, attempted to low format the drive, which failed. The drive I have been using is an old Seagate ST125. Note the jumper wire is not factory but a mod I sorted out that makes the drive act like the old Mini-scribe drives that came from the factory. These scopes only supported a few different drives. The KALOK KL320 was another one it supported. I inserted my backup drive and again, fails out. That drive has been in storage for maybe 15 years or so.
I had converted this scope to use rechargeable batteries for the backup. I had forgot that the low format would fail if they were dead. So after a full charge, low format, reinstall, the scope now works. I went through the same process with the second drive and it works as well. Putting it back together, it no longer resets when the internal memory is selected and does not show garbage when you try and display the contents.
So I tried your suggestion of using the storage and delayed trigger using about a foot or so of coax for a delay. Works like a charm, well, for now. :-DD
joeqsmith:
For fun, I took the ECL output into a DC block, into a Mini-Circuits 8,1GHz tripler and then into a wideand amplifier. The hope was to get a faster edge. Loaded waveguide is C2 and the bladed probe, C3. The WM8500A has a BW of 5GHz, or roughly 70ps. Signal level is a bit low but the homemade probe seems to do a fair job tracking it.
Granted, you can't use it for high voltages and at low frequencies, even a basic 10X probe would present less loading but if you work with high speed digital, it may be difficult to match the price performance of a resistor and some coax. Well, and apparently a really good ground. :-DD
lasmux:
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.
Glad you managed to fix your scope :) I've never been good at fixing old oscilloscopes. Last time I took apart my HP 54111D, I managed to achieve nothing but waste time and stress out about killing myself on the CRT.
Nice tuning of that delay haha. Very nice match between the source/measured signal. But yeah, damn dude, that resistive probe is working a charm. Really nice. As always, I think there is a place for active probes above resistive probes. Where you want reduced signal loading, probably sub 2GHz, and for frequency response stability.
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.
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.
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