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DC coupled 2.7 GHz Active Probe Project - Now Available!

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joeqsmith:
Tektronix P6202 vintage active probe attached to C4. 

lasmux:

--- Quote ---Beyond these, I also felt the overall quality of the probe is good.   The size of the body fits the hand well.  It was provided with a good variety of tips.   The hard carry case is a bonus.       

--- End quote ---
That's good to hear!


--- Quote ---One thing you may want to consider is a way to guard the input from ESD when the probe is being handled.  Hence my use of the bug rug.

Also, with there being no auto power off and having left the probe on a few times already,  if it wasn't clear, I have been using the probe with a lithium ion rechargeable battery.   These only output 7.4V when charged and it seems to be fine. 
--- End quote ---
The tips (and power inputs) are actually already ESD protected. The signal socket has a series 1MOhm resistor going into the op amp, which itslef has inbuilt ESD protection.
Interesting thought about an auto power off function. I'd need to see what is available. Maybe a danger with that is that it powers off whilst someone is taking a measurement, without them realising. Of course the alternative is using a 9V battery every time you forget to switch it off for a few hours. I like that you've used a rechargable 2S lithium battery :)


--- Quote ---One thing about SJL's scope, they advertise it as being 6GHz.  The unit they provided not only is easy to verify, they surpass it with a wide margin.   IMO, you appear to want to advertise the product right at the limits of what you could measure using a $1000 VNA and what ever standards it came with.  Personally, I wouldn't do this.  I would be much happier buying a product rated to 2GHz but seeing it perform at 2.5GHz. 
--- End quote ---
I do agree with the sentiment. Almost all of the probes in the first production batch were over 2.7GHz by my measurements. I guess the issue is that different peoples test kit will measure different bandwidths, which may leave them dissapointed. I'll consider that.


--- Quote ---The whole hand placement on the body of the probe IMO effecting the measurements needs to be addressed.   Then again, maybe not.  It's similar to the noise level.  I can tell you that my first dive into active probes, I would have been working on analog designs with higher voltage levels.  This probe would have been a very welcome addition.  It far out performs my first commercial active probe.  I would say it has a place, even as it is today.
--- End quote ---
I've done a bit of testing on this today, and unfortunately there's not an obvious fix. I tend to get somewhat consistent frequency response results when holding the probe in various different positions by hand, with different levels of contact, but letting go of it completely does seem to affect the response negatively for some reason. I think it'll have to stay as it is for the time being at least, and I'll need to update the documentation/website to make it clear that it's performance is characterised during handheld use, and using a clamp/stand will have an effect on the frequency response. I do agree with you however, that it is still a very useful tool irrespective of this. I'll keep looking and maybe figure out a fix at some point.


--- Quote ---At the selling price, I see it as a win. [...] Buyers just need to be aware of the limitations.
--- End quote ---
Yes for sure.

lasmux:

--- Quote from: joeqsmith on May 27, 2024, 03:21:12 pm ---A similar setup using a PECL driver but with a 10MHz fundamental.   C1/M1 is directly looking at the output of the PECL driver through the waveguide and about 6" of coax.   Consider this is still not what the actual signal looks like as we just added a fair bit of capacitance which is dampening that fast edge.     

C2/M2 is the Lasmux probe   
C3/M3 is the PP002A/AP-1M probe/buffer
C4/M4 is the HFP3500

I took the data zoomed out a ways to better show the noise and settling times.   Then zoomed in, aligning the edges.

--- End quote ---

Nice plots.

A couple of observations:
Noise is definitely apparent there on the lasmux probe. The probe noise combined with the relatively low PECL voltage transition amplitude (400mV) makes for a noisy signal when zoomed out.
On the zoomed in signal, the rise time of the original signal is around 1ns and the HFP3500 and the lasmux probe seem to track it fairly well, albeit with a small DC offset on the lasmux probe. Not surprising as it's well within the bandwidth of both probes. The P6202 has a bit of overshoot, and the PP002A probe doesn't seem to have the bandwidth to keep up with the edge.

joeqsmith:

--- Quote from: lasmux on May 27, 2024, 03:48:18 pm ---The P6202 has a bit of overshoot, and the PP002A probe doesn't seem to have the bandwidth to keep up with the edge.

--- End quote ---

I have the OEM tools for the P6202 to compensate it but you have to remove the plastic cover from the tip and expose the delicate circuitry (mechanically not very sturdy).     

I dropped the fundamental down low enough so at least we could see something beyond a flat line with that 10X probe.  It does make for a nice filter. 

Attached showing the updated spreadsheet for some of my probes.  Note the dynamic range of my active probes are all less than half of yours.  Maybe between you and SJL, you may want to give up dynamic range to improve the noise.   

lasmux:
From the very simplified schematic I posted much earlier in the thread (see below), that would involve reducing the ratio R2/R3. R3 is 20k, so there's an attenuation of 50x on the input. The signal is then amplified by the (not-unity-gain stable) op amp back up to a 10x attenuation factor. This becomes 20x a net attenuation by the 50 ohm output impedance/oscilloscope impedance. The issue with reducing the attenuation on the input is that the capacitive divider of the parasitic capacitance across R2, and the C1 + parasitic capacitance across R3 has to maintain the same ratio for the AC gain. The parasitic capacitance across R3 is great enough that C1 would have to be a negative capacitance to maintain the correct ratio, which obviously isn't an option. Reducing the ground around that node to reduce the parasitic capacitance just makes external noise pickup a lot worse.

Another option is to reduce the impedance of the op amp input node so external noise pickup is less of an issue. So say reduce R2 to say 100k, and reduce R3 to 2k. It makes it a 100k DC resistance probe though, not a 1M, although that's probably a reasonable tradeoff if it actually helps. I'd have to try it to see what the change is.

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