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DC coupled 2.7 GHz Active Probe Project - Now Available!
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lasmux:
Thanks for the review joeqsmith!! Excellent overview of it!

I'm glad your experience was overall positive, albeit with a couple of niggles. I love the array of probes you have to compare it against! And of course, good to see the new resistive probes in action also.



One thing that struck me was how well the Lecroy PP002A passive probe performed when you were loading the 1GHz clock signal later in the video. Despite it having a 14pF (!!!!) tip capacitance. It actually performed about the same as the reviewed 0.7pF active probe. I think this is because the clock is such a narrow band signal, and when I've done testing on passive probes, I've seen that their applied load is very frequency dependent. Maybe at 1GHz, the Lecroy load is particularly gentle on the probed signal? For example, the below graph was the S11 reflection coefficient (blue line) of a load I measured last year, with an HP 6pF 500MHz passive probe attached across the load. See at 1.5GHz, there's a pretty good reflection coefficient. At 600MHz, not good.




As you noted, I forgot to post a more detailed method for measuring the probe frequency response. Here it is:

There are three graphs provided with the datasheet (two of these measured for each individual probe in the test report). In the datasheet these are figures 1 top and bottom, and figure 2.

Datasheet figure 1 is captured on a libreVNA, S21 and S11, with a custom coplanar waveguide connected to port 1, see photograph. The VNA has been calibrated with the coplanar waveguide in place, so the reference plane is at the end of the coplanar waveguide (where the load is fitted in the photograph). For the through calibration, the load and SMA-BNC adapter are removed to allow the blue SMA cable (port 2) to connect to the coplanar waveguide SMA. I've found that SMA-BNC connectors are often not very good at higher frequencies. It took me a few attempts to find one which didn't lead to nasty reflections.



For the effective impedance measurement (datasheet figure 2), a custom SMA 50Ohm load was built so I could place the probe directly at the termination point, so the probe acts as a perfect(ish) parallel load to the termination/VNA reference plane. This was constructed from four 200Ohm 0402 resistors soldered to a trimmed SMA PCB edge connector. The custom load has a very similar return loss compared to a commercial load, measured up to 4GHz. The globs of solder on the load are there to allow the probe tips a larger targets to contact when taking impedance measurements.

The VNA is calibrated with the custom load, and then the probe is placed across the load.

The VNA is then set to output the impedance magnitude. The effective impedance of the probe is then seen as a load directly in parallel with the 50 ohm VNA load, so the probe effective impedance is calculated as 1 / ( (1 / measured impedance) - (1 / 50)).
joeqsmith:
Thanks for letting me have a look at it.


--- Quote from: lasmux on May 25, 2024, 10:34:21 pm ---One thing that struck me was how well the Lecroy PP002A passive probe performed when you were loading the 1GHz clock signal later in the video. Despite it having a 14pF (!!!!) tip capacitance. It actually performed about the same as the reviewed 0.7pF active probe. I think this is because the clock is such a narrow band signal, and when I've done testing on passive probes, I've seen that their applied load is very frequency dependent.

--- End quote ---

For those who did not watch just some background:
The oscillator was in series with the waveguide and then attenuator before connecting to the scope.  What was shown was what difference the scope sees when the test probes were attached to the waveguide.   For the PP002A, I used the provided ground spring.   During the video, I did not terminate the probe to an actual scope.

Here I show it attached to the scope simulator used during the review as well as the actual scope.  It's basically a wash but best to prove it.  When looking at the return loss of the PP002A using the same technique used to measure your probe,  you can see how it peaks at 500 then starts to drop.   I suspect some of this is due to the long spring. 

Had I used a 500MHz clock, I would imagine your probe would have far out performed it.   Similar, had I used a higher source impedance driver,  I could have made your probe look much better than the home made resistive probe.   This is why I started out by using that book to cover some of the basics about loading.   

Also lets be very clear and state the obvious.  Had I shown what the scope measures when the using your probe vs the LeCroy probe on that 1GHz clock, there would be a MAJOR difference!!   The only way I could have done this test is to use a homemade 1Meg/50ohm buffer.   That would have been meaningless.   The only other option would have been to use that LeCroy buffer with the WaveMaster along with the PP005 as that is a combination someone could possibly still purchase.   

Again, a bit difficult to know what to show. 
lasmux:
Yeah I totally understand that it's difficult to show all the context details. Not critisizing, I was just originally puzzled.

It was good to see a lot of the performance specifications get some external validation. I really appreciate all the work you put into the review!

Would the following be a fair summary of your conclusions?
Pros
Pretty good probe frequency response, with similar bandwidth to what's specified.
Low tip capacitance leads to low impact on DUT signal, with performance in line with specification.
Really good price/performance ratio. Multiple times cheaper than comparable used probes from ebay.
9 hour battery life.
Not a proprietry connection to test equipment.

Cons
Noise level is higher than other probes, so less useful for analogue work. This is unfortunately unavoidable due to the input design.
Needs to be handheld to achieve measured probe response linearity. This is interesting as all of my testing has been handheld so far, which is maybe why I didn't observe this. I will look into this further for the next production run, although no promises of any improvement. If I can't fix this I'll stick a note in the datasheet about this.
A bit tricky to clamp when not using handheld. Is this a good thing given the above issue :scared:
Offset ground pin can be a bit fiddly. I agree it can be a bit tricky. It works best when you pop the ground pin in a ground via nearby the signal to be measured.

Conclusion.
General mark of approval, especially at price point. Best for high frequency digital measurements. Less useful for analogue measurements due to noise.
joeqsmith:

--- Quote from: lasmux on May 26, 2024, 10:43:31 am ---Yeah I totally understand that it's difficult to show all the context details. Not critisizing, I was just originally puzzled.
measurements due to noise.

--- End quote ---

No problem.  I just wanted to be clear about what was shown and why.  IMO, the two primary criteria are 1) don't mess with the circuit I am probing. 2) let me see what is actually happening.  Then comes all the other things like tip selection, body styles...  Of course, no probe meets those first to constraints, so it's a compromise depending on what I am trying to measure.   

Just to drive that point,  shown with the PP002A 350MHz 10X probe connected to the AP-1M buffer, then to the WaveMaster.  During the review I showed how the loading for this probe was on par with your probe when attached to the 1GHz NEL oscillator.  But when use the scope and probe to measure the signal, we get a flat line with a bit of offset (M3).   Not at all useful.   

Compare that to your probe (M2).   The signal is high enough that the noise isn't a factor and with that higher BW, we get some details about the clock. 

Now compare that with the HFP3500 (M4).   What's the right answer?  I would say I need a better scope and probes to answer that!   


--- Quote from: lasmux on May 26, 2024, 10:43:31 am ---It was good to see a lot of the performance specifications get some external validation. I really appreciate all the work you put into the review!

Would the following be a fair summary of your conclusions?
Pros
Pretty good probe frequency response, with similar bandwidth to what's specified.
Low tip capacitance leads to low impact on DUT signal, with performance in line with specification.
Really good price/performance ratio. Multiple times cheaper than comparable used probes from ebay.
9 hour battery life.
Not a proprietry connection to test equipment.

--- End 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.       

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.   

***


--- Quote from: lasmux on May 26, 2024, 10:43:31 am ---Cons
Noise level is higher than other probes, so less useful for analogue work. This is unfortunately unavoidable due to the input design.
Needs to be handheld to achieve measured probe response linearity. This is interesting as all of my testing has been handheld so far, which is maybe why I didn't observe this. I will look into this further for the next production run, although no promises of any improvement. If I can't fix this I'll stick a note in the datasheet about this.
A bit tricky to clamp when not using handheld. Is this a good thing given the above issue :scared:
Offset ground pin can be a bit fiddly. I agree it can be a bit tricky. It works best when you pop the ground pin in a ground via nearby the signal to be measured.

Conclusion.
General mark of approval, especially at price point. Best for high frequency digital measurements. Less useful for analogue measurements due to noise.

--- End 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. 

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.   

At the selling price, I see it as a win.  Hobbyist are not typically doing cutting edge designs at home and may just need something better than their 10X probe.  IMO this probe could very well fit that need.   Buyers just need to be aware of the limitations.
joeqsmith:
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. 
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