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PRP1 - Low cost 2GHz power rail probe

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tszaboo:

--- Quote from: temperance on August 23, 2024, 10:27:11 pm ---I had an other fancy pot in my collection. Brand new.

Vishay PRV6

Edit: the remainder are some low cost pot's not worth testing.

--- End quote ---
I quickly measured the trimpot that I removed. The 4th Digit on the 6.5 digit DMM was already jumping around.
It kept on doing this even
Measured an ordinary 0805 resistor, only the last digit moves. I also measured the pot that I wish to use in the final design. I noticed some effect, that after moing, it was drifting for more than a minute to the same direction. I set  it up to ~10KOhm, and it would drift to 9981 Ohm I'm not sure if this is a heating up effect of the DMM, or something mechanical. Anyway after the drifting, only the last digit changes again, so it's 100 times less noisy than the trimpot. Unfortunately my 34401A doesn't do ratiometric measurement.

So rather than giving up, I placed the pot in the fridge for about 10 minutes, and after taking it out, connected the DMM across the two end terminals. It went from 11560 Ohm to 11146 Ohm, Unfortunately Alps doesn't specify the tempco of their pots. But I use it in ratiometric mode, and wiper current is minimal, so there shouldn't be any drift and low frequency noise happening.

And here is the interesting part: I grabbed another 10 turns trimpot from the same type. And it didn't have drift and noise.
So I have to assume handling it there is a failure mode when it becomes noisy.

To be not only off topic, I made quite a bit of progress on the probe. Found the issue of the full scale 24V problem, added ESD protection and ferrites to the USB line. In my investigation, the USB outputs of scopes didn't always play nice with the probe, they were injection noise, now it's not measurable on the scope I have.

temperance:
The pot drift/stability might have something to do with the track lubricant which lifts up the wiper ever so slightly while turning the pot. The RPV6 pot I tested is pretty expensive but worse than the 91-95. But the RPV6 pot is meant as a position sensing pot and probably contains more lubricant.

I'm interested in such a probe. My last design containing a 2.4 GHz RF soc went trough RED testing with a very small margin for spurious emission. (probably caused by noise from the fly back power supply finding it's way into the PLL)

I was thinking this topic would be gaining more attention.

tszaboo:

--- Quote from: temperance on August 23, 2024, 08:54:27 pm ---Is there a reason why the front and back are made of aluminum? You could use FR4 and no extra manufacturing steps would be required.

--- End quote ---

These two prototypes were (quite badly) modified by me with a drill and some sawing and filing.
I was also waiting for the supplier to give me a quote for the modifications. It's not too bad, but I think I'll go with FR4 end plates for the first batch. I'll don't want to end up in a situation where the silkscreen is bad, and I have to reorder the housings.

Some more measurements!
This is the USB ripple when the PRP1 proto 2 is connected to the USB port. This is the 5V voltage directly from the USB.
The voltage is measured with the other PRP1!



After manual modifications to the input circuit:


And the output of LDO on the board:


As you can see the PRP1 was capable to measure this voltage ripple.
The actual modifications reduced the ripple from 52mV peak to peak to 3mV. The rest is handled by extra ferrite beads, and the PSRR of the Opamps. The ripple there is on the output of the LDO is barely 600uV, and they have more than 30dB PSRR at these frequencies. The noise is not visible on the output

tszaboo:
Another day spent well. I've made quite a bit of progress, schematics is complete I think. I was focusing a lot on the coarse-fine adjustment, improving the linearity of the two potentiometers. The fine should have about the range to be able to change a signal in the active range now, so the coarse only needs to get the signal in view.
I also spent time on the mechanical design, pictures are below. Instead of 2 wire assembly and 2 PCBAs it's now 1 PCB. Added an LED indicator. And spent 4 hours staring at potentiometer datasheets trying to figure out how to get the right one, then settling for something close to it.
With this I'm off to BOM setup.

tszaboo:
I'm ready with everything to order the next prototype.
I'm only waiting for some samples from TI to test out some more parts, just making sure that the probe schematic is optimal.
Also I'm still waiting for some Coax cables, because RG316 with low quality connectors is not a good choice for the connection between the scpe and the probe.


I dare to ask: Who is interested in getting one of these?
Price 250 EUR + Shipping + paypal fees

Contents:
PRP1
SMA-BNC coax cable 0.8m for connecting scope (TBD likely RG142 or LMR240)
SMA-UFL coax cable for connecting DUT
SMA-pigtail coax cable for connecting DUT
SMA-2.54mm square header coax cable for connecting DUT
USB cable
Optionally P57 terminator (OK performance up to 200 MHz)

I'm asking because there isn't a lot of price difference between ordering 2 or a few.

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