PRP1 - Low cost 2GHz power rail probe

[tszaboo]

I'm calling this PRP1. It's a power rail probe, that is oscilloscope agnostic and low cost.
I've started working on this because normal power rail probes cost around 10 times as much as entry level oscilloscopes. I hope to change that with this probe.
It's still very much in development, and I release some info about it, hoping to organize a group buy.
Below is an image of the first 2 working prototype. I've some promising measurements from the prototypes already.



Specifications:
Max voltage offset: ±24V
DC Input impedance: 50kΩ
High frequency input impedance: 50 Ohm
Signal attenuation of 1.3:1
Transfer function within 1dB to 2GHz
Estimated noise: 42 µVp-p RMS to 20MHz
Active signal range: +/-350mv
Course - Fine adjustment
USB Type C 5V power input - run it from your oscilloscope directly
SMA input and output. SMA to BNC cable to connect to oscilloscope
Typical power consumption of 20mA

(Specifications are yet to be verified by independent third party.)

[tszaboo]

Example probing setup. PRP1 measuring power supply ripple of an ESP32 board with a Siglent oscilloscope, using external 50 Ohm termination.


Dual probing power supply ripple with PRP1 and standard oscilloscope probes. Siglent oscilloscope
CH2 is  standard oscilloscope probe, CH3 is PRP1


Noise level of PRP1
CH2 is input terminated with 50 Ohm, CH3 is PRP1


Dual probing power supply ripple with PRP1 and standard oscilloscope probes. Keysight MSOX 1 GHz oscilloscope
CH1 is  standard oscilloscope probe, CH3 is PRP1. Note the lower noise levels of the PRP1

[tszaboo]

Low frequency transfer function:
Note this measures up to 10MHz, which includes the low frequency crossover point of the probe.

High frequency transfer function:


Some TODO, to declare the status of the project:
Some trivial changes to the PCB, that are patched on the prototype
Prototype only reached 23.6V, needs a slight adjustment to the references
If possible, bring U6 underneath the internal shielding can
Going from 2 PCB solution (potentiometers) and wiring to one PCB
2 Potentiometers too big to fit on front panel, second potentiometer should be fine adjust type
Maybe go from end-launch SMA to TH, to bring the connector closer to the middle of the enclosure. On the other hand it's ascetic only, and it would reduce performance.
Drawings of enclosure to outsource the cutouts. So the end product doesn't look like I hacked some holes on it with a drill and some files. I've contact with the manufacturer
Quality SMA to BNC cable is needed. I've found half the time the cable, and the BNC connector made 4x the noise than the PRP1

[tautech]

Nice, OP image needs resizing....good you've done it.

Dunno what it is but we have one coming soon too, a 4 GHz version.

Wanna know the specs of what you're up against ?

[tszaboo]

Nice, OP image needs resizing....good you've done it.

Dunno what it is but we have one coming soon too, a 4 GHz version.

Wanna know the specs of what you're up against ?

Thanks. I'm eager to find out.
There are many benefits of using a factory built probe, it tells you on the screen what the voltage you are looking at, and with the active probe it's a better interface.
Still, I hope people with low-end scopes or budget will be able to use a probe like this.

[tautech]

Nice, OP image needs resizing....good you've done it.

Dunno what it is but we have one coming soon too, a 4 GHz version.

Wanna know the specs of what you're up against ?

Thanks. I'm eager to find out.
There are many benefits of using a factory built probe, it tells you on the screen what the voltage you are looking at, and with the active probe it's a better interface.
Still, I hope people with low-end scopes or budget will be able to use a probe like this.

Snippet attached.
To be released next month.

[jonpaul]

overload, transient protection?

j

[tszaboo]

overload, transient protection?

j

The parts are rated to 50V. Right now I don't have any output clamping, nor do I have capabilities to test surge.
In the commercial designs they have output clamps, diodes, or double diodes. I'm still searching for the right part.
I'd like to point out that even commercial probes are very scarce in info when it comes to protection.

[Neganur]

Guessing the keysight probe was source to your inspiration?

[tszaboo]

Guessing the keysight probe was source to your inspiration?

Inspiration came from:
Keysight probe that I used before.
Lecroy article " Buld your own power rail probe"
https://www.signalintegrityjournal.com/blogs/8-for-good-measure/post/1898-build-your-own-low-cost-power-rail-probe

Circuit stellar article by Andrew Levido
https://circuitcellar.com/research-design-hub/projects/building-a-power-rail-probe/
My schematic differs from this published one quite a lot, even though it might have started like it.

Honestly, I wanted to build one for a while, and I picked it up because a GPS receiver was having power supply issues, and I had trouble measuring it.

[JPortici]

Nice, OP image needs resizing....good you've done it.

Dunno what it is but we have one coming soon too, a 4 GHz version.

Wanna know the specs of what you're up against ?

Thanks. I'm eager to find out.
There are many benefits of using a factory built probe, it tells you on the screen what the voltage you are looking at, and with the active probe it's a better interface.
Still, I hope people with low-end scopes or budget will be able to use a probe like this.

and one may wonder if there were APIs for the probe interface. Maybe siglent would be prone to provide a generic class of devices (I2C device id = 0xNN means power line probe, probe reports calibration factors in such format and responds to command in such other format)
Ah, that would be indeed wonderful.

Now back to lurking

[nctnico]

Why not use a push-button electronic potmeter? I have been tinkering with the idea to add an offset to my HF differential probe and I drifted towards using an electronic potmeter. The advantage is having less noise (no wiper noise), a fine and more stable adjustment and no mechanical components.

[tszaboo]

and one may wonder if there were APIs for the probe interface. Maybe siglent would be prone to provide a generic class of devices (I2C device id = 0xNN means power line probe, probe reports calibration factors in such format and responds to command in such other format)
Ah, that would be indeed wonderful.

Now back to lurking

From what I've seen, everyone keeps their probe interface locked down, probably even with serial numbers. The profit margin on active probes is a magnitude more than the scope itself, it's what makes the money.

Why not use a push-button electronic potmeter? I have been tinkering with the idea to add an offset to my HF differential probe and I drifted towards using an electronic potmeter. The advantage is having less noise (no wiper noise), a fine and more stable adjustment and no mechanical components.

I did consider that. I think the potentiometer is the noisiest part in the design (and sometimes the BNC connector). At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers. And it's done with that in the existing probes. For now I've shelved the idea, because I like the idea of having a product with no programmable parts.

[nctnico]

There are up/down button potmeter chips which don't require any programming / software. Just hook up some buttons to pins and off you go.

[tszaboo]

There are up/down button potmeter chips which don't require any programming / software. Just hook up some buttons to pins and off you go.

I made a quick search and for example the MAX5450 would be suitable. It's 256 tap, that would bring the resolution of nulling to 180mV. So at least two would be needed.
So browsing through the datasheets of these, you would need to press the up/down button 256 times to swipe across the range. I don't think it would be a good user experience.
I also recall using some AD digipots, and they had quite bad tempco. These seem fine though from that point of view.

[nctnico]

There are up/down button potmeter chips which don't require any programming / software. Just hook up some buttons to pins and off you go.

I made a quick search and for example the MAX5450 would be suitable. It's 256 tap, that would bring the resolution of nulling to 180mV. So at least two would be needed.
So browsing through the datasheets of these, you would need to press the up/down button 256 times to swipe across the range. I don't think it would be a good user experience.
I also recall using some AD digipots, and they had quite bad tempco. These seem fine though from that point of view.

You still have the offset control of the DSO itself so 256 taps would be overkill. 64 would be more than enough. Also, do you really need a 24V range? It would make more sense to have a 4V and 24V range which then translates into a 64 tap digital potmeter having plenty of resolution either way.

[temperance]

At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

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.

Edit: the Circuit cellar schematic might have more problems. It is meant to be powered from alkaline AA cells. Some battery chemistries show some ultra low frequency noise where the voltage changes all of a sudden 50...150 mV or even more for some lithium batteries. Those transient lasts from several seconds up to hours.

[nctnico]

At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

I get that, but long term potmeters tend to wear. Back when I was designing audio gear, I always filtered a potmeter to provide a DC level to a chip which did the actual control work (like changing the volume). In this case a potmeter would need a filter + buffer in order to get rid of nasty potmeter effects. But you can still get a slow wandering DC offset.

[tszaboo]

At least my ten turns trimmer pot was definitely making more noise than these Alps potentiometers.

I don't know what your schematic looks like but I assume you are just injecting an off-set into some amplifier input. Adding a filter might work. (but not with X7R capacitors) The circuit cellar schematic has a mistake in the schematic. The filter cut-off varies with the pot setting from no filter to some filter when the pot is adjusted to the center position.

I prefer pot's over push buttons. Faster and less fiddling.

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.

I have to agree, pot is also my preference. There are a few issues with that circuit, that make it less than optimal. For example your offset voltage constantly changes as your battery discharges.
I was using a 10 turns trimpot to bring up the circuit and test it. They were creating strange low frequency drift. I have to assume, that pushing the plastic housing of the trimpot introduces stress that will upset uV level signals. The time constant was seconds, no amount of capacitors will filter that. With these potentiometers it works much better.

[tszaboo]

You still have the offset control of the DSO itself so 256 taps would be overkill. 64 would be more than enough. Also, do you really need a 24V range? It would make more sense to have a 4V and 24V range which then translates into a 64 tap digital potmeter having plenty of resolution either way.

With +/- 24V that the probe can offset out, and +/-500mV,  you can only ever investigate 1V around the setpoint with the probe. You need to bring the output signal of the PRP1 close to 0 otherwise any ripple could be clipped. With 48V/64 tap, the resolution is only 0.75V.
It's also worth noting that while you sweep the offset voltage range, nothing happens on the oscilloscope. If your potentiometer is set to 0V compensation, and the signal is 20V, the pot needs to be turned all the way to 19.5V before the signal moves. You can do this with a fraction of a second with a pot, but with a digipot, it's pressing buttons and nothing happens. Not a very nice user experience.

[temperance]

They were creating strange low frequency drift. I have to assume, that pushing the plastic housing of the trimpot introduces stress that will upset uV level signals.

Interesting observation. But probably hard to avoid as the contacts must settle after tweaking. But some ten turn pot's are better than others.

One of the better pot's around:
https://www.bourns.com/products/potentiometers/industrial-panel-controls/product/3310
https://www.bourns.com/products/potentiometers/industrial-panel-controls/product/91-95

Those are used in pro audio equipment where those pot's are exposed to extreme vibration. (like next to stack of subs.)

But of course that doesn't say anything about the effect you've noticed.

I have some of those laying around (10K linear). Now I'm curious...

[temperance]

Data for the 9195 attached.

That's a 5K pot.

 DMM-5 Run 4 2020-08-06T15.00.35.xlsx (440.12 kB - downloaded 12 times.)

[temperance]

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.

[Neganur]

👍 nice

[hpw]

I'm calling this PRP1. It's a power rail probe, that is oscilloscope agnostic and low cost.
I've started working on this because normal power rail probes cost around 10 times as much as entry level oscilloscopes. I hope to change that with this probe.

Specifications:
Max voltage offset: ±24V
DC Input impedance: 50kΩ
High frequency input impedance:b50 Ohm
Signal attenuation of 1.2:1
Transfer function within 1dB to 2GHz
Estimated noise: 42 µVp-p to 20MHz
Active signal range +/-500mv
Course - Fine adjustment
USB Type C 5V power input - run it from your oscilloscope directly
SMA input and output. SMA to BNC cable to connect to oscilloscope
Typical power consumption of 20mA

I did some early experiments as using an LNA (about 20dB gain) with 2 * +/- 9V  batteries to measure the power ripple on various digital ADC/DAC. Differential, as to use similar PIN header connections as LeCroy AP034 differential probe.

It started wit the differential AD8129 OPAMP with capacitors coupled and alike Pin header connections, this means no DC support but with higher load.

While on ADC/DAC VRef is a critical on low PN frequencies as low as 10mHz. In addition the ripple on ADC/DAC digital or analog part. The ripple tells you much what is on the chip going on (or not should be drawing in any case).
The GND current with unknown current on DUT & LNA & DSO must be avoided.

The critical goes than on high RF frequencies as the CMRR of the LNA amplifier will go down the hill.
Only MISIC show nice CMRR but never checked how they really performs. The 1GHz as for -3dB down really about 600MHz.

At the end it shows about 3 different LNA devices required as:

- 0.0xx mHz to 100Hz (large input capacitors required)
- 10Hz to view xxMHz as for the average use
- xxxHz to RF a 1GHz with good CMRR required as the beef test with high ADC/DAC with 100MHz locks.

Important to check with asymmetric connections on a DSO with good ADC resolutions as SDS2000 Plus 10Bits and input probe scaling for the LNA.

The FFT size & FFT Window is by this DSO not optimal for all ranges (SDS7000 as with 32M samples at best).

Later on a nice rtHz plot is an must do as BIN export and import with an FFT Analyzer.

Some parallel entries found on diyaudio Equipment forum