I would like to build a stand-alone microvoltmeter based on a HP85025 probe.
The probe contains a preconditioning circuit generating a 27.778kHz squarewave proportional to the input signal, and this signal should be amplified, filtered, demodulated and displayed.
No rocket science, obviously, and I could come up with a suitable circuit relatively easily, but I try to avoid reinventing the wheel whenever possible which is why I would like to have a look at a HP schematic, to see if I can recycle some or all of the ideas.
Unfortunately, it is difficult to find real, functional schematics for this processing block.
All I find are general, high-level block-diagrams like this one:
Has someone seen more detailed documentation on the subject?
The probe contains a preconditioning circuit generating a 27.778kHz squarewave proportional to the input signal, and this signal should be amplified, filtered, demodulated and displayed.
No rocket science, obviously, and I could come up with a suitable circuit relatively easily, but I try to avoid reinventing the wheel whenever possible which is why I would like to have a look at a HP schematic, to see if I can recycle some or all of the ideas.
Unfortunately, it is difficult to find real, functional schematics for this processing block.
All I find are general, high-level block-diagrams like this one:
Has someone seen more detailed documentation on the subject?
Hi Elvee,
In a former life I had a service manual for the HP8757D that used these probes. Try a search for manuals for the that scaler analyzer. I did a casual search and found some manual info at Agilent, but the schematics weren't included. The relevant circuity resides with the analyzer, and the probes themselves are almost passive as I recall. I understand that there's a HP forum group, but I haven't searched that far. Good luck!
Best regards,
Steve
In a former life I had a service manual for the HP8757D that used these probes. Try a search for manuals for the that scaler analyzer. I did a casual search and found some manual info at Agilent, but the schematics weren't included. The relevant circuity resides with the analyzer, and the probes themselves are almost passive as I recall. I understand that there's a HP forum group, but I haven't searched that far. Good luck!
Best regards,
Steve
I'm not sure what you mean by "square wave proportional to the input". Is the duty cycle proportional to the input? If so, a simple RC averaging circuit could be a detector.
Artek Media is a good place for service manuals: https://artekmanuals.com/
Tom
Artek Media is a good place for service manuals: https://artekmanuals.com/
Tom
The probe is not just a "dumb" piece of kit: it includes a crystal oscillator, a number of opamps, chopper switches, a PAL for the control, etc. etc.
The end result is a 27.778kHz squarewave having a p-p amplitude proportional to the input signal.
I have never seen a HP8757 manual including the probe processing circuit schematic, neither in paper form nor its electronic version.
Thanks anyway for all the answers.
I'll brew my own version of the processing circuit: there is nothing particularly difficult, but having a knowledge of prior art is always helpful
The end result is a 27.778kHz squarewave having a p-p amplitude proportional to the input signal.
I have never seen a HP8757 manual including the probe processing circuit schematic, neither in paper form nor its electronic version.
Thanks anyway for all the answers.
I'll brew my own version of the processing circuit: there is nothing particularly difficult, but having a knowledge of prior art is always helpful
Reminds me of the HP8405 vector voltmeter. Maybe you can find some inspiration from it.
https://ia800205.us.archive.org/16/items/hp_8405_service_6/8405_service_6.pdf
https://ia800205.us.archive.org/16/items/hp_8405_service_6/8405_service_6.pdf
There is a little bit about the HP 85026a directional bridge in the attached. A little more about synchronous detectors also attached. Don't know if any of that will help.
Here's a link to a Keysight page. https://www.keysight.com/us/en/product/85025A/detector-10-mhz-to-18-ghz.html
Just to elaborate a bit, and as best I recall, the probes themselves incorporate some DC biasing of the detection diodes to improve the detection threshold. Beyond that, the probes have had an opamp buffer to provide modest gain and drive the connecting interface cable to the scaler analyzer.
A typical system had a microwave sweep generator (controlled via HPIB) that was "chopped/modulated" with a ~28kHz drive so that offset biases were absorbed, expanding dynamic range several decades. Again, my recollection is that the 28kHz originated within the sweeper and drove the scaler analyzer where demodulation took place. Very often, several detector probes were synchronized from a presumed common 28kHz source.
Good luck with your project.
Just to elaborate a bit, and as best I recall, the probes themselves incorporate some DC biasing of the detection diodes to improve the detection threshold. Beyond that, the probes have had an opamp buffer to provide modest gain and drive the connecting interface cable to the scaler analyzer.
A typical system had a microwave sweep generator (controlled via HPIB) that was "chopped/modulated" with a ~28kHz drive so that offset biases were absorbed, expanding dynamic range several decades. Again, my recollection is that the 28kHz originated within the sweeper and drove the scaler analyzer where demodulation took place. Very often, several detector probes were synchronized from a presumed common 28kHz source.
Good luck with your project.
Be careful, microwave radiation is harmful.
You could think of a frequency divider circuit, but components for those tend to be difficult to find.
Have you tried sellers of military surplus equipment, those might be used in the maintenance of radar equipment.
You could think of a frequency divider circuit, but components for those tend to be difficult to find.
Have you tried sellers of military surplus equipment, those might be used in the maintenance of radar equipment.
Do you have the probe? And what are you trying to measure? Microwave, RF or LF and Audio? Or DC? The probe looks typical of HP overkill. It seems to have a diode detector and a chopper for when the source is not modulated. The chopper is typical of RF power and volt meters. The Boonton 4200 has a similar circuit with the chopper in the instrument, instead of the probe. The datasheet for the HP instrument suggests that they moved the chopper from the instrument into the probe and have a version that will work with conventional sensors. The current generation Boonton RF power sensors have it all in the "probe" with a USB connector for power and data.
The microwave part of the sensor is the real bitch (having attempted to repair one). Weird resistors and very delicate construction.
The microwave part of the sensor is the real bitch (having attempted to repair one). Weird resistors and very delicate construction.
The probe has two modes: AC and DC.
Depending on the mode, the 27.7kHz modulates the source, or is internally generated to chop the DC output.
The detection cannot be synchronous, because the chopping frequency is generated internally, and there is no reference output (or it would need to be self synchronous, which seems unlikely).
The selectivity is achieved through a sharp filter, hence the necessity for an accurate crystal reference
That's the simple, quick and dirty option, but a good filter like in the original equipment will certainly improve the dynamic range
I have the probe, and its frequency range starts at 10MHz, thus certainly not LF. I intend to use it to measure UHF, and "modern" SHF signals, like Bluetooth, WIFI, 4/5G, etc.
I do not intend to use its full 26GHz capability
detection can be synchronous with another 27,7khz signal but you need a quadrature demod so generation of a sine and cosine of 27.7khz. for example using a dds or an xtal of double freq. you could feed the resulting demodulated 2 channels into an audio dsp for further signal analysis. or use any different freq and demodulate the low IF in the dsp.
I would spend $7.50 on the service manual for the probe and another $11.50 for the service manual for the HP8757 mentioned in Post 2.
Or blow an entire $15 and get all five manuals for the HP8757. Like you said: No need to reinvent the wheel.
https://artekmanuals.com/manuals/hp-manuals/ I've gotten quite a few manuals from them over the years. They're good quality.
Tom
Or blow an entire $15 and get all five manuals for the HP8757. Like you said: No need to reinvent the wheel.
https://artekmanuals.com/manuals/hp-manuals/ I've gotten quite a few manuals from them over the years. They're good quality.
Tom