On Nov 18, 2018, at 15:48, Artekmedia <manuals@...> wrote:

Mike

almost all of the 848x series power sensor heads use a thermocouple and not a diode. The replacement thermocouples were at one time available directly from HP/Agilent. They were crazy expensive (like $750) as I remember) plus you needed an eye loop and the nerves of a Swiss watchmaker to replace them. What is essentially being attempted here is to make a diode version of the power sensor head as a replacement since the heads have become scarce and expensive in their own right.

Why you might ask did the then HP choose a thermocouple? I suspect that the thermocouple is more linear than a diode sensor and generally flatter over a wide frequency range. If your willing to trade off dynamic range and wider bandwidth of the thermocouple then the diode may be a a useful solution to lack of power heads available these days .. .I am hopeful that Tobias is successful.

Hope that helps
Dave
NR1DX

On 11/18/2018 2:35 PM, n2msqrp wrote:

I've been following this discussion about diode power sensors.


I assume the diode sensors fail because of blown diodes form overload. Has any repaired the faulty sensors by replacing diodes?


Mike N2MS

On November 17, 2018 at 6:33 PM "Ed Breya via Groups.Io" <edbreya@...> wrote:

Hi Tobias, I looked at the OP again, and realized that you already knew about the "mount" resistor - I didn't notice that the first time. I grabbed that 8484A manual pdf that Orin linked - thanks Orin, I have lots of 8484As, and the manual somewhere, but can never seem to find it. Now I have a fresh copy.

Anyway, after refreshing my memory on these sensors, and looking also at the 8481A schematic, I'd recommend studying both types - the TC and diode models have a different arrangement for the auto-zeroing function. The circuit you posted puts the AZ feedback to the detector, as in the 8481A TC circuit, but that won't work right. With the TC type, it is a low impedance source, so the AZ current can go through, and offset the signal. With a diode detector, you should use a circuit like in the 8484A, which applies it to the ground-side JFET switch in the chopper. The diode detector is a high impedance source, so you don't want to put the AZ feedback current there.

Good luck,
Ed

--
Dave
Manuals@...
www.ArtekManuals.com

I've been following this discussion about diode power sensors.


I assume the diode sensors fail because of blown diodes form overload. Has any repaired the faulty sensors by replacing diodes?


Mike N2MS

On November 17, 2018 at 6:33 PM "Ed Breya via Groups.Io" <edbreya@...> wrote:

Hi Tobias, I looked at the OP again, and realized that you already knew about the "mount" resistor - I didn't notice that the first time. I grabbed that 8484A manual pdf that Orin linked - thanks Orin, I have lots of 8484As, and the manual somewhere, but can never seem to find it. Now I have a fresh copy.

Anyway, after refreshing my memory on these sensors, and looking also at the 8481A schematic, I'd recommend studying both types - the TC and diode models have a different arrangement for the auto-zeroing function. The circuit you posted puts the AZ feedback to the detector, as in the 8481A TC circuit, but that won't work right. With the TC type, it is a low impedance source, so the AZ current can go through, and offset the signal. With a diode detector, you should use a circuit like in the 8484A, which applies it to the ground-side JFET switch in the chopper. The diode detector is a high impedance source, so you don't want to put the AZ feedback current there.

Good luck,
Ed

On November 17, 2018 at 6:33 PM "Ed Breya via Groups.Io" <edbreya@...> wrote:

Hi Tobias, I looked at the OP again, and realized that you already knew about the "mount" resistor - I didn't notice that the first time. I grabbed that 8484A manual pdf that Orin linked - thanks Orin, I have lots of 8484As, and the manual somewhere, but can never seem to find it. Now I have a fresh copy.

Anyway, after refreshing my memory on these sensors, and looking also at the 8481A schematic, I'd recommend studying both types - the TC and diode models have a different arrangement for the auto-zeroing function. The circuit you posted puts the AZ feedback to the detector, as in the 8481A TC circuit, but that won't work right. With the TC type, it is a low impedance source, so the AZ current can go through, and offset the signal. With a diode detector, you should use a circuit like in the 8484A, which applies it to the ground-side JFET switch in the chopper. The diode detector is a high impedance source, so you don't want to put the AZ feedback current there.

Good luck,
Ed

The 12-pin circular connector used on the power sensors is an Amphenol.? The old part number was 91-T-3638.
Now, it's part of their "C 091 A/B/D" series circular connectors.? Several variations are listed in the Amphenol catalog:


Mouser has several slightly expensive options (a few of which appear in this link -- note that widening the Mouser search filter will supply more results):


It appears that an eBay seller is selling them at a lower price.? His text says they are the 12-pin versions, although his photo shows the 14-pin version.? It's hard to tell which of the two connectors he is selling.? However, it appears the two additional pins shouldn't matter since it's the female part of the connector -- you would leave those two extra pins unconnected.

Hi Guys,

sorry for my late reply.

Today I was able to make some further progress. Robert was right in his post (/g/HP-Agilent-Keysight-equipment/message/92641) - the autozero problem I encountered was indeed related to the position where I connected the autozero signal. In principle, I have now the same schemativ as the 8484A diode power sensor.
After moving the autozero signal, I still had the problem that the autozero did not work properly, so I further analyzed the circuit of the 8484A and realised that it has less gain than my circuit, so I changed the base resistor of Q1 () to 1k Ohms and the autozero problem was fixed - pressing the autozero button perfectly allows the meter to zero the sensor. However, with this modification, my sensor had a different gain, and therefore the readings on the meter had an offset. I was able to re-adjust the gain with the trim pot and when I put a 10dB attenuator at the sensor input, I was able to have the same readings on the meter as on the signal generator I used to test the sensor. However, as I said, it is not terribly accurate, at very low power levels, it was around -1dB off, and at high power levels, it seems like the diode transitions from square law region to linear region and therefore the readings were also off a bit. But this is no big deal; one can simply add attenuators in front of the sensor such that it is ensured the diode is in its square law region.

Thanks at Orin for the 8484A sensor schematic. I have never found it, even though I was of course looking for it.

Joarez (/g/HP-Agilent-Keysight-equipment/message/92646) says that the capacitors need to be there to prevent the circuit from generating switching abberations. This is probably true because the 8484A manual tells that the capacitors were factory selected individually for each sensor, so they are probably related somehow to the JFET switches. In my sensor, I currently don't have these capacitors, and this is probably another reason why the readings are not 100% accurate. It could also be related to the JFETs I use - I chose the first ones I could get, but somewhere in my junk boxes I should have a pile of 2N4856, which are probably better. However, I would prefer SMD devices since I will make a PCB in SMD design. I got a bunch of MMBFJ310, but I have not enough of them to make a lot of experiments :-/


Thanks also to rmc321 and Rainer for the link to the connectors. I was once told that HP used their own connectors, so I didn't even try too hard to find the connectors, but very good you found them! I could have saved my valuable power sensor cable if I had known it earlier :-)

I think I am soon going to make another dead bug prototype and in parallel design a PCB and probably my own housing. I will share the design files as soon as I am ready.

Tobias HB9FSX

The 12-pin circular connector used on the power sensors is an Amphenol.? The old part number was 91-T-3638.
Now, it's part of their "C 091 A/B/D" series circular connectors.? Several variations are listed in the Amphenol catalog:


Mouser has several slightly expensive options (a few of which appear in this link -- note that widening the Mouser search filter will supply more results):


It appears that an eBay seller is selling them at a lower price.? His text says they are the 12-pin versions, although his photo shows the 14-pin version.? It's hard to tell which of the two connectors he is selling.? However, it appears the two additional pins shouldn't matter since it's the female part of the connector -- you would leave those two extra pins unconnected.

On Sat, 17 Nov 2018, 17:30 Peter Gottlieb <hpnpilot@... <mailto:hpnpilot@...> wrote:

This is an interesting project.? As you say the meters themselves are dirt
cheap.? I have seen blown sensors sold cheap as well and maybe it's
possible to
build your circuit into one of those which would have the advantage of
connectors and case already taken care of.? If a small inexpensive SM
board is
made to do this it could easily fit and be so inexpensive that it wouldn't
even
be worth the time to troubleshoot a failure, just replace the board if it
gets
blown out.

Peter


The worrying thing is the Chinese counterfeiters will probably do just that. eBay seller yixunhk springs to mind - someone called them the HP rebirthing centre. I can see them "rebirthing" power sensors. ?

Dave