Hi guys,

as you all know, the HP 435A, 436A, 437A and so on are quite nice power meters and the meter itself is availabe at quite low prices. However, the power sensors are quite expensive. I own a 8481A? and a 8484A, both in nice and working condition, but I don't want to damage anything so I treat them like raw eggs and handle them extremely careful. So I always thought whether it would be possible to make a own diode power sensor which is compatible to the HP 436A. This would have the advantage that the 436A could be used (and therefore its HPIB capabilities), and if it works with the 436A, it will do so also with the other meters (and possibly others, I don't know). Further, the homebrew power sensor would be cheap and if it breaks, no matter, it can be repaired easily.

So today I spent a few hours on making my own power sensor. I checked the schematic of the 8481A power sensor, and I did basically the very same. I came up with this schematic: (I don't want to waste valueable web space for the group, so I uploaded the high resolution pictures to my webspace)



The letters on the right side are the connector pins for the HP 436A, of course. Then I made a quick 'dead bug' style prototype, as follows.... (please don't laugh at me :-) ).



I connected to the 436A and I also installed a SMA 30dB attenuator at the input and connected my power sensor to the power reference of the 436A. At first, the readings were quite off a bit, but using the little trim pot on my PCB, I was able to precisely adjust the gain of my sensor such that the reading on the power meter was exactly 0.00 dBm. I then connected a signal generator to the power sensor and tested various power levels between -10dBm and +20dBm. Incredible, but my power sensor was accurate to 0.1dB! I also tested different frequencies between 1MHz and 2.6GHz. The flatness was not extremely good, but between +/-1dB. So, basically, the concept seems to work. I also tested different ranges for the power sensor - there is a 'mount resistor' inside the HP power sensors (8481, 8482, 9494, ...) which tells the power meter which range the sensor uses; the meter then internally adjusts the gain according to the power sensor range.

It appears that it should be possible to home brew such power sensors. However, at the moment, I have two issues:
a) the auto zero function does not work - even if I run the autozero feature, the power meter afterwards displays -13dBm anyway, so I cannot properly zero my power sensor. At the moment, I do not understand why this is so. Has anyone on the list an idea?
b) I don't understand exactly how the amplifier using the NPN transistor works. When I was adjusting the gain of my sensor, I tried to change the collector resistor (330 Ohms), but this had almost no effect on the gain. The emitter resistor (1k Ohms) also had only little effect on the gain. Only when I placed the pot where it is now I was able to adjust the gain in a wide range. So it basically looks to me like this amplifier is a common emitter amplifier, with the gain being RC / RE, is that true? why then is a feedback from the emitter required to the meter?

Thanks for any hints, tips and so on,
Tobias HB9FSX

Robert,

do you own a schematic of the diode sensors? The schematic for the thermocouple sensors was hard enough to obtain. In the current service manuals, there are of course no schematics at all, and the HP journals of that time also don't show the actual schematics of the sensors.

Tobias

Sent from my Samsung Galaxy smartphone.

-------- Original message --------

From: "Robert G8RPI via Groups.Io" <robert8rpi@...>

Date: 11/17/18 20:09 (GMT+01:00)

To: [email protected]

Subject: Re: [HP-Agilent-Keysight-equipment] A homemade diode power sensor for HP meters

Interesting project. The diagram you show is for the Thermocouple "A" suffix sensor. While the principle and basic circuit is the same for diode and thermocouple, the A2 PCB is different so there must be a difference in the circuit. This may be related to your offset issue.

Robert G8RPI.

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

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.