Power sensor project

[Mrt12]

I got a HP 436A power meter with a 8481A power sensor. Both the sensor and the meter work fine and very accurately.
Today, I just wondered, whether it could be possible to reverse-engineer the power sensor interface, and make a own power sensor using a diode detector. I have already made several diode sensors, they work fine from a few kHz up to GHz, no problem, but it would be really comfortable if it was possible to connect the homebrew sensor to the 436A power meter and directly read off the dBm. Has anybody ever tried such a project? in my opinion, it should be possible because the schematics of the meter and the sensor are both available.

Apparently, there is a chopper inside the power sensor; I think, this chopper could be built e.g. with analog multiplexer ICs or similar. However, I wonder how the meter converts the voltage to the dBm reading. I think this could be the biggest problem. Can somebody explain it a bit more in detail?

I think it should definitely be possible because there are diode power sensors for the 436A, like the 8481D or similar, and the meter works with these.

[Tony_G]

The sensors use a resistor to indicate what the sensor dynamic range is. The meter, the 436A in your case, provides a 220Hz square wave that drives the chopper in the 8481A. The resultant AC waveform (RF input converted to a 220Hz square wave by the chopper) is scaled to reflect a value in the dynamic range that reflects the power sensed.

There is a full schematic for these in the 8481A manual available from the Keysight website.

I don't have the full manual for the 436A but the 438A manual shows the service information indicating that the sampled AC should be in the 100-150mV range when reference to the 1mW 50MHz reference source on the meter. I've never actually measured this so I can't confirm the accuracy of the manual.

The basic thing to remember is that it is the combination of the dynamic range resistor telling the CPU what to display and the measure AC referenced to the 1mW source that will end up displaying the value on the meter. for example the 8484A diode sensor has a dynamic range of -70 to -20dBm so when you attach it to the meter and the 1mW reference you do so through a 30dB attenuator and the meter knows that it is actually seeing the results for -30dB. As an aside the mount resistor is 10K for the 8484A and 0 for the 8481A.

TonyG

[Mrt12]

Thanks Tony, this is a helpful information regarding those 100 to 150 mV.
I just built the chopper according to the schematic shown on page 20, here:

http://f1chf.free.fr/hyper/hpsondes.pdf

I have no idea what transistors are used in the actual HP sensor, but I guess it is kind of an ASIC since it says "U1 FET ASSY".
I made a very simple detector using an attenuator, a BAT62 Schottky diode, and a simple chopper, using two 2N4856 JFETs, exactly the same circuit as shown in the manual mentioned above. This seems to work, at least I can generate the desired chopper waveforms. Next step is to make the necessary amplifier - do you think I can replace Q1 with an OpAmp? I don't understand this circuit, yet.

[Tony_G]

That FET assembly is a custom HP part with the FETs & components on the one substrate. That said, I don't think there is anything super special about them apart from them being low noise, tight tolerance items so you should be able to just duplicate the sensor circuit but you won't get the linearity or accuracy of the original.

The sensor input amplifier in conjunction with the power meter first amplifier basically creates a single Op Amp (check out figure 8-12 from the 438A manual for example) with those two amplifiers increasing the signal to the rest of the meter by a factor of 600. Of the top of my head, as long as you get a final gain of around 600 I don't see why you couldn't use a different amplifier design. I'd imagine that this was best practice with the components of the time rather than anything really unique but I haven't really looked at it either so YMMV.

For some good insight into the working and signals you might want to look at the manual - http://www.ko4bb.com/manuals/73.109.124.249/HP_436A_Service-MIL.pdf - Page 8-124 seems like a good place to start for specifics but you might want to start back at Service Sheet 1 and work through them to get a handle on the system. This manual also has some good troubleshooting instructions that should help you get your sensor right (things such as the A-D Converter input voltages for various RF Power indications etc).

Good luck with the project.

TonyG

[Mrt12]

Hi,
I managed to hook up my homebrew power sensor to a 436A, and it worked!
For the transistor amplifier, I used the original HP schematic, for Q1 I took a 2N2222A (I am sure there would be a better transistor, but this one was just around...). For the moment, I ignored the autozero circuit. The power sensor had an offset - the power meter always showed 13 dB more than the actual sensor input was, but this should be no problem. Linearity was also surprisingly good. My sensor worked fine up to a frequency of 1.25 GHz, I could not test it further yet. But I expect it to be usable up to approx. 3 GHz.
What I don't understand yet is how the Q1 amplifier actually works. I know in the power meter, there is another circuit which forms kind of an OpAmp together with Q1, but I have no idea how that OpAmp actually works.

Is it possible to get the HP Power Sensor connector somewhere? So far, I used test leads to connect my sensor to the meter, but this is not a long-term solution ;-)

[rfeecs]

You might want to look at Oct, 1975 HP Journal:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-10.pdf

It has a few articles on the 436A and power sensors that go in to detail on the theory of operation.

[Tony_G]

I managed to hook up my homebrew power sensor to a 436A, and it worked!

Outstanding - Congrats.

What I don't understand yet is how the Q1 amplifier actually works. I know in the power meter, there is another circuit which forms kind of an OpAmp together with Q1, but I have no idea how that OpAmp actually works.

That MIL Service manual I pointed you to on KO4BB has a schematic in it and a good "how does it work" description for the 436A First Amplifier - You should check that out.

Is it possible to get the HP Power Sensor connector somewhere? So far, I used test leads to connect my sensor to the meter, but this is not a long-term solution ;-)

You're looking for an 11730A cable to cut up for your sensor - You could buy a blown sensor as well and salvage the connector from it but it's probably cheaper just to cut one end off the cable.

TonyG

[Wolfgang]

Hi, 

there is another article from HP describing how they make the very wide range front-end so it can cover several decades of signal strengths. IIRC, it works by using a resistive attenuator with schottky diode strings attached at several points of the attenuator structure. The diodes for the low levels are at the front end of the attenuator, the diodes for the higher ranges are at the back. All diode outputs are sampled, and then a range switching logic decides which diode string delivers the most accurate result. HP has patented this (maybe the patent has run out). My U2204A USB power sensor uses this, and it works fine.