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
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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
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