Electronics > Metrology

HP437B with a new microcontroller system

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I have an HP437B meter. I don't have a probe for it. So I decided to modify it to be able to use an external probe on AD8307 (and similar). The assumptions behind the modifications are as follows:

* possibility of easy return to the original version
* use of the original keyboard and display
* support for the analog part to use the mode as in the original probe
* no HPIB support but data output to serial port
* additional OLED display in place of the tilt indicator (which in my case is damaged)

I also plan to make an equivalent of the HP848x series probes (based on a diode detector). I'm not interested in the range beyond 1GHz (at least for now). The board connecting the digital and analog parts has already been designed and I am waiting for delivery. For convenient connections, it is made of 4-layer laminate. By default, for the new control system to work, you need to reconnect the keyboard and display connectors. The rest on the IDC34 connector may be transparent. The system will consist of two uPs - AVR64EA32 and ATTiny414 for keyboard operation.

For the purposes of adaptation and checking the meter itself, I made a circuit corresponding to the HP8484A probe. I read some documentation about this probe and measurements performed in a similar way (Marconi 6290). There are many indications that crosstalk (symmetry) of the 220Hz signal driving the FET gates will be a problem.
I have also seen projects using repurposed probes as calibrators. As I already know, most problems will be at the ends of the range (from the background/noise side). But I'm really curious what the meter will show if I connect such a probe instead of the original one.
Will it pass autozero and what will it show for a 0dBm signal (with attenuator)? I think this will be instructive before I start implementing the new controls.

My first tests are behind me. I assembled the HP8484A probe simulator. The effect is that the HP437B shows a level of approximately -46 dBm. Everything indicates that there is some voltage on diode D1. Shorting C3 or TP1 to ground (signal) causes the meter to read approximately -65 dBm. What could be the reason? It doesn't seem to be about thermoelectric effects. Initial tests of the HP437B show that there is no damage and the analog path works.
There are many indications that the problem lies in the Q1-Q2 system, i.e. the circuit that converts direct voltage into alternating voltage. Or maybe in such a system (built this way) it is impossible to go below -50dBm?

Have you tried it with a known level input? I assume you've used not the original HP diode - so the forward voltage may be different. Not sure what the consequences of that would be.

It depends on the driving voltage levels for the Q1 and Q2 - there is the gate-source diode which may leak when forward biased and you will get a voltage on that input diode. Btw the 1N5711 is one of the best diode for the input detector up to 1GHz (HP5082-2800 eq. afaik), imho.


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