Any ideas what this circuit is doing?

[keith59]

Hello, my first post here.
I am more comfortable with digital circuits, but have been asked to test the attached circuit.
The circuit is only the input section, the output is through some general purpose transistors so is fairly easy to sort.
The question is how do the schottky diodes work in this configuration and how would you test them, what signals / levels would you apply? Note, the bottom line D3 anode is connected to 0V.

thanks in advance, Keith

 

[Nigel Goodwin]

Without any context, and most of the circuit missing, it's not easy to suggest what it might be doing?.

 

[keith59]

The only other part of the circuit, not shown, are 2 general purpose transistors feeding to an output.
The only other connections are to the collector of TR1, top right of diagram. One goes to a connector with a 3.9K resistor in-line, and the other connection goes to a connection +18V via a 100k in-line.
I can't post the full circuit for confidential reasons.

 

[AnalogKid]

Can you tell us what the circuit is intended to do? Do I get a prize if I guess correctly?

Three-stage limiter - ?

Also, and this is important, are the input signals considered to be/treated as analog or digital?

ak

 

[rjenkinsgb]

The whole setup appears to be for noise immunity or threshold control; it will need a significant voltage at either input before the transistor starts to conduct.

The input diodes drop some voltage, D9 means there is a significant base to 0V current via the 2k2 at anything over about 0.6V on the base, meaning something like 25:1 voltage division from the input, and the the 2.5V ref in the emitter circuits so over 3V at the base before it starts to conduct..

 

[keith59]

Thanks for the reply AnalogKid, to be honest I don't know what it does, not a helpful really.
I suspect the inputs would be treated as analog because it is something to do with radio, but could equally be digital.
The reason why I am confused is the link across the cathode's of the 2 schottky diodes, why link them?

 

[AnalogKid]

The two input signals are being summed into a couple of filter capacitors.

D3 - The symbol is Schottky diode, but the partial part number comes back as an active voltage reference from Zetex.

???

ak

 

[keith59]

The whole setup appears to be for noise immunity or threshold control; it will need a significant voltage at either input before the transistor starts to conduct.

The input diodes drop some voltage, D9 means there is a significant base to 0V current via the 2k2 at anything over about 0.6V on the base, meaning something like 25:1 voltage division from the input, and the the 2.5V ref in the emitter circuits so over 3V at the base before it starts to conduct..

When you say "significant" would you like to put a finger in the air figure on that? Would it be a DC level or a sinewave?

Arrg, digital is much easier, it's either 1 or 0!

 

[rjenkinsgb]

When you say "significant" would you like to put a finger in the air figure on that?

Very roughly, somewhere around 45 - 50V at the inputs.

 

[augustinetez]

The HSMS2085's are Broadcom RF Diode Schottky - 2 Independent 70V 1 A (2 diodes in 1 package) so that ties in with the OP's reference of radio related.

Without any other info, my guess is the circuit is related to some sort of control loop - probably to do with output power.