How to add offset dc adjustment to a circuit (YIG filter driver)

[hafrse]

Hello,

I have an old spectrum analyzer with outdated calibration, and need to fine adjust the offset voltage (with a multiturn pot ) of the preselector YIG filter, attached is the circuit diagram.

PSDR+ (Preselector control voltage)
PSDR- is ground from the controller
+MTXV is the supply voltage to the YTF (+26V)

The fine adjustment offset should be around U10 I think, any suggestions how to add that on this circuit?

Many thanks!

George

 

[chuckey]

R36, 1K? or is it mean't to do something else?
Frank

 

[hafrse]

R36, 1K? or is it mean't to do something else?
Frank

Hello,

I tried there, it did not change any thing, obviosly it does something else
Thanks

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Hello,

More information:

The analyzer can however adjust the offset through a menu entry but after shutting down the analyzer, the values are read from the calibration data which are outdated. I need to do this procedure every time I power the analyzer ...

I measured now the needed offset at TP-10 (the PSDR+ signal from the AD converter) , the offset range of +/- 250 mV is enough.
So now, how to add an offset at TP-10 of +/- 250 mV with a multiturn pot?
Many thanks!
George

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Attached is the schemtics for the source of the PSDR+ signal (TP-10), output signal is the right from Q13 and Q14

 

[chuckey]

How about measuring the DC around the circuit as it is. Then actuate the menu driven off set control and alter the offset a lot then measure the DCs again. This will give you a clue as to what the off set actually does, could put a saw tooth on a pedestal some where else for instance.
Frank

 

[hafrse]

Thanks for the information, I actually did, when I rotate the offset soft konb, the offset dc goes up and down about 200 mV . I also measured what offset I need to get the YIG filter linear in almost all frequncies, it is around +40mV , so the needed offset is a very stable positive dc of around 40mV .
George