[time-nuts] RF isolation amplifier question

Bob kb8tq kb8tq at n1k.org
Sun Oct 14 16:31:41 EDT 2018


Hi

What you have there is known as a cascade buffer amplifier. The common base
stage combined with the common emitter has some well known issues at high 
frequencies ( like UHF / microwave). That’s the tradeoff for having really good 
isolation at HF and usually at VHF. 

Put a sniffer loop near the circuit and look with a good wide range spectrum analyzer. 
Drive the circuit over a range of input levels. Try terminating the output with odd loads. 
Watch for birdies popping up and then fiddle things to kill them. 

As others have noted, those may not be the best parts to use for this. I would not replace
it with something simple, it should have a lot of isolation when it works correctly.

Bob

> On Oct 14, 2018, at 11:54 AM, <cdelect at juno.com> <cdelect at juno.com> wrote:
> 
> Hi,
> 
> I removed this amp from the EFOS2 Maser as it was intermittent.
> 
> Of course now I can't get it to fail on the bench!
> 
> I have fabricated a new PC board to replace it and have the components on
> hand.
> 
> Note the 12pfd cap on the collector .
> 
> Some notes I have indicate is select at test and that is it a glass
> capacitor.
> 
> The as built unit has a johansen 50pf that looks to be an early version
> of surface mount with leads soldered on.
> 
> How do I determine what value to install on the new amplifier?
> 
> Cheers,
> 
> Corby<efosiso.jpg>_______________________________________________
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