[time-nuts] RF isolation amplifier question

Dr. Ulrich L. Rohde ka2weu at aol.com
Sun Oct 14 17:00:36 EDT 2018


Sometimes a 10 Ohm resistor from Base to RF ground will rescue you.

73 de N1UL 

Sent from my iPhone

> On Oct 14, 2018, at 4:31 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 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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> 
> 
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