[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:
> 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.
>> On Oct 14, 2018, at 11:54 AM, <cdelect at juno.com> <cdelect at juno.com> wrote:
>> 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
>> Note the 12pfd cap on the collector .
>> Some notes I have indicate is select at test and that is it a glass
>> 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?
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