I am looking for a Buffer amplifier circuit at 4GHz,I need
a very good isolation(better than 50dB) between locked
VCO output(4GHz) and next stage (an analog phase modulator).
Experimentally I have observed that 20dB isolator is not
enough to isolate these two stages.I get over modulation
( 3 times of the expected modulation) when I connect these
two stages via a 20dB isolator.
As per datasheet VCO(Teledyne-Cougar) OC4500,frequency
pulling for this VCO is 55MHz at (VSWR 1.67:1 or RL 12dB).
---------- Post added at 06:29 ---------- Previous post was at 06:27 ----------
I think a C-Band BJT can do this job.May be a AT42070.
A single stage common base/gate transistor can give up to 30dB isolation, maybe more depending by the transistor, and if the bandwidth is narrow and the circuit and the layout are well done.
You can use one of this stage followed by an isolator.
Does that means, that although it being a 4GHz problem,if I am using a buffer amplifier, I need not worry
about 50 ohms matching at VCO o/p and buffer o/p?
As VCO o/p sees high impedance always and won't get loaded, similarly buffer amplifier has lower impedance
than the next stage i/p and it doesn't get loaded too.