Buffer Amplifier at 4GHz at PLL output

[afz23]

Hi,

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.

Kindly suggest circuit for this.

 

[biff44]

been there, done that!

You can use 15 dB amps followed by 15 dB pads, until you get the isolation you need.

You can do the LO at 2 GHz and double to 4 ghz, should give you around 50 dB isolation

You can send the LO to the modulator as optical (i.e. laser diode driving photodetector) and get 90 dB.

There are other ways too, involving the modulator design.

Rich

 

[afz23]

Hi biff44
I understand the first suggestion, but how do you get
50dB isolation by doubling LO frequency? please elaborate .

 

[vfone]

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.

 

[afz23]

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.

Is this the correct explanation?

 

[biff44]

With something like a schottky diode push push frequency doubler, load changes at 2 x Fin are highly decoupled from the input port at 1 x Fin.

If you really want suppression, you could run at 8 GHz, and use a digital divider chip.