linear PA output impedance to drive SAWs ...

[CTT]

Hello,

Is there anybody who already designed a PA that should drive a SAW filter ? Should we provide a particular output impedance to the SAW to
reduce VSWR ?

Lots of design papers says that you should transform the load to the optimum load to have best efficiency. Suppose I have a 50Ohm SAW
filter that is terminated with a 50 Ohms impedance and that my optimum
load is 10 Ohms. Suppose also that I use an open collector output + a RFChoke. I need to use an additional impedance transformation (1:5)
to obtain the require 10 Ohms but if I do that the source impedance that
sees the SAW will not be 50 Ohms at all ... is that correct ?

Is there anybody who designed and measured a linear PA + SAW filter already that can help me ?

Thanks

CTT

 

[radiohead]

You're gettin into the area of non-linear design here. It is correct that the output impedance of the matched PA does not equal 50 Ohm. The output impedance is cylic, and varies during the period. You can use simulators to define it. And you can use simulators to measure the quality of the filtering involved by just plugging the filter into the simulation of the PA.

At least the filter is matched at one side, this is one good thing. But you might not see a nice 50 Ohm at the other side of the filter if the losses are high, beware of that!

Also, take care you do not put too much power into the SAW. These are acoustic devices with vibrating structures. More power = heavier vibrations = DESTRUCTION! Watch the datasheet.