I am fairly new to RF design.  I've actually had some experience in RF board layout, assembly and tuning.  Oddly though, I don't know some of the more high level detials.

As an experiment I am going to build a transmitter to operate in the 2m band.

I would like to implement BPSK modulation.  In the past I have built a transmitter that performs the BPSK mod. in the IF.  This doesn't seem to make a whole lot of sense to me, beacuse a 90 degree phase shift at this IF(45 MHz or thereabouts).  This wouldn't produce a 90 degree phase shift at the tx frequency in the 70 cm band.

Is it okay to PM the IF frequency, or does it have to be done at RF?  Thanks for your help!

-John