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Design >> RF Design >> IQ vs. IF modulation
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Message started by pilotcj525 on Apr 7^th, 2007, 10:50am

Title: IQ vs. IF modulation
Post by pilotcj525 on Apr 7^th, 2007, 10:50am

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

Title: Re: IQ vs. IF modulation
Post by mg777 on Apr 7^th, 2007, 12:56pm

Indeed you can PM the IF:

cos(ω₁t + φ) × cos(ω₂t) = (USB) cos((ω₁+ω₂)t + φ)

and the φ gets carried through. The physical picture will be clearer if you write φ = ω₁.τ, where τ is a time delay.

M.G.Rajan
www.eecalc.com