planning superheterodyne receiver IF frequency

I would like to ask the experts on how to plan superheterodyne receiver IF frequencies.

I am using double conversion. RF input is 9.3 GHz and final IF frequency is 70MHz. How do I choose first IF frequency?

Dixon's receiver book mentioned the choice is dependent on bandpass filter rejection, but I don't quite understand. In the book, it says that the IF frequency must be at least twice the bandpass filter bandwidth away.

Can any expert enlighten me? (with some example calculation please.)

What is the bandwidth at 9.3GHz ?

You are avoiding two problems.

Image response has two aspects. One is the selectivity of the system before the mixer. The second is the presence of high power signals that may be at the image frequency. The usual advice for image rejection is to have the IF at least 10-20% of the RF.


The more complex problem to avoid is IMD products in the mixer. You can make a spread sheet program (or get a free one somewhere) that calculates what these products are that would fall in the IF passband (not just the 3 dB bandwidth, but the 60+dB bandwidth.)

check this book about transceivers architectures it may help u

khouly

Thanks for the replies.

The final IF bandwidth is 20MHz.

Yes, I have a spread sheet to calculate mRF +/- nLO1 +/- nLO2 to calculate the mixer spur products due to the RF, LO1 and LO2.

The question is what should my IF1 be. Should it be 1GHz, 2GHz, 3GHz, or 900MHz, 800MHz. After deciding the rough frequency, I can then perform mixer spur calculation.

One aspect of selecting the first IF is on the air signal strength at that firequency. Another is readily available low cost parts.