high injection side local oscillators

[prakash3705]

i read it in most books that a high injection side local oscillator is preferred over low injection side at mixer stages in heterodyne receivers.this is not only the case with AM signals (where variation ratio becomes very high)but even at rf signals having freqs in GHz.
can anybody pls explain why is the case???????

thank u........

 

[betwixt]

It has nothing to do with the kind of modulation, that is more to do with the choice of IF.

You can use either at any frequency. At high IF, the oscillator stage is usually easier to implement and image response becomes better.

Brian.

 

[biff44]

Also, mixers generate output tones that are (M x IF) +/- (N x LO). By allowing the choice of low side or high side LO, you can pick the one where the least amount of unwanted spurious signals are generated. (just keep in mind that some types of modulation might get reversed, since you are ADDING or SUBTRACTNG the IF, depending on the math).