I know that IF frequency is the difference between the incoming signal and the local oscilator frequency in a frequency synthesizer. My question is why in a frequency synthesizer we need the IF frequency>
- posted
18 years ago
I know that IF frequency is the difference between the incoming signal and the local oscilator frequency in a frequency synthesizer. My question is why in a frequency synthesizer we need the IF frequency>
You need a frequency synthesizer to generate all those different local oscillator frequencies, so that, no matter what frequency you are wanting to receive, the IF frequency is the same. The IF gain section has fixed tuning.
The I.F. is at either the _sum_ or the _difference_ frequency; it's up to the designer.
The mixing (multiplication) of the frequencies that results in the sum and difference, is not done in the synthesizer; it is done in the Mixer, a.k.a. Converter stage.
In a receiver using an I.F., a synthesizer is commonly used to generate the variable (tuning) frequencies, whereas the fixed frequency for the Mixer is typically generated by a crystal oscillator.
If you elect to design a Direct Conversion receiver, you use only the variable frequency tuning oscillator (or synthesizer) and demodulate received signals directly to baseband audio - there is no I.F.
Don
You don't need any mixing and thus any intermediate frequencies in some synthesizers. Google for "direct digital synthesis" (DDS) to see how this can work. Also, single-loop phase-locked-loop (PLLs) synthesizers use only a reference, a divider, a phase comparator and a voltage controlled oscillator.
Even a Direct Conversion receiver needs a mixer (demodulator).
Don
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