RF Up/Down Converters

How to tune variable local oscillator in a dual stage down converter. I've heard that comb generators can be used as frequency synthesizers. Any body know how to use this as a variable local oscillator.[/quote]

is also called as VCO ( voltage controlled oscillator)
Wiki it
http://en.wikipedia.org/wiki/Voltage...led_oscillator

Thanks for the link. Do we need to give any control signals(TTL) to VCO to change oscillations?
If so we shud be knowing what is the input frequency we are looking for. But if u take comb generator synthesizer, there we use solid state switches to select frequency out of the Comb output.
My question is, if u r down_converting a signal in dual stages, by using Variable LO in 1st stage and fixed LO in 2nd stage, on what basis this Variable LO is tuned so that all incoming signal in a BANDs of frequency will be down_converted to a Fixed Narrow band frequency?
If you guys have any examples please send me...
thank you

It seems you are not familiar with VCOs...

1.-Most VCOs the frequency is a function of an analog voltage. So, no there are no TTL levels...although you could use them...but that gives you very poor frequency selectivity unless you have lots of bits.

2.-Comb generators produce an spectrum that pretty much looks like a comb in the frequency domain...lots of lines at multiples of the fundamental frequency.

3.-By themselves, comb gen. or vcos do not downconvert anything... the basics is that you have to tune the VCO to the frequency you need or make the comb generator produce the spectral line (frequency) that you need to downconvert your signal with some extra hardware (mixer or sampler)