Mayank
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Hi aaron_do, Nice & Simple Idea, BUT
IF YOU DO NOT USE A PLL, INSTEAD JUST AN OSCILLATOR
1. VCO frequency drifts heavily with changes in Process & Temperature. a.> For Process, you would need sm kind of trimming. b.> For controlling Temperature Drift, you would need some Bias cktry designed so as to compensate temp effects.
2. Even after doing the above mentioned stuff, You can never be phase aligned in Open-Loop Process. So only FSK & no kind of PSK/OFDM/phase sensitive modulations can be applied.
3. Even if you compensate point no. 1, there will still remain certain drifts in freq with PVT which you need to put a spec on i.e. you need to calculate the limit of unwanted freq variations that your FSK can tolerate.
IF YOU USE GENERATE operating pt from PLL & PUMP SIGNAL INTO VCO VARACTORS;
1. When you will change your signal bits, VCO freq will vary immediately. This kind of variation will compete against VCO's single Period Jitter. You gotta take care of that.
2. PLL will try to negate the effect of Signal that you apply to the varactor. So if your signal is DC (i.e. it sits at a particular value & doesnt change for a long time), then even though you would expect your VCO Freq. output to be Fop_point + ( some gain value * your signal i/p ) , BUT over a long time, PLL will stabilize the vco freq to PLL's op. freq. So you cant see that output.
-- Mayank
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