What do you mean by open loop vco modulation.

as the vctl drifts, the VCO frequency changes.

Or are you talking about how much VCO frequency changes with Process.

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

Quote:
Yeah, Some for of 0/1 Padding OR Frequency Hopping OR Spread Spectrum Teechniques would be required; I Suppose.

References :--

http://www.google.co.in/url?sa=t&source=web&cd=4&ved=0CCUQFjAD&url=http%3A%2F%2F...

http://www.google.co.in/url?sa=t&source=web&cd=3&ved=0CCAQFjAC&url=http%3A%2F%2F...

& many more on ieeexplore.

i am currently not working on this. BUT do suggest me if you find any good reading material on this.

--
Mayank.

for a VCO the time varying signal on the varactor will have the following equation
Vout(t)=A*sin(wt+kv*integ(x(t)))

which coincidentally is the same for FM where kv is replaced by "m" for modulation index.

With that in mind wide band fm power spectral density inherently follows a bessel functions of the first kind which are probably solved somewhere on the internet. Where amplitude of x(t) and KV determine the bandwidth and spectral content.  I believe this spectral leakage in the transmitter will ultimately be the limiting factor in ACPR and thus be the limitation of the circuit.

http://rfcooltools.com