Oscillator drift

[walid]

I read in a pdf file what appears in fig.oscillator drift below.
as you see there is 6 points to take into account to make the osc. stable, I'll
concentrate on the first one labled (a); the c/L ratio.
what I want is to explain it numerically for FM 100MHz.
thanks.

[walid]

The next thing is; it is not usual to make an oscillator on 100MHZ,

Why you may don't hear about FM Tx osc.

your answer is bad, but thank u

[Guest Alun]

The large L to C ratio means using a larger capacitor and smaller inductor rather than the other way round, the reason for this is because capacitors are more perfect (i.e. they have a higher Q than inductors.

[walid]

Hi Alun
If this is a VCO used in a FM  Tx transmitt at 100MHz,
the tank circuit contains these C and L,
tell me an example of their values.
thank you.

[Guest Alun]

At the bottom are all the formulae you'll need, now you can do some calculations.
Keep F = 100MHz.
Now calculate C for the following values of L:
120nH
100nH
80nH
60nH

You'll see that if you want to reduce the value of L while keeping the frequency at 100MHz you'll need to increase C. Now the value of C you calculated for L = 60nH will be bigger than it was for l=120nH, i.e. the LC circuit would have a higher C to L ratio. Capacitors are more perfect than inductors because they have a higher Q and have low parasitics and this is why large C to L ratios are prefered.

[audioguru]

If you are calculating my FM transmitter's oscillator frequency, don't forget to add about 10pF stray capacitance to represent the pcb wiring capacitance, the transistor's collector capacitance and a contribution of the feedback capacitor.

If the value of my 0.1uH inductor is correct, then the oscillator in my FM transmitter should tune 80MHz to 160MHz with 10pF of stray capacitance added to its 5pF to 35pF trimcap. No wonder the trimcap's adjustment is so sensitive. I should add some capacitance to reduce the frequency range and reduce the inductance so it tunes only the 88MHz to 108MHz FM band. ;D

Hey! I just looked up through my skylight and saw a hawk circling very high, probably looking for my little white dog as a tasty snack. Luckily my dog is on my lap. 

[Guest Alun]

If you are calculating my FM transmitter's oscillator frequency, don't forget to add about 10pF stray capacitance to represent the pcb wiring capacitance, the transistor's collector capacitance and a contribution of the feedback capacitor.

Damn, I should've sent you a pm telling you not to say this, I wanted Walid to do the calculations and wonder why the capacitance value is smaller, then ask about it on the forum or hopefully figure it out for himself, but you've ruined it audioguru. :'( You're just too good and the trouble is don't you know it. ;D

[audioguru]

I remember somebody in another post asking why a sim or calculation has the max oscillator's frequency as high as 225MHz. Nobody knows about stray capacitance.