How to calculate the crystal Value in Oscillators?

[Shahriar]

Hi
In many crystal Tx oscillators, I always see that if the oscillator frequency is for example 98Mhz, the crystal
value is sth around 12MHz.(a little bit more or less).
I have heard sth. like second or third harmonic but do not know anything exact.
My questions are:
1- how to calculate the crystal value...
2- when the oscillator has a common-base Amplifier the crystal is placed in base(or Gate if mosfet). WHY?

ThanX in advance
Shahriar

[prateeksikka]

2- when the oscillator has a common-base Amplifier the crystal is placed in base(or Gate if mosfet). WHY?

it is connected to base because we can exploit transistors high value of beta(B)
to use it as a good amplifier only in this configuration.
integral multiples and not same value is used to avoid frequency distortion which may push it away from resonance

[Shahriar]

Dear prateeksikka

Thank you very much for your answer but
I found a Application note which contains answers. (I attached it)
The reason is: "The practical frequency range for Fundamental mode
AT-cut crystals is 600 kHz to 30 MHz. Crystals for fundamental
frequencies higher than 30 to 40 MHz are
very thin and therefore fragile. Crystals are used at
higher frequencies by operation at odd harmonics
(overtones) of the fundamental frequency. Ninth overtone
crystals are used up to about 200 MHz, the practical
upper limit of crystal oscillators" {Copied from this PDF}


ThanX again
Shahriar

[prateeksikka]

but this does not explain that why do we need higher frequency or nth harmonics of the fundamental than what is required.
;D i.e your own question why dont we adopt a crystal of same value and why of a higher one?

[Shahriar]

Sorry, I forgot to attach the PDF..

Building Crystals for High frequencies is difficult and Expensive. For example if You want  to build a
150Mhz Oscillator, you will never find such Crystal, you should use a 30MHz Crystal and design a
5th Overtone Oscillator.
Everything is explained in PDF...

ThanX
Shahriar

00826a.pdf