Fundamental trap and 3rd overtone of crystal

Hello, I am quite confused about using the fundamental trap to remove fundamental mode oscillation. I use sa612 and in datasheet application circuit the advised values to remove fundamental frequency are 1nF and 0.5 to 1.3 uH. This values remove the frequency of 4.4 to 7.1 MHz.

However the proposed crystal is 44.545 MHz in 3rd overtone. Does not that mean the fundamental frequency must be 44.545 / 3 = about 14.8 MHz, so frequency around 14.8 MHz has to be removed, isn't it? I am using 48.9 MHz, so the fundamental trap should be at around 16.3 MHz. If that's not the case, then how do I calculate the values needed for LC in series?

if that 1nF was .1nF it would make more sense.
Frank

The 1nF cap is just a DC blocking capacitor.
The series 22pF cap connected to the ground is the capacitor part of the resonant circuit.
For example: 0.58uH with 22pF resonates on 44.54MHz

Ok, that make sense now. So if I use 48.9 MHz, and this resonant circuit is acting as bandpass, therefore I can use something like 330nH and 32 pF to pass 48.9 MHz, is that correct?

Yes, it will resonate on 48.9MHz. For tuning is good to have at least one component variable (inductor or trim capacitor).

Ok, thanks a lot. I have a variable inductor, so I will make use of that.

the oscillator circuit needs the right phase shift between pins 6 and 7 to work. That 22 pf cap at pin 7 does some magic to let that circuit in the schematic work at the 3rd overtone and not the fundamental. So if you are at 45 mhz, there is a good chance just varying the 10 pf and 22 pf caps will get it to come up at 45 mhz every time, instead of 45/3.

If you are having real trouble you could play around with an L-C network. Maybe a series L-C resonating at 45, in series between the crystal and ground.

Looking at the simplified schematic, the oscillator is a colpitts using the 10p and 22p capacitors. The 0.5uH - 1.3uH inductor is across the 22p cap, and its purpose is to make the capacitor look inductive at the fundamental frequency you are trying to suppress, as if you replace one of the caps with an inductor the circuit will not oscillate. The inductor must be big enough so that the parallel combination with the 22p is still capacitive at the frequency you want.

So to operate at 45Mhz the inductor should resonate with the 22p cap somewhere between 15MHz and 45MHz. If you set it at 30MHz using 1.3uH then the parallel combination of the 22p and 1.3uH looks like about 12pF at 45MHz and should be a good starting point.

This is explained here and **broken link removed**