# Crystal oscillator impedance

Discussion in 'The Projects Forum' started by simo_x, May 12, 2011.

1. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Hi all, I have this crystal oscillator

which works fine.
The circuit is taken from the TX-2B datasheet at page 9 first circuit.
The oscillator works fine, but I am interested in calculate his total impedance.
If I am not wrong, in a first moment the crystal is pushed by the current coming from R1 and then is pulled back by the L1C2 network.

As I can see from the crystal's datasheet used, between 27 & 30Mhz in 3rd overtone the ESR of the crystal is about 150 Ohm.

If the 27MHz frequency is applied to L1 & C1, I obtain the following impedance,

XL = 2π · 27MHz · 2,2uH = 373,22 Ohm
XC = 1 / [ 2π · 27MHz · 80pF ] = 73,68 Ohm

and adding the Crystal ESR, I obtain the following resistive network

but I am not sure it is correct.. Maybe (or off course) the situation is more complex and, as I expect, I am out of way.
Can you help me to determine the total circuit impedance please?

The real circuit, with power supply of approximately 9.19V consumes about 29.2 mA.

Regards,
Simon

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2. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Hi, someone can help me here, please?
Thank You.
Simo

3. ### Wendy Moderator

Mar 24, 2008
21,341
2,918
Truth, I've built my share of crystal oscillators, and have never worried about it. If you want isolation put a CMOS gate between the oscillator and the load. The imput of the gate is extremely high impedance, the ouput medium to low impedance.

Why is this an issue?

4. ### simo_x Thread Starter Member

Dec 23, 2010
200
6
Hi Bill, it is my interest determine the impedance of the oscillator to understand better how it works..
Regards,
Simo