Need to explain about principle of comparator based crystal oscillating circuit.

Thread Starter

sun0727

Joined Aug 2, 2020
2
Hi everyone,

I am a newbie in electronic field. Now I am trying to design a circuit to drive QCM (Quartz crystal microbalance). In some literature I have seen some comparator-based designs like this:

Xnip2020-08-05_22-46-03.jpg

In fact, some similar designs can also be found in the datasheets of high-speed comparators like LT1671 , LT1720

LT1671-4147.png LT1720-4217.png

It looks just like a relaxation oscillator with a Crystal on it. I have learned the basic principle of a relaxation oscillator from hier http://www.analogzoo.com/2015/01/relaxation-oscillator-design/

But what I don't quite understand right now is:

- How does quartz help the circuit to produce stable frequency?
- How to choose the value of C1 and R1, for example, I put a Quartz with a Base-Frequency of 10 MHz on it?


Thank you in advanced, and I am looking forward feedback from you all.

Best regards
 

Papabravo

Joined Feb 24, 2006
14,205
The answer to your question is that a piece of quartz behaves like a circuit with inductance an capacitance and resistance. Something like the following:

QZS100K.png

L1, C1, and R1 are called the "motional" parameters of the crystal, The values are chosen so that L1 & C1 are series resonant at the desired frequency. 100 kHz. in this case. The C2 capacitance models the mounting of the slice of quartz -- that is it is an actual capacitor inside the crystal package.

The crystal acts a a bandpass filter, passing signals over a narrow range of frequencies. The RC just has to get you close for startup.
The RC is also providing some low pass filtering to prevent the crystal from operating at one of it's odd harmonics (just a guess - I haven't looked closely at it)
 
Last edited:

Thread Starter

sun0727

Joined Aug 2, 2020
2
The answer to your question is that a piece of quartz behaves like a circuit with inductance an capacitance and resistance. Something like the following:

View attachment 214078

L1, C1, and R1 are called the "motional" parameters of the crystal, The values are chosen so that L1 & C1 are series resonant at the desired frequency. 100 kHz. in this case. The C2 capacitance models the mounting of the slice of quartz -- that is it is an actual capacitor inside the crystal package.

The crystal acts a a bandpass filter, passing signals over a narrow range of frequencies. The RC just has to get you close for startup.
The RC is also providing some low pass filtering to prevent the crystal from operating at one of it's odd harmonics (just a guess - I haven't looked closely at it)
Thanks for your reply, could you talk more about the R1-C1 part.

I have just simulated this circuit, for example I put a 10MHz crystal in the circuit and when I adjust R1 in a wide range (R1 = {2k, 20k, 47k}, C1 = 15pF), I find that the output frequency also changes drastically. I don't quite understand how their values should be chosen in an actual circuit.
 
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