Crystal questions

[zachtheterrible]

I've been wondering about crystals. Not much information in my '99 ARRL handbook (unless i missed it). Or if someone could point me to a good tutorial on crystals that would be great.

Here's what I'm wondering.

1. How easy is it to replace an LC circuit with a crystal? Would one need to make changes to the circuit in order to allow a crystal, or is it not very hard? The point of this question is to see if a crystal is basically like an LC circuit.
2. What is meant by a crystal reference?
3. How does one take for example a 1MHZ crystal and use it to make 3MHZ?
4. If I remember right, a series LC circuit will only let past a certain frequency, and a parallel LC circuit will let past everything accept a certain frequency (did I get that part right?). So does a crystal behave as a series or parallel LC circuit when used as a filter? Or can a crystal not be used as a filter?

Thanx for the info :wink:

 

[Russlk]

1. The equivalent circuit of a crystal is a very large inductance with series and parallel capacitance. A crystal has both series and parallel resonant modes, usually close together in frequency, but some are optimized for parallel or series operation. Since the crystal is like a tuned circuit, you can put capacitance in parallel with a parallel resonant crystal and change its frequency (a little bit). Parallel resonant crystals always have a shunt capacitance specified that will put it at the specified frequency. You can make a Colpits oscillator using a crystal but it would be too dificult to tap the crystal to make a Hartley oscillator.

2. Older signal generators, using LC oscillators, have a crystal oscillator that is rich in harmonics. The operator could zero-beat the LC oscillator with a harmonic of the crystal for calibration.

3. A stable oscillator generates sine waves. If you distort the sine wave (clip it) harmonics are generated. You amplify the desired harmonic. Since a square wave has only even harmonics, you would want an asymetric wave for the 3rd harmonic.

4. An LC circuit (parallel resonant) is high impedance, so if it is in parallel with the load, the frequency passes, if it is in series with the load the frequency is attenuated. The series LC circuit is low impedance, so if it is in parallel with the load the frequency bypasses the load and if it is in series with the load the frequency passes.

Narrow band filters are made with crystals, the crystals are used like tuned circuits.

 

[pike]

To answer question 3, I would say you would need a PLL phase lock loop.

This really is just a VCO (voltage controlled oscillator). the crystal is there to compare the frequency, and a phases detector (which does the comparing) calibrates the voltage on the oscillator so the crystal to VCO frequency ratio always stays the same.

knowing you, are you trying to build an FM transmitter or an FM reciever??

 

[David Bridgen]

2. What is meant by a crystal reference?

A crystal reference (oscillator) gives a frequency against which another may be compared.
Russ has given one example.
Another example of its use is in a phase-locked loop.

3. How does one take for example a 1MHZ crystal and use it to make 3MHZ?

Some crystals are used in what is called an overtone oscillator. An overtone is a frequency at an approximate, not exact, multiple of the fundamental.

 

[zachtheterrible]

Hey, thanx everyone, that was very helpful!

Parallel resonant crystals always have a shunt capacitance specified that will put it at the specified frequency

I am not very sure what you mean by this russ. I found a model of a RLC model of a crystal. Could you draw it in?

Hee hee pike, you know me too well :lol: . Actually I wasn't even thinking of building anything! I was just curious. hmmm, but now that you mention it . . . I imagine it would be pretty simple to replace the LC in my bug with a crystal to get a stable frequency and a higher Q. That would be very cool, but how hard is it?

Thank you so much everyone!

 

[Russlk]

This is a crystal oscillator, it is a Colpitts circuit. The 30 pF and 220 pF caps are in parallel with the crystal. The capacitance is probably a little more than the specified amount, but it is close in most cases.

 

[JimB]

Zack

Look here:

www.quartslab.demon.co.uk

They have a LOT of info about crystals and oscillator circuits.

In answer to your questions:

1 Depending on the circuit, the LC tuned circuit can be directly replaced by a crystal. If you are thinking of doing this with one of your el-cheapo bug transmitters, the answer is a reserved no.

2 A "crystal reference" is just that the circuit or system uses a crystal as a frequency (or time) standard.

3 The simplest way (as said elsewhere), is to drive an amplifier into distortion (distortion = harmonics) and pick off the required harmonic using a tuned circuit.

4 Crystals are often used as filters, either individual crystals, or, built up into a unit with well defined characteristics.
Crystal filters also tend to be expensive.

Crystals can be used in a series resonant or parallel resonant mode.
The series and parallel resonant frequencies differ by a few hz.

JimB

 

[zachtheterrible]

If you are thinking of doing this with one of your el-cheapo bug transmitters, the answer is a reserved no.

figured as much :roll: otherwise im sure it would have been done.

I forgot to upload the crystal model that i found (its not the first time), but from what you said russ, it is just capacitance used to change the frequency a little bit?