Testing crystals

[Terry]

"The manufacturer of the second crystal in the last table I posted, has marked the crystal as 4463.619 kHz, yet when tested in the circuit I made, it was oscillating at 10071.5 kHz. Why, or how, would a crystals properties change so that it would start oscillating at about 3 times its original frequency."

Actually, it was marked 3373kHz but I see what you are saying. However, if there was something in your test set-up to encourage it to oscillate at the third harmonic, it wouldn't be nearly 1½% out, so some other mechanism is at play here. I've already mentioned over-driving the crystal - note also Lawrence's comment - which could cause all sorts of problems.

Think of the crystal as a clock pendulum. It looses energy so you give just enough of a kick on each cycle to compensate for it and it will swing at a constant speed for ever. What would happen if you kept giving it a hefty swipe instead? A crystal doesn't like it, either!

As I keep saying, try a different test circuit and, for MW, include a tuned circuit. Provided you haven't caused any lasting damage, you could end up with a completely different result.

As for the 21.8MHz crystal, someone's been telling porky pies! No way will it ever oscillate below 1605kHz!

[Murphyv310]

What I want to understand is how to get the crystals to give out lower harmonics, so that they can be used as the manufacturer intended.


Mike

Mike.
The manufacturer would never use lower harmonics. A third overtone crystal is basically the third harmonic up.... of the fundamental or what the crystal is cut to.... never below.
If you Google "Crystal oscillator Sub harmonics" you should get plenty of info. I already stated that sub harmonics are very low indeed and in fact not a desirable phenomenon. They often are caused by the inaccuracies of manufacture. If you have a crystal with say for arguments sake 5 MHz it should oscillate at that frequency, you should be able to check it with a frequency counter without any tuning, conversely a crystal marked 15 MHz which is a third overtone 5 MHz crystal, will require a bandpass 15 MHz tuned circuit to operate correctly.

There is something massively amiss in your tests Mike as they don't make any logical sense. Mind crystals can be ruined by excess voltage across them unless they are specifically designed for that scenario.

[Murphyv310]

But being as I cant get either to oscillate reliably in my circuit I wonder if he was making it all up. I guess they need to be tried in a circuit like Trevor's pantry TX to be sure.

Mike

Hi Mike.
Those crystals are well by their use by date. I've a bucket full of them and due to the fact they are ancient and unused they are basically "dead". All the stripping and hitting them with a hammer is a waste of time, don't ask me why as I haven't a scooby.
You could try one of the crystal suppliers but they don't usually oblige making 9 KHz spaced MW crystals.

[Murphyv310]

Hi.
Refugees Idea with a generator and scope works. Personally I'd build a valve testing circuit as older crystals have more of a chance to run with valves than transistors.

[pwdrive]

The 'scope idea I linked to does work, I remember checking colour sub carrier xtals back in the day.

I've improved the odd xtal in the past by cleaning them with meths if they are of the strip down design, one in an HRO receiver was, I think, the last one I did.

Lawrence.

[Crackle]

Hi Mike.
Those crystals are well by their use by date. I've a bucket full of them and due to the fact they are ancient and unused they are basically "dead". All the stripping and hitting them with a hammer is a waste of time, don't ask me why as I haven't a scooby.
You could try one of the crystal suppliers but they don't usually oblige making 9 KHz spaced MW crystals.

I threw a dead crystal onto the floor, it made a nice rattling sound afterwards.

Trevor, I know the results of my tests were strange, hence the discussion here, but they are a true record of what happened as I saw it.
I will try and have a go with the scope method.

cheers
Mike

[Crackle]

The video shows how to do it with an external signal generator and a fast scope.
A dip in the scope waveform will be seen for a series tuned crystal and a dip in the waveform will show a parallel tuned crystal.

Ref
I was a bit confused by the last sentence in the quote above.

cheers guys, thanks for your input.

Mike

[Refugee]

It is easier to see if you try it.
A series tuned crystal is almost open circuit to all but its frequency just like a coil and capacitor connected in series. The capacitor is high impedance to all frequencies below resonance. Go above resonance and the coil becomes high impedance.
With a parallel tuned crystal the coil and capacitor do the same thing but in parallel so they almost short out all but the tuned frequency.

The can with the frequency printed on the outside contains a very special tuned circuit that is like a pair of capacitor plates with a glassy dielectric between them that works like the coil.

[Crackle]

Hi Ref
So should your statement have read "A peak in the scope waveform will be seen for a series tuned crystal and a dip in the waveform will show a parallel tuned crystal.
I will study the youtube video all the way through it may help.
thanks
Mike

[pwdrive]

It depends on how you connect things up, if the xtal is connected in series between the source and the 'scope then there will be a peak on the 'scope at the xtal series resonance and a notch at it's parallel resonance, it's just like any series or parallel tuned circuit except that there is no "real" inductor or capacitor or resistor inside the crystals structure, eg: L,C and R as represented within the xtal are motional only.

Couple of video's here that will explain the basics of xtals:

https://www.youtube.com/watch?v=HGKn5TYGe_Q

https://www.youtube.com/watch?v=lQI-3tl4wDk

Lawrence.