Testing crystals

[Pat Pending]

Mike.

If the crystals are very old they may well be quite away from their marked frequencies, for your purpose that may not be a problem and a trimmer capacitor can be used to trim the frequency by a small degree. What circuit you use will depend on whether the crystals you are testing are series or parallel resonant, parallel resonant crystals have a high impedance at resonance and can be tested with a Pierce oscillator, a series mode crystal have a low impedance at resonance, and are more suitable for use with a colpitts oscillator.

Andy.

[Terry]

My initial thoughts were, did it matter too much if the fundamental frequency was not a multiple of 9, so long as one was not using a synthesised tuner which has the channel separation fixed.

Nothing to do with synthesised tuners, Mike.

Let's say you have a crystal which is 1kHz off a multiple of 9. You set up your little transmitter and all is fine. But there is a 200kW transmitter on the authorised channel. You are well beyond the ground wave coverage, so not a problem - until night time, when you start to receive the sky wave.

Your receivers will now see two signals and mix them together to generate the sum and difference products: f1 + f2 and f1 - f2.

The sum, we aren't interested in, but the difference, 1kHz will appear in the output of your receiver(s) in addition to the programme material you wish to listen to.

Obviously, being an exact multiple of 9 is not all that is important as it mustn't be a channel used by a local transmitter!

Crystal harmonics are perfectly OK. In which case is the multiple that should be a multiple of 9.

[Pat Pending]

Mike.

If the crystals are very old they may well be quite away from their marked frequencies, for your purpose that may not be a problem and a trimmer capacitor can be used to trim the frequency by a small degree. What circuit you use will depend on whether the crystals you are testing are series or parallel resonant, parallel resonant crystals have a high impedance at resonance and can be tested with a Pierce oscillator, a series mode crystal have a low impedance at resonance, and are more suitable for use with a colpitts oscillator.

Andy.

A couple of circuit ideas

[Crackle]

Hi Andy
Thanks for the circuits, they give me something to work with.
What sort of markings would you expect to see on a crystal to indicate if it was series or parallel?

thanks
Mike

[Murphyv310]

Hi.
Both of Andy's circuits should test the majority of crystals. In addition though I'd say a lot of older crystals that have been lying unused for many years may not work, I've no idea why this phenomenon occurs, some of the ones I have are the big chunky ones that come apart, they have springs to hold the contact onto the crystal and look 100% perfect but wont respond on any circuit valve or semiconductor.

If you are intending to replicate my micro TX project then most of the more up to date crystals will work fine, a series fed crystal being the right choice.
I also have one of the bangood frequency counters which has the facility to test crystals and it gives the fundamental frequency of the crystal on the display, again it's fussy and prefers relatively modern crystals.

[Mark Hennessy]

The PDF I posted in #5 contains working circuits, complete with component values.

To answer the questions regarding series/parallel, the Wikipedia article is a good start: https://en.wikipedia.org/wiki/Crystal_os...ance_modes - that link jumps to the section regarding series/parallel modes, but the rest of the article is worth perusing.

Don't forget you can build these with CMOS inverters, preferably the ones with Schmitt inputs. E.g. the 40106B or 74HC14. For example: http://qrp-popcorn.blogspot.co.uk/2016/0...lator.html

To be honest, much as I like using transistors whenever I can, I wouldn't mess about with them here unless there was a really good reason to do so. The 40106B will run from 3V up to 18V, so is suited to battery operation. The ICs are dead-cheap if you haven't already got them: http://cpc.farnell.com/texas-instruments...dp/SC12812

[Terry]

... a lot of older crystals that have been lying unused for many years may not work, I've no idea why this phenomenon occurs, some of the ones I have are the big chunky ones that come apart, they have springs to hold the contact onto the crystal and look 100% perfect but wont respond on any circuit valve or semiconductor ...

I remember those! Massive things!

I recall a conversation with a man who had visited a shop in Lisle Street at the tail end of the post war government surplus boom where he found large quantities of these crystals - all individually tested, it was claimed.

He selected the ones he wanted and took them to the counter - it was at this point that the 'individual testing' took place.

Each crystal was plugged into an oscillator to check for activity - most of his selection proving to be very inactive!

No problem, said the vendor, and he proceeded to hurl them onto the floor with considerable force! He calmly picked them up again and plugged them into his tester - every single one now worked!

So, it might work for you, Trevor - you never know!

[Murphyv310]

Well worth a try Terry, I've nowt to loose.

[pwdrive]

There's also a good article on Xtal basics here:

http://www.bliley.net/XTAL/docs/eng_bull...lletin.pdf

Lawrence.

[radioman]

You might find this useful as well :-
  tech-notes.pdf (Size: 252.92 KB / Downloads: 4)

Andy