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Author Topic: Testing crystals  (Read 3364 times)
KC5HMC
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« on: April 25, 2009, 10:25:10 AM »

Hey all,
 Is there a tester for crystals that would tell you where they are oscilating?

Thanks
Herb   KC5HMC
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AA4PB
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« Reply #1 on: April 25, 2009, 11:52:45 AM »

The same crystal will oscillate on different frequencies in different oscillator circuits. You need to know what mode the crystal is to work in, what the load capacity is, etc. It's not as simple as using a multimeter to measure the resistance of a resistor.
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WX7G
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« Reply #2 on: April 25, 2009, 11:55:04 AM »

There are many ways to do this. I will assume you have no test equipment other than a receiver. I like to use a CMOS inverter to build a crystal oscillator for testing. Take a 4050, a 4011 inverter connected, or another CMOS variety inverter. Connect the crystal from the inverter input to inverter output. Parallel the crystal with a 10M ohm resistor. Place a 22 pF cap from inverter input to GND. Power it up and tune the RX to find the fundamental frequency of the oscillator.
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K7AAT
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« Reply #3 on: April 25, 2009, 11:57:25 AM »


  Such a tester is extremely easy to build.  Have you tried Google?  A quick check of  "crystal tester"  brought up all kinds of simple circuits such as found at:  http://www.geocities.com/w4jbm/xtl_test.html

  Ed   K7AAT
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N6AJR
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« Reply #4 on: April 25, 2009, 12:26:36 PM »

Some times they are written on the top of the xtal
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AC5UP
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« Reply #5 on: April 25, 2009, 06:09:36 PM »

N6AJR could be on to something.............. Wink
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AC5UP
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« Reply #6 on: April 25, 2009, 10:05:32 PM »

While we're on the subject, if I connect two 10 MHz crystals in series, would they oscillate at 20 MHz?

If I connect the same two crystals in parallel... 5 MHz?

Just Askin'.
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KC5HMC
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« Reply #7 on: April 26, 2009, 05:11:09 AM »

N6JR wrote,
Some times they are written on the top of the xtal

Yes, but I want to test them to see how close they are to spec.

AC5UP Wrote
While we're on the subject, if I connect two 10 MHz crystals in series, would they oscillate at 20 MHz?

If I connect the same two crystals in parallel... 5 MHz?

Never thought about this. Now I want to know!
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AA4PB
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« Reply #8 on: April 26, 2009, 06:43:52 AM »

Like I said, if you want to know if the crystal is accurate you have to also know the other specs and design the test oscillator accordingly. The simple test circuits people are describing will tell you if the crystal will oscillate or not, but it won't give you an accurate test.
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KL0S
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« Reply #9 on: April 26, 2009, 08:35:48 AM »

You may want to check out Doug DeMaw's "A Tester for Crystal F, Q and R" in the January 1990 issue of QST (ARRL members can download it from the new QST Archives section).  The article was updated and reprinted in Doug's "W1FB Design Notebook."

I built one a couple of years ago....you can get the PCB from FAR Circuits.  Nice addition to the test gear at the workbench.

Dino KL0S
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KM3K
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Posts: 295




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« Reply #10 on: April 26, 2009, 12:52:41 PM »

To AC5UP...
The answers are No and No.
A 10 MHz crystal, when cut for 10 MHz as its fundamental-frequency, will oscillate at ~10 MHz, and can be made to work on its 3rd (~30 MHz), 5th (~50 MHz), 7th (~70 MHz), 9th, etc overtones.
I used "~" in the previous sentence because as someone in an earlier posting wrote, the frequency you measure when testing the crystal in an oscillator circuit will depend greatly on something called the "load-capacitance", which is the over-all capacitance in series with and parallelling the crystal.
It is progressively more difficult to have an oscillator work on the higher overtones.
BTW, here is a definition of RF...."RF is when amplifiers oscillate and oscillators won't."
Incidently, take a look at Array Solutions AIM-4170 for testing crystals.
Although I have one, I have not had the need yet to test crystals.
The AIM-4170 is one mean box; I've had outstanding agreement with calibrated Agilent network-analyzers.
Hope this helps.
73
Jerry, KM3K
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VK2TIL
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Posts: 318




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« Reply #11 on: April 26, 2009, 02:49:45 PM »

",,,,,, if I connect two 10 MHz crystals in series, would they oscillate at 20 MHz?
If I connect the same two crystals in parallel... 5 MHz?".

No.  Each crystal continues to act on its own so the frequency will still be 10 MHz.

I have verified this experimentally with VNA sweeps.

Apropos the "crystal tester"; if you just want to check oscillation, a simple oscillator will do.  Such a tester has been described by W1FB and is found in older (c.1980s I think) Handbooks.

For parameter testing, the G3UUR tester is described in EMRFD and here;

http://w7zoi.net/G3uuralator.pdf

Don't be put-off by the maths; it's really quite simple to build & use.

For an overview of crystal-parameter testing, see this excellent paper;

http://www.cliftonlaboratories.com/Documents/Crystal%20Motional%20Parameters.pdf

It includes the G3UUR method.
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AC5UP
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« Reply #12 on: April 26, 2009, 03:25:23 PM »

Now we know why none of us are making a living writing technical humor... No market for it. Wink
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VK2TIL
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« Reply #13 on: April 26, 2009, 04:18:20 PM »

Like John Steinbeck's character "Doc", most of us assume that anyone who asks a question really wants to know the answer.
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WA3SKN
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« Reply #14 on: April 27, 2009, 07:53:54 AM »

A "Pierce" oscillator is a fairly simple build.  However, it does stress the crystal.  But it will probably determine whether the crystal is "cracked" or not... that is, whether the crystal is good.
As for two crystals in series... forget it! They are tuned ciruits. There is a lot to crystals and the handbook covers it very well, so I recommend reading it.
73s.

-Mike.
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