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Author Topic: Plans for a crystal calibrator?  (Read 27666 times)
KA0HVE
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Posts: 117




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« on: September 19, 2013, 06:35:33 AM »

I'm just beginning to build stuff for my station.  So far I've built a 10 watt dummy load and an OHR WM-2.  A crystal calibrator for a future xcvr would be nice to build now.  Should I just grab the plans out of an ARRL publication and build it?  Any better plans available elsewhere?

What's a good source for crystals and sockets?
« Last Edit: September 19, 2013, 07:26:19 AM by KA0HVE » Logged
KA4POL
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Posts: 1978




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« Reply #1 on: September 19, 2013, 07:43:05 AM »

It depends on what you plan to do. There are frequency standard stations with more than good calibration frequencies. nowadays much is done with DDS chips.
I have good experience with the ARRL handbook circuits.
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WX7G
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Posts: 6038




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« Reply #2 on: September 19, 2013, 07:48:09 AM »

Digikey stocks crystals and crystal oscillators.
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WB6BYU
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Posts: 13242




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« Reply #3 on: September 19, 2013, 01:03:39 PM »

The first step (as always) is to decide what features you need.

It's easy to build a simple oscillator to find the 100kHz points on the dial.  The better
designs provide for additional division down to 50, 25, 10 or 5 kHz as well.  (It is better
to have switchable steps rather than, say, fixed at 10kHz intervals, as that allows you
to figure out the exact frequency of each marker.)

To some extent your needs will depend on the dial calibration of your rig:  when the
dial is pretty linear and covers enough of a range, just having 100kHz points is usually
good enough.  But if your rig only covers, say, 7.015 to 7.045, then even 50kHz points
won't help you any.  And if you want to be able to tune to 7.040kHz, then having both
50kHz points (to find 7.050) and 10kHz points (so you can tune down one marker to
7.040) will come in handy.

Sometimes having the markers pulsed (or at least the option for it) will make them easier
to find among other signals.


Probably the most important step is to find a design that uses currently available ICs, and
draws little enough current that you can run it on batteries.  The standard CMOS CD4xxx
(or MC14xxx) series will run directly from a 9V battery, while the 74HCxx types should run
off a pair of penlight cells.  TTL logic types (74xx, 74LSxx, etc.) require a 5.0V regulator
and usually draw more current, but still will be better than older designs that use something
like the MC9xx or other logic families that are pretty much obsolete.

The basic circuit is simple enough:  one logic gate serves as a crystal oscillator, perhaps
some other spares pressed into service as buffers, then a couple programmer counter/divider
chips to divide by some combination of 2, 5 or 10 as needed.  An extra gate (or 3) can be used
as an RC oscillator to provide an optional pulsed output.  If you just want a fixed frequency
crystal without the switched steps (which might use an in-band crystal rather than 100kHz)
then a single FET or bipolar transistor should do the job, though I find the CMOS logic gate
crystal oscillators are quite simple to build, so sometimes use them instead unless I need to
make the circuit physically small.
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G3RZP
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« Reply #4 on: September 20, 2013, 03:34:55 AM »

My first thought on reading the question was a 1 MHz crystal, a 6SJ7 and a pair of 6SN7 multivibrators to get to 10kHz......

How much easier it is with a couple of HC7490s or equivalent!
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K6AER
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Posts: 3515




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« Reply #5 on: September 25, 2013, 08:03:38 AM »

Crystal calibrators were a great item in the day of old tube radios.

Today's radios are much more accurate (by 100 times) than the crystal calibrator will be over normal temperature usage. Checking you radio calibration against WWV (10 MHz) is more than accurate for normal operation.
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WB6BYU
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Posts: 13242




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« Reply #6 on: September 27, 2013, 01:05:11 PM »

But for a simple homemade QRP rig that tunes 6.95 to 7.15 MHz with a variable capacitor
rather than a digital dial, a crystal calibrator can be a big help in making sure that the
calibration is still on, as well as finding the Extra subband limit at 7.025 MHz (assuming
you can generate 25kHz markers.)

If you have a reasonably accurate digital display, however, there often is no need for
a crystal calibrator.
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AA4PB
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Posts: 12836




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« Reply #7 on: September 27, 2013, 03:37:44 PM »

Many of the newer radios use either DDS or PLL which is locked to a reference crystal therefore they are normally as accurate as the crystal calibrator.
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KA0HVE
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Posts: 117




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« Reply #8 on: September 30, 2013, 01:05:03 PM »

I use older equipment from around 1980 with analog dials.  Plus, any homebrewing I do may not necessarily have digital displays.
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N3KXZ
Member

Posts: 77




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« Reply #9 on: November 10, 2013, 08:21:05 PM »

Search Google with the terms "crystal marker generator" and you will get lots of possible sources. Also, when you determine which crystal you need you might PM me- I may have a few crystals I can give you. Personally, I wouldn't worry too much about sockets.

Keith Ostertag
N3KXZ
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