On Sun, 12 Dec 2010, Stuart Longland VK4MSL wrote:

Hate to hijack a thread …

On Dec 6, 2:55*am, highlandham wrote:
With your tuneable range ,for 10.100-10.150 MHz you can use any crystal
in the 18.495 to 18.985 MHz range ,probably even a low cost
microprocessor type.

Out of curiosity, what's the difference between the "microprocessor"
type crystals and other crystals? Is it just frequency stability or
is there something more fundamental?

Crystals traditionally were not commodity items. They were ground
on demand. Rare exceptions would be frequencies so common that
it wouldn't be a waste to make them ahead of time, so 100KHz
crystals were pretty standard.

WWII kind of spoiled people, because there was so much surplus
afterwards that for a long time, one could get a crystal "off the shelf"
because it existed as military surplus. You'd either live with a crystal
"close enough" or open it up and grind it so it fit. All those surplus
crystals allowed for people to make crystal filters when single sideband
became popular in the fifties, a lot of VHF work was able to use surplus
crystals in the 8 or 6MHz range (when the transmitters would be a string
of multipliers to get up to the desired frequency), there was even
enough that worked on the HF ham bands.

When 2M FM came along, that was a shock. Suddenly "close enough" wasn't,
since it was channelized, so there was an illusion that one suddenly
needed to have crystals ground to frequency, when that had been the
case all along. It didn't take many years before that got old, which is
when frequency synthesizers really took off in amateur radio.

An odd exception seemed to be CB crystals, where you often could buy off
the shelf, or at least on a very short wait. But they too were ground
just like all the rest, only the cb set's manufacturer saw there'd be
demand so they built up stock.

It was the rise of digital circuits that caused the growth of commodity
crystals. ICs would be designed to use a specific frequency, that often
looked pretty odd until you divided it down, and these ICs were used
enough that the crystals became available off the shelf. Some companies
tried to make use of existing commodity crystals, such as the very common
color subcarrier frequency 3.58MHz, but that didn't always work.
Initially it was a fairly small set of commodity crystals, but more than
the 100KHz and a few other frequencies seen before. "Microprocesser
crystal" is probably a misnomer, since they could often use whatever
frequency was available, but other things needed very specific things.
The variety of crystals multiplied as new ICs and gadgets came along,
until there was quite a few.

Commodity crystals likely are lower spec'd, certainly you have to make
the circuit work with the crystal rather than have a circuit and spend
the money to have a crystal ground to fit that circuit.

But the real reason they are cheap is they are mass-produced, because
the demand is there. So long as you can make do with what other people
want or need, then you can get cheap crystals. It's not unlike the
decades after WWII when there were all those surplus crystals, you lived
with what there was (but there were so many, it often wasn't a hardship).

Michael VE2BVW