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Basic Information On Temperature Compensation and Modulation Capability
for Channel Elements, ICOMs, and TCXOs
Plus some tips on ordering crystals
Compiled from contributions from Eric Lemmon WB6FLY, Jeff DePolo WN3A, Robert Meister WA1MIK, Doug Marston WB6JCD, Jeff Kincaid W6JK Neil McKie WA6KLA, Kevin Custer W3KKC, and Mike Morris WA6ILQ
Edited and HTML'd by Mike Morris WA6ILQ
This page deals with the subject of replacing the crystal in any type of Temperature Compensated Crystal Oscillator (TCXO), be it a Motorola "Channel Element", a GE "Integrated Circuit Oscillator Module" (commonly called an ICOM), or an RCA TCXO. Here we're going to use the term "element" just to make writing this article easier. We're going to describe the problems that can arise by rebuilding them yourself. Most hams are frugal (some are downright cheap) by nature, always wanting to save a nickel anywhere they can. Replacing the crystal in the precision frequency determining element is a necessary task when taking a crystal controlled radio and moving it from one frequency to another. Money can be saved by recrystaling the elements yourself, but there are problems that can arise when doing so. Many folks don't know about the issues when buying a $10-$15 crystal and stuffing it into the element themselves. Sometimes the issues are with the crystal itself, sometimes they are with the element, and lastly, both can contribute to the end result.
One comment I have heard over and over again - places like Sentry and International do not just recrystal an element, they remanufacture it. Unfortunately, the crystal houses and several manufacturers manuals refer to the remanufacturing process as "compensating" the element despite the fact that temperature compensation is only one factor in the remanufacturing process.
A very perceptive comment on rebuilding elements, from an email to Repeater-Builder from Eric Lemmon WB6FLY:
...There still seems to be a tendency to seize upon temperature compensation as the primary reason for sending the element to a crystal house, and that is not the case. In fact, the temperature compensation is not the most important issue at hand.
I believe that "compensation" of a crystal in an element comprises a number of actions:
It should be obvious that few, if any, of the above actions can be performed on a crystal by itself, since most of the components that directly affect the operation of the crystal are not in the crystal. In other words, whether or not the radio is in a temperature-controlled environment is hardly the point; it is more important that the crystal perform properly in the oscillator circuit. That's hard to verify if you don't send the oscillator circuit to the crystal house!
Notes on Temperature Compensation:
The first thing to realize is that the crystal frequency varies with ambient temperature. Period. It's a simple law of physics. The frequency the crystal oscillates at is also determined by the circuitry around it, and the values of the components in that circuitry. The crystal manufacturers even have a name for this characteristic - you will see references to a crystal "operating into a 32pF load" (or some other number). If the load value changes, the frequency changes. The method used in temperature compensating a crystal is to deliberately use other temperature sensitive oscillator components to vary the capacitance across the crystal and the load on it, thereby counteracting the temperature-caused change in crystal frequency. The temperature sensitive components are carefully chosen so they exactly counteract it, and over as wide a temperature range as possible.
A bit of physics: Crystals are made by putting a slab of polished quartz between two conductive plates. Picture a sandwich made from two pieces of bread and a slice of cheese in the middle - the slices of bread are the plates, and the cheese is the quartz crystal. Raw quartz has a grain, just like wood, and the angle of the cut relative to the grain has a lot to do with the characteristics of the finished crystal. There are several preferred angles to cut quartz for crystals, and the companies that do communications crystals use the cuts that are optimized for frequency stability. Companies that make crystals for digital circuit clock oscillators use cuts that maximize the number of crystals per slab (i.e. make them to a price target) and rely on the fact that any frequency error is divided as the crystal frequency is divided down to a target frequency. A drift of 100 Hz on a 1 MHz crystal divided down to 10 pulses per second is a drift of one millisecond. You will never see a 1 ms drift (even if it was 10 times that - a full kilohertz) if that crystal is in the digital LCD clock on the wall.
This situation is the exact opposite of the communications world where crystal frequencies are multiplied up to a operational frequency and every frequency error is multiplied as well. This is how the modulator in an FM transmitter works - a UHF transmitter using a 12 MHz crystal multiplied 36 times only has to wiggle the crystal frequency about 140-150 Hz to get a full 5 kHz deviation. A 900 MHz transmitter with a times 48 multiplier needs a 50 Hz wiggle to get a full 2.5 kHz deviation. 50 Hz is more drift than most crystals have during warmup!
In short, you need to get your crystals from a company that understands the technical requirements of the communications crystal as opposed to a microprocessor (or timepiece) clock crystal, and from one that operates towards a quality target, not a price target.
Note from WA6ILQ:
By the way, if your crystal is in a totally temperature stable environment, then no temperature compensation is needed. This is why two-way radios in the 1940s through the 1970s used crystal ovens - small plug-in housings that kept the crystal at 80 degrees C (about 175 F).
Repeater sites are a whole different situation - I've personally seen cabinet interior temperatures as low as -20 degrees C (-5 F) and as high as +50 degrees C (+130 F) at sites that had no environmental controls. I've also seen sites that have air conditioning / heat pump systems (i.e. cooling, heating and humidity control) that kept the equipment within a 10 degree window year around. And note that just because the building has a nice environmental control system that does not mean that you can scrimp on the crystals or the elements - the temperature inside the radio, where the crystal or element is located, is often much warmer and more widely variable than the room temperature. The power supplies make great heaters and the RF shields make great airflow shields too.
So from the above, you can see that the performance of the element during extreme temperature swings is only as good as the crystal installed in it and the compensation that was done at the time the crystal was installed in the element. The element only achieves its specified stability if the element is matched to the crystal. If the crystal tends to swing upwards in frequency as the temperature rises, the element has to swing lower, and by the same amount. Likewise, if it swings down in frequency, then naturally the element has to swing upwards. And in order for the two swings to cancel, they have to be exactly matching. Another way to think of this behavior is "temperature to frequency tracking". The desired end result is a crystal and element combination that maintains the specified frequency and stability, and operates properly in the circuit for which it will be used, and does it from one temperature extreme to the other. And the previous discussion is only concerned with the oscillator - and has not touched on if the crystal can be modulated, or if the modulation is equal above and below the center frequency, or having a stable output level, or a pure signal.
When placing a new crystal in an element yourself, you take the chance
that it may or may not work correctly (and the odds are stacked against you).
When someone re-crystals an element, it's a game of chance if the values of
the existing compensation parts that were installed when the element was
manufactured (or was last recrystaled) will accurately track the new crystal
to specification. , There's a chance it could, there is also a chance it could
make it worse! Many hams do this with success, others have problems.
However, the average ham does not have the test equipment to validate the
new combination of crystal and element.
Note from WA6ILQ:
Personally, I'll recrystal an element for bench test elements, my personal base or mobile stations, however anything that goes at a commercial site or on a mountaintop goes to International. This is especially true about the one site that is inaccessible, except by helicopter, from first snowfall to late March or early May. And any element for a radio that is on a commercial, Red Cross, GMRS, public safety or military channel goes to International.
Many folks will choose to recrystal their elements themselves to save money, and, in some situations, that's fine. However, just because you have an element rated at 5 Parts Per Million (PPM) (i.e. a GE "5C" element) doesn't mean the element with your new crystal inside it will provide you with 5 PPM stability. As you now realize, when you change the crystal in an element, you will almost always need to make other component changes inside the element to ensure its designed temperature stability. And it's more difficult with a 2 PPM element (the ones required in the GMRS service), and even worse on a 1 PPM element used at 800 MHz, 900 MHz or 1200 MHz.
Note from WA6ILQ:
And what was the source of the element?
I've seen the following scene way too many times: Joe Ham finds a retired
GE or Motorola station in surplus, and buys it. He carts it home then
discovers that the station has no elements. None of his friends have any
available (even if they know what they are). So he shops on eBay, or has a
friend pick up a set at Dayton. He orders crystals from South Driftal Inc
(or some other lowest-price place) and mounts them in the elements. He fires
up the station and has nothing but troubles.
So what's wrong?
1) He does not know if the station had problems before he bought it. It may have had more bench time than in-service time. I've also seen used stations where every major assembly had different problems (i.e. a good spare station had been progressively cannibalized to fix other stations, and all the bad assemblies put into that one cabinet that was then sold to an unsuspecting ham... I can see the ebay auction now - "station originally purchased as a spare but never placed in service!").
2) He does not know if the elements are good. They may have been at Dayton because they were bad and someone wanted them off of his shop shelf.
3) He does not know if the cheap crystals even work in that circuit, if they are temperature stable and if the transmit element can modulate adequately and symmetrically.
This is why time and time again you see the comment on the Repeater-Builder mailing list: always get the old elements with the station and verify the operation of the station on the old frequencies. If you can get an extra set of elements so much the better. A half hour with a dummy load and a service monitor (or a good signal generator and a spectrum analyzer) will let you know if you have a good radio BEFORE you go setting it up on amateur channels. Then take some KNOWN GOOD elements (maybe the original elements) and send them to the crystal house for recrystalling. They will be tested for proper operation, including output level and spectral purity, before your new crystals are installed, and then tested again afterwards before the actual temperature compensation process. In other words, your elements are first tested on their existing frequency/frequencies, then remanufactured on your new frequency/frequencies then tested to make sure they meet original specifications.
Back to elements and compensation...
Note that the compensation components in the element are dependent on the characteristics of the individual new crystal, and those characteristics have to be measured (with extreme temperatures) after the crystal is made. The components are then selected by hand and installed. Hence, this has to be redone whenever the element is recrystaled. Most of the cheaper suppliers build a crystal, install it, and change the primary parallel capacitor so the channel element resonates "on frequency" with the trimmer device centered. This one new component (the capacitor) has nothing to do with temperature tracking or stability, modulation capability, modulation symmetry, proper output level (amplitude) or the spectral purity of the output signal. Just for reference, the term "modulation symmetry" means that for a given modulating audio voltage that the carrier shifts an equal amount above and below the idle carrier frequency (i.e. in FM, not AM).
Speaking of modulation capability...
A crystal with the wrong cut may not exhibit enough rubberiness for full and undistorted deviation to occur (yes, we invented the word. Can you think of a better one?). This situation happens when a crystal vendor cuts a crystal for absolute stability (by making the crystal stiff) and doesn't realize that it will be used in an FM transmit element. Even if the crystal does make enough deviation, it may not swing the same amount on each side of the center frequency, or the modulation may otherwise be distorted. International Crystal and Sentry are the only companies (and ICM bought Sentry in January 2005) that I know of that actually test the modulation capability and ensure modulation symmetry in their finished elements. ICM and Bomar are the only companies that I know of that actually change the temperature compensation components (capacitors and resistors) to properly temperature compensate the element, and then test for the five characteristics mentioned above. The symmetry is very important as poorly made or poorly compensated crystals don't have a symmetrical response to deviation voltage, causing the average center frequency to shift during modulation. In other words, you may be on frequency with no audio, a little off frequency at 900 Hz deviation, a little more off frequency at 2 kHz dev, and a lot off frequency at 5 kHz dev.
The actual schematic of the RCA TCXO or the GE ICOM is not in the manuals (the schematic in the LBIs is simplified, and on it the frequency compensation area is just a block diagram), but the schematic of the equivalent Motorola Channel Element is below, courtesy of the reverse engineering capability of Scott Zimmerman N3XCC, of Repeater-Builder:
Look carefully at the schematic. Q1 is the oscillator. Q2 is a buffer-tripler. X1 is the crystal). Go north from there, and across to C3, the trimmer. Then find L1 and C7. Nothing north of the crystal but C3, L1 and C7 is needed to make an oscillator. Now find the AFC pin, C2, D1, R1 and R2. Those components are all that is needed to make the frequency move with the AFC voltage. Everything else that is north of the crystal is compensation.
Anybody want to trace out a GE Icom? We'll publish it, with credit or without, your choice.
To deal with temperature stability, the crystal and element must be heated and cooled under strictly controlled conditions, and the frequency must be measured accurately at each temperature extreme. New compensation components are then chosen and installed in an attempt to minimize the overall frequency change of the assembly. This must be repeated until the desired stability is achieved. It's a trial and error process; each heat-and-measure then cool-and-measure then component-swap cycle could take several hours, so the worker is working on or testing another element while the first one is heat-soaking or cold-soaking... in fact he or she may be rotating as many as a dozen elemeents across his/her bench and through the chiller and the oven at any one time. And remember that in most electronic circuits the intent of the designer is to select components that have a minimal effect from temperature changes - here the objective is the exact opposite in that the components in the compensation area are specifically and carefully chosen for their changing characteristics at varying temperatures.
Once the temperature compensation process is complete the element is tested again for proper RF output level and spectral purity on the new frequency. If it is an AFC receive element they test it for proper direction of shift with AFC voltage, plus the sensitivity to that voltage and proper range. If it is a transmit element and has an internal FM modulator they test it for modulation sensitivity, symmetry and distortion under ambient temperature and at hot and cold temperature extremes.
When you are ready to recrystal an element and are calling around to get prices, you need to ASK what each company does when "compensating" an element. To recycle an old line, you can't compare fruit unless you care comparing apples to apples and oranges to oranges. Not all will go through the time-consuming process of hand-selecting components and thermal cycling in the two environmental chambers (one for heating, one for cooling, and both use air drying equipment so that there are no condensing humidity droplets to cause problems). Considering what you are going to spend on the entire repeater, paying a little extra for a professional top quality crystal house to manufacture the crystals and do the compensation is definitely worth it. If you are serious about your repeater, especially if it's going to be located in a hilltop building with no climate control, do yourself a favor and send the element(s) back to the crystal house and pay to have them compensate the element to the crystal. After all, those people have the thermal chambers, the air drying equipment, stock the wide variety of the proper temperature sensitive capacitors and thermistors, and have the expertise to use all of them to their best effect because they do it 8 hours a day, 5 days a week, for 52 weeks a year.
Crystal and Element Specifications:
Individual crystals and channel elements are built to certain stability specifications, measured in Parts Per Million (PPM), and refers to how many Hertz a crystal or element can drift (vary in frequency) with changes in temperature from a known reference point. In a GE MASTR II, that's 80 degrees Fahrenheit. A 5 PPM crystal / element combination can drift 5 Hertz for every million Hertz of operating frequency. So, a 146.000 MHz element can drift 146 times 5 which equals 730 Hz. This means a 146.000 MHz element can vary plus or minus 730 Hz and still be within the plus or minus 5 PPM specification. Holding to this same specification, a UHF transmitter operating on 445.000 MHz can vary 2,225 Hz and still be within specification (but an experienced ear can hear someone that is 1 kHz off frequency). And remember that the plus and minus 2,225 Hz tolerance at UHF is only 61.8 Hz at the 12 MHz crystal frequency (due to the times-36 multiplication)... and it's even tighter if the exciter uses an 8 MHz or a 6 MHz crystal (about 30 Hz).
Yes, that means that your 12 MHz crystal oscillator signal can drift at most about SIXTY-TWO HERTZ each direction of the center frequency from the coldest winter night to the hottest summer day and stay legal at UHF. Warmup drift in most equipment is more than that.
The tighter frequency tolerances and 2.5 kHz devaition limit on 900 MHz makes it even worse - one 927 MHz repeater transmitter that I worked on has a multiplier of times-48. One PPM at 927.5 MHz is 927.5 Hz. Divide that by 48 and you have 19.3 Hz. This means that your 927 MHz repeater transmitter oscillator has to stay within 20 Hz of frequency ! And the 2.5 kHz deviation at 927 with a times 48 multiplier only needs about 50 Hz of wiggle for full deviation. Yes, a drift of 50 Hz will push the transmit signal right out of the bandwidth of the user's receiver.
From the above, the FCC rules notwithstanding, it's plain to see why people choose 2C elements (0.0002% ) for UHF repeaters and mobiles (resulting in a maximum of about 900 Hz variation) and 1C for 800 and 900 MHz (800 Hz or 900 Hz variation). It's also plain to see why you should spend the extra money and have a reputable crystal house redo your repeater elements so they are properly rebuilt and temperature compensated. Also remember, you get what you pay for. I have heard the complaint many times "ICM wants $50 to replace the crystal in the element". Now you know why, and that it's not all that simple. It's more than just mounting and soldering the crystal, it's ageing the new crystal plus many hours of heating, measuring, cooling, measuring, installing compensation components, and repeating the cycle until it's absolutely right. And then testing for modulation symmetry, modulation sensitivity, minimal modulation distortion, adequate RF output level, output purity, AFC voltage sensitivity and range.
So go ahead and buy the crystal for $20... Can you do the rest at home
for $30 more ?
Especially considering that a round trip to your repeater site (depending on where it is) may take six hours and cost a tank of gas ? Do the arithmetic. For me a round trip to one site will cost 2/3 of a tank of gas, and a tank is 20 gallons. A trip to a different site requires me to coordinate schedules with another gentleman who has a 4x4 truck - a round trip there costs about 30 gallons plus meals. How many round trips will it take to equal the cost of having International rebuild an element?
An emailed note from WA1MIK: I recently sent two Motorola MICOR
0.0005% receiver channel elements to ICM. They charged $19.95 for each crystal
and $30 to install, temperature-compensate, and test each element. The entire
process should take about a month, and more than half of that would be devoted
to the compensation and testing phase.
(Follow-up: elements sent in 1/19/07, received back 2/15/07. Worked perfectly.)
Another emailed note: In mid 2009 a pair of rebuilt UHF 2C ICOMs was around $180, including tax and shipping.
A third emailed note: In early 2010 a single rebuilt UHF 2C ICOMs was quoted as $83, plus tax and shipping.
Don't take the above quoted price as an absolute - redoing a 0.0005% element (i.e. a 5C) is the least expensive, an 0.0002% element (a 2C) costs more, and a 0.0001% element (a 1C) (used at 800 MHz, 900 MHz or 1200 MHz) costs the most.
Comments on crystal aging:
Most new crystals will drift downward in frequency in the first 6 months of operation (but some will go up) - this drift is very visible on UHF and can be seen even on 28 MHz. This drift is due to the crystal "aging", and is quite normal. In the mid 1980s crystal manufacturers adopted a technique called "pre-aging", where they cut the raw quartz crystal material into blanks, then run the blanks in a power oscillator circuit under very elevated temperatures for several weeks. Then this pre-aged material is processed into crystals. Once the crystals are cut close to frequency they run them in a heat / cool cycle for a week or so, then the final manufacturing process puts them on frequency and in the metal case. This technique produces a finished crystal that might age an additional 3 to 5 kHz (at UHF) total, if that much. Then, if the finished crystal is headed for a channel element, it is installed, compensated (which includes a number of additional heat / cool cycles), tested, and shipped. Note that rebuilt elements get much more run-time and more heat / cool cycles than plain crystals. This is one reason why compensated elements don't age as far as individual crystals.
In closing, do not be surprised if a properly manufactured crystal (or a remanufacturered element) drifts a little in the first 6 months, but it will not be very much.
Even if your brand-new UHF crystal drifts as much as 7 kHz (on the UHF frequency, not the fundamental) during the first 5 to 6 months of operation, it does NOT mean that the crystal is defective. Just plan on resetting the frequency a few times in the first 6 months of continuous operation (which may take a lot longer on a transmit crystal since it doesn't get as much "run time" as a receive crystal). Once the crystal ages it will settle down.
As a personal example of the above aging, back in the early 1980s I moved a UHF Motrac repeater from GMRS (467 / 462 MHz) to the 443 / 448 MHz range. The newly crystaled receiver drifted 11 kHz in the first 5,000 hours of operation (four adjustments of about 5 kHz, 4 kHz, 2 kHz and 400-500 Hz across six to seven months), and hasn't budged 800 Hz in the 20-25 years since. Interestingly enough, when the crystal was placed in service the adjustment was almost at one end, and when the frequency stabilized the adjustment was very close to the middle of the range. You can avoid this by having a "test fixture" on your bench operating 24x7 to "burn in" the crystals / elements while you work on some other part of the system that you are building. The channel element / ICOM "test fixture" can consist of just a a channel element or ICOM socket (or two), a power supply (which can be nothing more than a 9 to 10v DC "wall wart" transformer),a 3-terminal voltage regulator, and a resistor (as an output load for the oscillator or element). If you are running crystal based radios then it can be just an extra oscillator or exciter board, the wall wart and a load resistor. Make sure the element is seeing nice clean stable DC of the correct voltage. All you are after is a way to run the element(s) (or crystal(s) full time when you aren't in the shop.
My personal method is to set a brand-new crystal / ICOM / channel
element on frequency in the test fixture and wait a few days and see which direction it
heads as it ages (90% have gone downward in frequency). I then set it back on frequency
and a little further in the same direction so the next bit of aging brings it back towards
center of channel. Once the new repeater is finished I take it to the hilltop and mark my
calendar - I then check the frequency at a week, two weeks, a month, at 2 months and at 6
months, plus any other opportunities when the service monitor is handy.
Notes on Ordering Crystals:
See the Suppliers Page for a list of the crystal houses that really do Element Remanufacturing and Temperature Compensation.
Thanks to Neil McKie WA6KLA for letting me plagiarize his crystal ordering rules. He developed them from personal experience in over 35 years in commercial 2-way radio at a mix of privately run and government shops. The notes I'm typing these from are dated 1983.
Crystal ordering rule #1: When I make an order I call the crystal house on
the phone and verbally order the crystal(s) and give them a credit card number
to place the charge against. They give me an order number (which I repeat back
to them), and send the order to production that day. While they are grinding
the crystal(s) I'm boxing and shipping them the element(s). On each order I always
ship them one extra element - there have been times when they've found a dead
or otherwise bad element, and I've always received the spare back with the
remanufactured element(s) (and it beats waiting the extra time while you locate
and ship them another element). If I'm sending over ten elements of the same
type I include two extras. If I am having them do multiple types of elements
(for example, both a receive and a transmit element) I include a spare of each
type. There have been times that a single order has been for eight or nine
frequencies and sent them a dozen elements. And I include a note in the box
with the elements noting the order number, listing the channel elements and
the frequencies I am ordering, and explaining why the extra element(s).
I also put a piece of masking tape on each element I am sending them with the
order number and the new frequency written on it in ink, just in case the note
gets separated from the box of elements. All it takes is something like
"Order # 65-12345, rebuild to receive on 444.000 MHz", "Order # 65-12345,
rebuild to transmit on 449.000 MHz" or "Order # 65-12345, Extra receive element
in case one is bad".
In several hundred orders Neil never had a problem with this procedure. And both of us always received every one of the extra elements back, occasionally with a "NG" (not good) note taped to one of the elements.
Crystal ordering rule #2: I never give them the crystal frequencies (i.e. let them figure them out) - I give them the target frequencies (receive and / or transmit), the radio type and the channel elelement model number (i.e. Mitrek KXN1086B element to be moved to 449.000 MHz) and on ham radio orders I always mention the receiver injection direction, even if it's unchanged (note that in most cases when you move a GE MASTR II receiver from commercial high band to 2M or from commercial UHF to 440 you will want to flip over the injection from low side to the high side). The rock chippers need to know this!! Even on 220 MHz MICOR receiver conversions I just tell them that I want it to receive on a certain frequency, the multipler used to be 9 and is now 12, I want low side injection, and I still let them caculate the crystal frequency. One result of this policy is that the correct channel frequency is stamped on the crystal case along with the crystal frequency.
The only time I've told ICM what frequency to cut the crystal to is the time I had them take a UHF 2ppm element and put a 10.00000 MHz crystal in it, compensate it and bypass the internal tripler (and if I'd had a 1ppm element I'd have used it). I installed the resulting custom element into my Yaesu YC355D counter as a replacement for the the 10 MHz time base reference. My counter had a bad crystal and it drifted all over the place... The decision was a no-brainer... a custom element from International cost about $5 less than what Yaesu wanted just to estimate the repair. My own testing after the mod was installed revealed that the element was dead on from the coldest temperature my kitchen food freezer could do to over 125 degrees F (50 degrees C). Supposedly the counter wasn't an end-user repairable piece of equipment despite the fact that the design was straight out of Popular Electronics magazine (and that article was based on a Texas Instruments applications note). Yaesu wouldn't sell me just the crystal, they "had" to install it on their bench in their shop). Sorry, Yaesu... the channel element mod was a lot cheaper and the counter became a lot better piece of equipment (much more accurate).
Crystal ordering rule #3: Never, ever ask the crystal house to push the delivery
time. Plan on four weeks, occasionally five (if the crystal house has a large bunch
of orders) to deliver properly cut and aged crystals (or rebuilt channel elements).
If it shows up in three weeks, say a thank you prayer to your personal higher power
and add a bit extra to the sunday collection plate in thanks. If you absolutely have
to get a rush crystal, order both it AND A REGULAR CRYSTAL AT THE SAME TIME.
The reason is best given in a story... this dates from the days of GE MASTR Pro radios with separate crystals in sockets (i.e. not in channel elements or ICOMs) but it is as true today as it was then.
Neil told me about the time he did a mountaintop service call on a dead community repeater and discovered that the receiver crystal was bad. He had fought this battle before... He got his lineman's phone out of the service truck, clipped across the phone line at the repeater site and called International Crystal right from the repeater site (annd as he tells the story - and note this is pre-caller-ID days, you know that you've been in the business too long when the order desk lady at International recognizes your voice, calls you by name, and asks if it's a company order or a personal ham radio order).
He ordered both a rush crystal and a regular crystal. The rush crystal arrived via airmail special delivery in five or six days. Neil drove to the site, installed it and set it to frequency. The next day he drove to the site and reset the frequency. Two days later he drove to the site and reset the frequency. And every three or four days for the next three weeks either he or one of the other techs was at the site setting the frequency. THIS IS WHAT UNAGED CRYSTALS DO - THEY DRIFT. When the replacement regular crystal arrived (exactly four weeks and one day from the day he ordered it) Neil was very happy to drive to the site and install it, and set it on frequency (and yes, it required a couple of resettings before it stabilized).
Then he drove back to the shop, put the rush crystal in a spare receiver and let it run. And run. And run. Months later it was still drifting! Neil took a three pound sledge hammer, beat that crystal paper thin and gladly tossed it in the trash. The rush crystal bought the company three weeks of income from the community repeater airtime, but cost a a couple dozen service trips, however it wasn't usable even with months of 24x7 cooking time!
The lesson? Give the crystal house time to do a proper job. UNAGED / RUSHED CRYSTALS ARE JUNK.
The other lesson? If your day-to-day income depends on crystal-based devices (like a bunch of community repeaters) then you need to have ready-to-go spare receivers/exciters/crystals/elements on the shelf.
The following was extracted from a discussion on the repeater-builder Yahoo! Group (reproduced with each of the participant's permission):
What exactly are ICM and other crystal manufacturers doing when they "compensate" or match an element with a crystal to get it netted on frequency?
I've had mixed success. Some crystals and elements tune on frequency just fine, while others don't. I've had mixed luck padding extra capacitance on MICOR elements, but Mitrek elements use the inductor instead.
For example, I have a KXN1052 that's 20 kHz high after dropping in a new crystal. I can pad the trimmer, but then the element won't produce more than 3 kHz of transmit deviation.
What's their secret? The only thing in the element is resistors and capacitors! I'm sure those of us capable of working on a repeater are capable of changing a few components.
= = = = = = = = = = = = = = = = = = = =
Each channel element, regardless of the manufacturer, contains a number of resistors, capacitors, and perhaps a few inductors. The capacitors have specific TCs (Temperature Coefficients) that are chosen so that the capacitance variation with temperature change is exactly complementary to the reaction of the crystal. When performed correctly, the capacitors change value with temperature just enough to cancel out the frequency drift of the crystal.
However, a full compensation of the crystal holder (channel element, ICOM, etc.) includes more than temperature compensation. The technician also verifies that the crystal can be set exactly on frequency with the included trimmer, that the output amplitude meets the minimum specification, and that the crystal is "rubbery" enough to be modulated to the required deviation level, and the modulation is equal on both sides of the idle (carrier) frequency.
As you might expect, full compensation of a channel element to a particular crystal is an exacting and time-consuming process. That's why ICM charges more for the compensation ($30) than the crystal costs ($20).
When a radio user orders just the crystal and puts it into a handy channel element, the components inside that channel element may or may not match the characteristics of the new crystal. As you and many others have discovered, the new crystal could be such a poor match to the channel element that it may be impossible to get it to operate on frequency. Even if you can add or remove some shunt capacitance to tweak the crystal to frequency, that shunt capacitance is not temperature compensated. It may work fine, and it may not.
Both Motorola and General Electric operated their own crystal manufacturing facilities for many years. Since each company had complete control over the making of both the crystal and the channel element that contained it, they could evolve the processes to optimize performance and longevity. Let's say that Motorola found that their MICOR channel elements worked best with crystals that were made for a 25 pF load rather than a 30 pF load. If you have one of these original MICOR channel elements that you want to re-crystal, it is likely that ICM, Bomar, or Crystek will ship you a nominal crystal, since they have no way of knowing that your channel element is not nominal but has already been compensated to the original crystal, which may have a non-nominal load capacitance. How can they know, if you don't send in the channel element? Also, since the crystal house never had the chance to test your channel element first, they have no obligation to make changes to your crystal if it doesn't work properly once you install it.
Given that a full compensation is a one-time charge, I personally have every hilltop crystal I buy given the full compensation in a channel element I send to the crystal house. I think it's a prudent investment. Not everyone agrees...
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Ok, but back to my original question: if I send an element to ICM to be re-crystaled, do they cut the crystal first to "their" standards, then modify the element to make it work, or do they measure the element first in some fashion, then cut the crystal to match the specific element's characteristics?
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To the best of my knowledge, the crystal is always manufactured first to meet ICM's nominal specifications for a stock element, and the channel element compensation components are then modified as necessary to perform satisfactorily with that crystal.
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And don't forget, a channel element or ICOM that was previously compensated for another crystal may work poorly or not at all with a new crystal. If the crystal house does not have the element in their hands, they have no choice but to make your new crystal to "nominal" specifications. And that is the problem, because none of the used channel elements or ICOMs are a "nominal" oscillator- every one was individually customized for the previous (probably original) crystal.
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The following was extracted from an International Crystal OEM crystal catalog:
ICM can recrystal customer supplied oscillator modules (channel elements) used in most modern commercial two-way communications equipment. The service we offer includes the following steps:
The oscillator modules (elements) are checked for proper operation when received. If repair is needed (transistor, capacitor, etc.), it will be replaced at a nominal charge. An oscillator module that has been damaged and/or unrepairable will not be accepted for frequency modification, and will be returned to the customer with no action taken.
After the new crystal has been installed, the module is tested for nominal frequency and trimmability at room temperature, overall frequency specifications over the specified temperature range and RF output. If the module uses a compensation network, ICM will make adjustments necessary to insure that the frequency drift due to temperature does not exceed the original manufacturer’s tolerances. Some modules have the crystal and a few compensation components only, not the entire oscillator assembly; in these cases the unit is spot checked using an oscillator of the required type. In addition, some modules such as offset type units are actively modulated; these units are set up on a test fixture and tested to insure proper deviation.
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Text contributions by (in alphabetical order) Kevin Custer W3KKC, Jeff DePolo WN3A, Jeff Kincaid W6JK, Eric Lemmon WB6FLY, Neil McKie WA6KLA, Robert Meister WA1MIK, Doug Marston WB6JCD.
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This page originally posted on 01-Febuary-2007.
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