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Basic Information On Temperature Compensation and Modulation Capability
for Channel Elements, ICOMs, and TCXOs
Generically referred to in this article as "Elements"
Compiled by Kevin Custer WJ8G
This page deals with the subject of replacing the crystal in a channel element, ICOM, or other type of TCXO, and the problems that can arise by doing so yourself. Hams are frugal (some are downright cheap) by nature, always wanting to save a nickel anywhere they can. Replacing the crystal in the frequency determining element is a common effort when taking a crystal controlled radio and putting it on the ham bands. 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 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 effectiveness of the process.
Notes on Temperature Compensation:
The first thing to realize is that the performance of the element during 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. These units only achieve their 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. 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.
When placing a new crystal in an element yourself, you take the chance that it may or may not work correctly. 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 last worked on will accurately track the new crystal to specification. There's a chance it could, there is also a chance it could make it worse! If it does work out, it's only by blind luck.
Note that the compensation components in the element are dependent on the characteristics of the individual crystal, and those characteristics have to be measured 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. Many of the cheaper suppliers build a crystal, install it, and change the parallel capacitor so the channel element resonates "on frequency" with the trimmer device centered, without regard to temperature stability or modulation capability and symmetry.
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/measure/cool/measure/component-swap cycle could take several hours.
Speaking of modulation capability...
A crystal with the wrong cut may not exhibit enough rubberiness for full or undistorted deviation to occur. This situation happens when a crystal vendor cuts a crystal for absolute stability (by making the crystal stiff) and does't realize that it will be used in an FM transmit element like the Motorola KXN1019B, KXN1052A or a GE MASTR II (PLL) FM ICOM. Even if it 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. ICM is the only company that I know of that actually tests the modulation capability and insures modulation symmetry in 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.
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. Not all will go through the time-consuming process of hand-selecting components. 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 back to the crystal house and pay to have them compensate the element to the crystal.
Many folks will choose to recrystal their elements themselves to save money, and, in some situations, that's fine. However, just because you have a 5C element doesn't mean the new crystal inside it will provide you with 5 PPM stability anymore. 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 insure its temperature stability.
Crystal and Element Specifications:
Crystals and elements are built to certain stability specifications, measured in PPM. PPM is Parts Per Million, 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 X 5; which equals 730 cycles. This means a 146.000 MHz element can vary plus or minus 730 cycles and still be within the 5 PPM specification. Holding to this same specification, a UHF element operating on 445.000 MHz can vary 2,225 cycles (yes that's 2.225 kHz) and still be within spec. It's plain to see why people choose 2C elements for UHF (resulting in a maximum of 890Hz) and 1C for 800 and 900 MHz (800 Hertz or 900 Hertz). It's also plain to see why people should spend the extra money and have a reputable crystal house redo your repeater elements so they are properly 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 hours of heating, measuring, cooling, measuring, installing compensation components, and repeating the cycle until it's right.
Note from WA1MIK:
I recently sent two Motorola MICOR 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.)
See the Supplier's Index Page for a list of those crystal houses that really do Temperature Compensation if you send an ICOM or element back to them with your order for a new crystal.
The following was extracted from a recent exchange on the repeater-builder Yahoo! Group (reproduced with 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 20KHz high after dropping in a new crystal. I can pad the trimmer, but then the element won't produce more than 3KHz 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.
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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.
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 crystal I buy given the full compensation in a channel element I send to the crystal house. I thinks 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, and the channel element is then modified as necessary to perform satisfactorily with that crystal.
If you have the time and are one of those people who can still read, additional details, editorial content, reader testimonials, and other commentary can be found here.
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Text copyright © Kevin Custer WJ8G 2002, 2004, 2005, 2007.
Text contributions by Mike Morris WA6ILQ, Eric Lemmon WB6FLY, Jeff DePolo WN3A, and Robert Meister WA1MIK.
This page originally posted on 01-Febuary-2007.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.