On 2/1/07, Kevin Custer <[EMAIL PROTECTED]> wrote: > Long overdue, here is a fairly comprehensive article on re-crystaling a > Channel Element, ICOM, etc., and the effects on stability, temperature > compensation, and modulation capability. > It's available from the Tech Index: > http://www.repeater-builder.com/rbtip/techindex.html > > Direct link to the article: > http://www.repeater-builder.com/tech-info/temperature-compensation.html > > Many thanks to those who helped with its publishing. > > Kevin Custer
Nice article, and I agree that getting the right capacitor value with the crystal is important since that keeps the crystal in the "center of the curve" in the standard GE chart shown in every discussion about ICOM's in their manuals. But I have a question... or maybe it's a comment... and this might be a long one... It's about the article's references to 5 PPM... and the "dangers" of using a 5PPM device in a UHF repeater. THERE ARE NO 5 PPM ICOMs when they're operated in the center of their temperature range! All 5C ICOMs (and even EC ICOMs!) are TWO PPM (2 PPM) devices inside the CENTER of their temperature range!! That's not mentioned anywhere in the article, and the article seems to try to scare people into using only 2C ICOM's on UHF! That's not correct... read on... In the MASTR II manuals, it's apparent from the text that the high and low side temperature compensation of a 2C or 5C doesn't even "kick in", until some awfully wild temperature extremes are breached. Here are some of the text items in the LBI to back up this statement: (from http://www.repeater-builder.com/ge/lbi-library/lbi-4561c.pdf) "Should the 5C-ICOM compensator fail in the open mode, the EC-ICOMs will still maintain 2 PPM frequency stability from 0C to 55C (+32F to +131F) due to the regulated compensation voltage (+5 volts) from the +10-Volt regulator IC." "At temperatures above and below the midrange additional compensation must be introduced." "The cold-end compensation circuit does not operate at temperatures above 0C. When the temperature drops to 0C, the circuit is activated." "The hot-end compensation circuit does not operate at temperatures below +55C. When the temperature rises above +55C, the circuit is activated." And of course, the kicker... "Service Note: Proper ICOM operation is dependent on the closely-controlled input voltages from the 10-Volt regulator. Should all of the ICOMs shift off-frequency, check the 10-Volt regulator module." Setting 10V *exactly* in your MASTR II's is important for frequency accuracy. It's rarely mentioned anywhere in articles, but it is carefully mentioned numerous times in the GE manuals themselves. Set those 10-Volt regulators accurately, folks. It's clear from the GE chart and description (and even the schematic that shows the high and low side compensation circuits with SWITCHES that turn them on and off!) that what compensates the crystal in the mid-range temperature area is NOT the high and low side temperature compensation circuits found in the 2C and 5C ICOMs... at all. It's the CAPACITOR that was chosen by the folks compensating the ICOM! So, in that sense your article is very VERY accurate. You should always choose to pay ICM or someone else to compensate your ICOMs if you have any desire to make sure they're dead-nuts on frequency and going to stay there. ANY ICOM compensated properly will exhibit 2 PPM accuracy in the middle temperature range of 0C to +55C (assuming you're feeding the EC ICOM with +5V on the COMP line, if you just *have* to use one of those). If your radio lives in a temperature controlled environment, the use of 2C over 5C makes NO difference at all, and you HAVE a 2 PPM accuracy with either one. In fact, if a constant +5V is placed on the compensation line, the EC's will also do this. For a "backyard" repeater living in someone's basement, the 2 PPM provided by a 5C is the same 2 PPM accuracy provided by a 2C. There's no difference at all as long as you're in the middle temperature range. The text of the article would indicate that there's some grave danger when using a 5 PPM device on UHF, but a 5C ICOM is a *2 PPM device* between 0C and +55C! If your system is staying between 0C and +55C, using a 5C will exhibit *2 PPM* stability according to the GE manuals. In fact, even a properly fed (solid +5V on the compensation line) EC ICOM will do that, too. They maintain 2 PPM accuracy IN THE MIDRANGE TEMPERATURES by the choice of capacitor by the crystal house when they compensate the ICOM. And that was the REAL point of your article, I think... and I agree... as does GE. If the capacitor in the ICOM was chosen to match the crystal... then things are good. But the text in the article about the difference between 2 PPM and 5 PPM makes people think the ICOM's labled "2C" and "5C" are only 2PPM and 5PPM accuracy. This is NOT true, and people shouldn't be afraid to use a 5C ICOM on a UHF repeater *in a temperature controlled environment*. The only time the extra compensation circuit does anything at all is above +55C or below 0C. The constant harping on the use of 2C elements on UHF by many people is 100% correct if the repeater is in a non-temperature controlled environment, and totally incorrect if the radio is living in a room that's always between 0C and +55C. We're using all 2C's "just because" on most of our systems, but I hate to see folks struggle to find 2C ICOM's for environments where they simply don't DO anything. (Or worse, waste a 2C on a basement repeater. Send them to your friends who have COLD and HOT sites, and use the 5C's you have a-plenty in every mobile rig! If the radio isn't getting below freezing or above 138F ... go with the 5Cs.) But YES... if you're serious about frequency accuracy, have the crystal house compensate the darn things... get the capacitor that's doing all the work between 0C and 55C "right"! But don't be afraid to use a 5C ICOM in a temperature controlled environment... it's really a 2 PPM device in that temperature range. Nate WY0X
