Crystal Oven Temperature Controller Module
For Two-Way Radio Applications
Model CH-25 Click Here for schematic, installation, and set-up instructions.
Ordering instructions on bottom of page.
This device will precisely control the temperature of a quartz crystal heated to an elevated temperature.
This product is mostly thru-hole for easy assembly. It does have one small surface mount IC, and the kit can be ordered with it preinstalled.
It can be used in either the transmitter, receiver, or both if two boards are purchased.
Can be used on reference oscillator crystals of most synthesized or programmable radios.
For amateur radio applications only, as use of this device does not ensure or guarantee FCC (or other governing bodies) Type Acceptance.
Price $25.00 each - in kit form.
Also available assembled and tested - $35.00 each.
US shipping and handling $7.50 in small quantity.
PayPal accepted - order below.
Actual Size CH-25 photo.
(Click image to enlarge)
What is it, and why do I need it?
Let's first put all of this in perspective. As of May 2017, ICM is no longer accepting orders, and has gone out of business. BOMAR had stopped making two-way radio crystals sometime in 2015 - BUT we hear they're making them again, however, without temperature compensation services. While many of us have attempted to recrystal elements ourselves, nothing guarantees your newly made crystal will be temperature stable. Why? Because the components installed inside the element were selected to make the original quartz crystal stable over changing temperature. Crystals are like diamonds, no two are alike. In the element, there are many components with hundreds of combinations which are hand selected to insure the crystal stays put with varying temperature. Some of these components hold the crystal stable in the "normal" temperature range, and some of them operate above and below this normal range.
Certainly, you can purchase a crystal, stuff it into an element, and have it oscillate, but will it stay frequency stable over changing temperature? Without doing some type of temperature compensation, that's a chance at best, with the odds being against you. The cost of the crystal isn't why crystal vendors were requiring so much money for element re-do's - it was the labor involved in doing the temperature curve testing on the new crystal. That testing determines the components inside the element that need changed. So, they'd change them, do additional testing and component augmentation again (over and over if necessary) until the element met or exceeded the stability standards/requirements of the element/customer.
So, what can we do to save on these costs? We can consider other options - like the CH-25 paired with a crystal custom made to your frequency, and built to operate at a higher temperature - like 60°C.
If we artificially hold the new crystal at a temperature above the maximum temperature ever expected inside the radio set, it will be temperature stable, thus providing frequency stability. The operating temperature one chooses, needs to be above any expected temperature from outside or internal influences to maintain the best frequency stability. So, obviously you want to elevate the temperature to some point that won't ever be reached due to heating of the radio set from normal or excessive transmit times or lack of cooling or ventilation, for whatever reason. We can do this easily by heating the crystal (and surrounding components) to an elevated temperature and holding it there. That's what the CH-25 does.
Those who are wanting to recrystal their own elements can do it without fear of it not being temperature stable. This process is fairly easy and inexpensive.
This is what we've been able to repetitively accomplish with a standard crystal and a $25 heater - 0.2 PPM temperature stability with little current draw.
CH-25 REV4 Review:
Click Here for a review of the latest revision of the CH-25 - with photos!
The latest board revision includes a few modifications. They are as follows:
The CH-25 is a precision unit, built on a quality printed circuit board, which is designed to heat a radio crystal to an elevated temperature and hold it there. Most radio crystals are 'AT' cut, and have a 'S' curve temperature/frequency response. The point in the middle of the S curve is called many things, but the most correct term is inflection point. Most crystals intended for unheated (TCXO) channel elements and ICOMs are built for a 25°C (or thereabouts) inflection point. For this project, it is best to order crystals for your channel elements or ICOMs which have an elevated inflection point of 60°C. Then, the crystal could be operated in the middle of its 'S' curve to maximize temperature stability.  This isn't a requirement as you may be wanting to stabilize an already purchased crystal.
The operating temperature one chooses needs to be above any expected temperature from outside or internal influences to maintain the best frequency stability. So, obviously you want to elevate the temperature to some point that won't ever be reached due to heating of the radio set from normal or excessive transmit times or lack of cooling or ventilation, for whatever reason.
This circuit will heat and hold the crystal and surrounding components to a very stable temperature, thus stabilizing the operating frequency. You may want to frequency stabilize a crystal which was cut for some other inflection point, like a cheap crystal insert that was built for your channel element, but you chose not to have it properly temperature compensated. Another application would be to frequency stabilize a crystal or reference oscillator in equipment that never had any temperature compensation or a higher frequency stability option.
This unit is normally installed inside the radio and the heater resistor(s) and temperature sensor mounted onto the crystal with a 5 conductor cable, or 5 wires (not supplied) in between. One module will ovenize one crystal. If you need to stabilize more than one crystal, additional units will need to be purchased - one for each crystal that needs stabilizing. The cost of a new crystal insert by itself for a typical amateur radio application is about $25.00 to $35.00. The installation and proper set-up of this module could result in temperature stability which could far exceed that of the crystal manufacturer, at an overall reduced cost. Of course, there is the consideration of your time to install this unit and to insulate the crystal and oscillator adequately, but most hams have more time than money for their hobby.
Installation procedures, and methods used to insulate the heater and crystal, will vary from radio to radio and what you find to work properly. I have been able to achieve a temperature stability of 0.2 (that's zero point two) PPM stability or 10 times better than a properly compensated 2C or 2 PPM element/ICOM. This has been achieved with crystals built for a 25°C inflection point. For new installations, I certainly believe it's better to have the crystal built for the application, but I also believe there is little reduction in stability if you were to use a ready-built 25°C crystal and hold it within a tight temperature tolerance. This is especially true of aged crystals.
Packaging the resistors and sensor can be done in a way that the original cover will go back onto the element. A slot can be cut in the side of the cover so the wires going to the circuit board have clearance. See the photo below for an example. I have found that spreading out the heat maximizes temperature stability. I include two 100 ohm resistors which are paralleled to make a 50 ohm heating element. I use epoxy (not included) to attach the heater resistors and LM34 sensor to the crystal case. Leads from these components are extended with hook-up wire (not included) and run to the CH-25 circuit board header pins. You can either use the headers, or simply solder fly leads from the board to the heater, sensor, and power source, if desired - your choice. Other types of resistors could be used to save space. Surface mount resistors could be considered as replacements to those supplied to possibly make packaging easier for your application.
There are a few other things that need to be said. Because this device heats the crystal and surrounding components, other components in the element/ICOM are not exposed to a great temperature change - so existing 'compensation components' aren't changing against ambient temperature changes. This should preclude you from having to change anything except possibly the fixed ranging capacitor - so the netting capacitor (trimmer) can be centered. In most cases, this is not necessary. Of course, you need to properly insulate the oscillator and surrounding components for best results. Many times a cover of some sort will need to be removed from the element/ICOM for placement of the heater resistor(s) and sensor. This may take up room that won't allow for the cover's re-installation. Certainly the wires connecting to the heater and sensor will pose a concern. Another caveat is that a new crystal will likely age at an accelerated rate when kept at an elevated temperature, so you'd want to let your crystal bake at operating temperature as long as possible before you do the final netting, prior to placing the radio into service.
Click Here for schematic, installation, and set-up instructions.
|Power Source: +9 to +14 VDC (up to 18V with the Zener installed) must be regulated and constant, not switched.|
|Easy to install crystal heater - an alternative to professional (expensive) temperature compensation.|
|Includes a LM34 temperature sensor and two 100 ohm 1/2 Watt resistors for heater.|
|Will work with other combinations of heater resistors down to 30 ohms - allowing for proper dissipation.|
|Low current draw, about 200 mA at startup (about 125 mA at setpoint) with 50 ohm heater and 13.6 V supply.|
|Low frequency drift when coupled with a good crystal, regulated power source, and adequate insulation.|
|Most crystal houses no longer perform full temperature compensation.|
|Circuit board will accommodate vertical or horizontal multi-turn potentiometer.|
|Small size - fits easily in most radios.|
|Crystal set-point temperature range is +30°C to +100°C or more, (adjustable with 25 turn pot).|
|Module operating temperature range is -30°C to +60°C.|
|Simple to hook up and set up.|
|Mating signal connector can be plugged on backwards without damaging the board.|
|90 day Limited Warranty. Repair available for units out of, or not covered under, warranty (my determination).|
|Mounts with double-sided foam tape or stand-offs (not included).|
|Hookup is made with miniature 5 pin male and 3 pin male 0.100 header pins (included).|
|Mating female sockets and spring contacts are included so you can make your own wiring harness (wire not included).|
|Kit comes with thru-hole resistors and capacitors, and is fine for the first time
The kit can be supplied with the surface mount parts presoldered onto the board - just ask - no charge for this service.
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Product of Kevin Custer - W3KKC, all rights reserved.
Specifications may change without notice.
Original concept and circuit design by Bob Dengler - NO6B
Images are property of Kevin Custer and Chuck Kelsey - WB2EDV
Board layout by Chuck Kelsey.
HTML May 24, 2016, W3KKC All Rights Reserved!