Crystal Heater How to Build

radio8z · 2012-02-22T19:30:16Z

Recently my part 15 AM station has been sounding a bit less than perfect audio wise when monitored on a digitally tuned receiver. Checking things revealed that the transmit frequency was low by 40 Hz. Initially, it was set exactly on frequency by using a trimmer capacitor so why the drift? The transmitter is mounted in a 4 x 4 inch electrical box and when the lid is in place the components heat slowly to about 105 degrees. This rise above the usual ambient of 75 degrees produced a downward drift of 40 Hz which is not acceptable. The drift was so slow that the transmitter would take two days to settle on a frequency. A means to deal with this is the subject of this post. Starting with a strip of shim brass a crystal heater was developed. The brass was cut and bent so that it would fit tightly around the crystal can. A thermistor and two resistors were then affixed to the brass with epoxy. This is shown in the first two pictures below. Next, a control circuit was developed to control the crystal temperature. This is shown schematically in the third picture below. More on this later... The fourth picture is a view of the assembled control circuit and the heater. The circuit was built using the "dead bug" style. The fifth picture is of the heater and circuit installed in my transmitter. This transmitter was detailed in a previous post here:In this view the control circuit is seen at the upper right of the main board and the heater is installed on the crystal. The heater controls the crystal temperature to within +/- 1 degree F of the setpoint and the frequency of the transmitter is now stable to within +/- 1 Hz. The frequency from a cold start is high by 60 Hz but is stable and on frequency after two minutes of operation. The reason it is important to keep the frequency of an AM transmitter to within +/- 10 Hz of nominal is that the signals from other stations will heterodyne and cause an audible tone with a frequency equal to the difference between the two signal carrier frequencies. If both (or all) on channel carriers are within 20 Hz the beat will be subaudible though even so some mixing artifacts can usually be heard as a growl. The operating temperature is set by R1, R2, R3, and T shown in the schematic. T is a thermistor that was salvaged from a PC power supply but any device with a resistance around 1k to 2k ohms at room temperature will work. In this case the T resistance was 1.7k. The target operating temperature was 110 degrees F so a 1K resistor (R1) was used based on the thermistor resistance being about 1k at 110 degrees. Almost any NTC thermistor will work if R1 is chosen to be near the thermistor resistance at the operating temperature. This is not critical but is simply a design guide. The temperature can be changed by changing any of the resistors (R1 thru R3) and in this design R3 was varied to adjust the setpoint. Lowering the resistance of R3 increases the temperature and conversely. The final value of R3 was determined by experiment to give an operating temperature of 110 degrees. This was chosen to be above the highest inside the box ambient which was measured to be 102 degrees. A potentiometer could be substituted for R2 and R3 to make adjustment easier but experience has taught that pots are not always reliable in DC circuits so fixed resistors are preferred. The LED gives an indication for heater status, lighting when the heater is on. This is handy but the LED is important to drop the voltage from the output of the op amp which is not zero in the heater off state. The drop across the LED insures Q1 will turn off when it is supposed to. Nothing in this circuit is critical and leeway for parts values can be used. The values for R5 and R6 were determined experimentally to give a maximum temp. of 120 degrees when constantly powered. This is to prevent a "melt down" in case something fails. The circuit was optimized for a 10 volt supply but higher voltages can be used and R5 and R6 values may have to be changed. The best operation occurs when the heater on time about equals the off time but this is not critical. It is hoped that enough detail has been presented for those who are interested in building this project. If not post your questions and I will try to help. Neil

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Anonymous

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both. hell that thing looks as good as a manufactured and certified tx.

Posted : 05/03/2012 6:08 am

Anonymous

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Give me a few days for this. I have a pretty schematic but the PCB layout and parts placement need a bit of work to be usable. This is because this unit was part prototype and part final production and there were some things which didn't fit as intended and there were some parts value changes.

Neil

Posted : 05/03/2012 7:42 am

Anonymous

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I suggest putting it all in the forum library. The library needs more content in there for AM stuff. Actually it could use much much more content considering.

RFB

Posted : 05/03/2012 7:47 am

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