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                                                   CCWN 75:5
                                                   
FREQUENCY STANDARD TEMPERATURE COMPENSATION by Chas Woodson

        I have built two of K4EEG' S "Universal Frequency
Standards" (Ham Radio, February, 1974, page 40) and had excellent
success with both. They have unusually stable oscillators but
even these oscillators need temperature compensation to meet the
accuracy requirements of CCW. A little effort with temperature
compensation has made the standards stable well within one cycle
at 10 MHz or 10 under ordinary conditions.

         To measure temperature I used an ordinary thermometer
attached by tape to the metal box in which the standard was
housed. I varied the room temperature from about 15 to 26 degrees
C by leaving the window open or by placing a reading lamp near
the standard. I found it took nearly an hour for the standard to
stabilize after a change in room temperature. The metal box
obviously has insulating properties as well as shielding
properties.

        Observations were made by adjusting a small variable
capacitor for oscillator zero beat on WWV at 10 or 15 MHz, and
then recording the capacitance (in degrees of the rotator from an
arbitrary point) and the room temperature. I vary the strength of
the oscillator signal to match that of WWV by adjusting the
length and placement of an "antenna" (about 1 w output of the
standard. The observations of temperature and relative
capacitance were then graphed and the trends were examined to
infer the type of compensation changes needed. Each graph is
labeled with the capacitance combination used and after a few
graphs the needed changes were obvious.
                                           
                                            
        Compensation is achieved by changing the temPerature
compensation coefficient of the total capacitance in series with
the crystal while maintaining the total capacitance at the
same value. For example, if more capacitance from the variable
is needed for zero beat with increasing room temperature, then
a decrease in the amount of negative temperature coefficient
capacitance in the total capacitance used is needed.

        I found my local TV parts store had a supply of NPO,
N1500, and N750 ceramic capacitors in values of 10 ppf and
below. I suggest purchase of 4 or 5 of each available value.

        The standards are so stable -that it is possible to chart
the changes in the received frequency of WWV. In case you
didn't know, as the propagation conditions change, e.g.,
the F layer goes up, the received frequency of WWV changes.
When the layer is rising, the frequency is lower due to the
dopler effect because, electronically, the transmitting station
is getting farther from you. I have observed changes as large
as .5 x 10-7 and these are easily noticed in the average of
several observations over a few days.