PIC Frequency Counter with Frequency Lock

This PIC software combines frequency counter and frequency lock functions. By adding couple of transistors and operation amplifier TL082, it is possible to lock the LC oscillator frequency. Frequency reference is formed from the measured frequency itself after the delay defined by parameter 0Dh, when the number of the consecutive samples (defined

by 0Eh) of measured frequency are within the +/-20 Hz. Then the actual value is trigged as frequency reference until the SW detects that the lock conditions are not valid. Frequency is measured every 100 ms. Next the frequency actual is subtracted from the reference and the difference is compared to value zero to calculate the deviation. If the result is zero, it means that no deviation and also no need to fine-tune the oscillator frequency. If the result is negative it means that the frequency actual is higher than reference. This detects the direction of the needed frequency correction. Next the rough value of the difference is calculated. If within 20 Hz then only short 2. 4 ms pulse is controlled. If bigger then 100 ms pulse is controlled. By means of these few calculations we have information how to fine-tune the frequency of the oscillator to hold the frequency actual equal as frequency reference. Simple, is it These pulses are controlled to digital outputs RB0 or RB3 according to the sign of the frequency difference. The outputs are never simultaneously on because it means almost short circuit. The controller is made using TL082 operation amplifier. The first amplifier is an integrator by means of the capacitor C8 and R23. A time constant is R23 * C8 = 22 M Ohms * 2. 2 uF = 48 s. So the control is slow and it only fine tunes the oscillator frequency and this is of course the purpose of the controller. The VFO itself...

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