Frequency Locked Loop for HF

The principle of operation is quite simple, a counter measures the VFO frequency, the new reading is compared with the previously stored value and an adequate correction is applied consequently. This process has been implemented in the past using several discrete TTL logics, now a single microprocessor can do the job much better, moreover offering a digital readout capability. The best way seemed to be a D/A converter, driven by the PIC so as to produce a control voltage towards a varicap diode, but it would have required at least a 10 bit (better 12 bit) resolution, a component not so cheap nor so easy to find. So I considered to employ a different microprocessor, the PIC16F628.
Frequency Locked Loop for HF - schematic

This device offers a built-in PWM module capable of delivering a 4 KHz square wave whose duty cycle may be software controlled with an accuracy of 10 bit. It was enough to add a simple RC integrator to obtain a well filtered control voltage, variable from 0 to 5V with a 1024 steps resolution. Such a circuit may control the VFO frequency in a 15 KHz range with a 15 Hz unit step, notice that a single frequency adjustment is applied only when a certain drift amount is measured and, unlike a PLL system, there is no stability problem, no elongation or settling time, so it is possible to obtain a quite clear” VFO signal, with a very low noise level. Just what I was looking for. In short, the main performance of this device are : · Frequency stability aligned to the reference crystal, with a maximum +/- 15 Hz drift · Digital frequency readout, with programmable IF value and 10 Hz resolution on the LCD display · Manual frequency tuning, trough a multi-turn potentiometer · Automatic Lock/Unlock function, unlock status is signalled by a LED diode and entered during manual tuning or in case of an excessive VFO drift · Programmable lock range. It is the maximum allowed value for a short term” (0.2 sec) frequency shift without exiting the locked status. This improves the capability to suit to various VFO circuits. · The overall (long term) drift compensation capability is limited to about 15 KHz (+/- 7.5 KHz),...

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