The schematic is fairly simple and straightforward and uses a PIC16F628A microcontroller for measuring frequency and a high speed comparator for signal amplification and conditioning. Timer0 is used to count the input signal at pinRA4. In order to achieve the maximum possible resolution, the input signal is probed for 0.125 seconds and the prescaler value is computed accordingly. The input of the schematic is feed through a high speed comparator.
The two inputs of the comparator are set at about 1/2 of power supply voltage with 15-25mV difference between them so any AC signal with higher voltage will start switching the comparator. The input is protected with 1k resistor and two diodes limiting the voltage to ±0.7 V. The input impedance for low frequenciesis equal to R1 – 47k. For VHF range maybe it is good idea to replace it with 50Ohm value. The schematic have fairly low power consumption – with no input signal the supply current is 7-8mA and goes up to 20mA with 200+MHz input signal.