The basic idea comes from the AN592 Microchip application note: `Frequency counter using
PIC16C5x`. where you may find a simple software wich implements a frequency counter using a
PIC microcontroller.I wrote a specifically designed software to improve the counter resolution, to handle the IF mode and value by means of an operating menu, to decode and edit the read frequency on an LCD display. The result was a simple and effective device, equipped with a free software available to those who could be interested.
The electrical schematic is very simple, given that most of the functions are implemented by the
microprocessor. It was needed only an amplifier stage to raise the input signal level from 200-300
mV p.p. to about 3 volts p.p., so as to drive correctly the RA4 (pin 3) triggered gate of the PIC. I
implemented a common emitter amplifier using a 2N2369 transistor, with a small inductance series
connected to the collector load, so as to improve the frequency response at the high frequencies. So
it was obtained a suitable gain from 100 KHz up to about 50 MHz, the lower limit being forced only
by the C10 capacitor. The R8 value is chosen so as to obtain about 1,6-1,8 V on the transistor
collector, such a value is necessary to drive correctly the PIC gate, and you may verify this voltage
after completing the assembly, and before inserting the PIC on its socket.
The time base is provided from a 4 MHz, parallel resonant, microprocessor crystal, if you have at
your disposal a professiona l frequency meter, you may tune accurately the frequency by adjusting
the value of C9, which could also be replaced by a little plastic trimmer, otherwise the reading will
be in any case within the quartz tolerance (typically 50 p.p.m. max).
The 78L05 regulator is well suited to feed the 15 mA required, however if you want to employ a
back-lighted LCD module, it will be necessary to replace it with a 7805 model, capable to supply
about 60 mA without excessive heating. On the 16 pin connector two pins are provided (15, 16) to
drive the LCD LED panel. The supply voltage should be in the 8-12 volts range, and you may
control the display brightness by turning the R9 trimmer, the maximum value being obtained with
the cursor completely turned toward the ground.
Voltlog #14 - Frequency Meter Kit Assembly
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