This is a very simple frequency counter. The original design comes from http://pandatron.net, but the site is currently unavailable. The heart of the device is PIC16C84 microcontroller and SAB6456 prescaler.
Frequency range: 5Hz - 30MHz
Sensitivity: < 100mV (typ. 30mV)
Input resistance : 1M
Resolution: +/- 1Hz
Frequency range: 20MHz - 1300MHz
Sensitivity: < 100mV (typ. 10mV)
Input resistance : 50
Resolution: +/- 100Hz
Input C [TTL]:
Range: 0 - 999s
The scheme of the frequency counter can be found here. On the bottom of the page you can find zip file containing the detailed description of the whole design (unfortunately not in English), including PCB design and source code for PIC. A or B input signal after forming and protecting circut is selected by 74HC4053 multiplexer and is directed to PIC. Input C feeds directly INT pin of the microcontroller. The whole device is controlled via 3-button keyboard and results are displayed on the standard 2*16 LCD.
The case is a standars steel box. Together with my frequency generator and digital oscilloscope it creates quite a nice laboratory measurement set.
Files and links: