In the early days of cubes, they were either single frequncy xtal controled or tunable VHF VFO's. The problem with the tunable VFO's was that they were not that stable and required constant retuning. The problem with the old single frequncy cubes is that you had to know the customers frequncy (for Jerrold or SA) before shipping, and that you had to build different cubes for different frequencies.

We at Group 42 solved the problem by designing a better cube. It is a harmonic generator that will hit all Jerrold frequncies, and can be easly adapted to SA and Pioneer Units. To date it has been widely copied in the grey market cable industry, making it the most popular FSK design.

The schematic is here.

R5 is not necessary unless you are leving the unit hooked up full time. D3 is a 1n914 diode.

It operates by taking the Manchester Encoded digital signal from the microcontroler and feeding it into the xtal oscillator driving the HC4060 chip. The voltage from the microcontroler is droped by D3 to .7 volts creating either a zero or .7 volt image accross D2 the variable cap diode. This changes the capacitance in the xtal oscillator driving the HC4060, thus pulling the frequency of the xtal. This creates a frequency shift, thus FSK.

The xtal oscillator is divided down to a 500khz (using an 8Mhz xtal) square wave that is feed out an F-Connector and into a cable box. Since the output is a nice square wave we get lots of harmonics, every 500khz up to 200MHz and beyond. This means a signal is present on all 3 Jerrold frequencies (97.5, 106.5 and 108.5 Mhz).

It can be adapted to other frequncies by making sure that the divided down value (ie 500khz) is dividable by the desired frequencie.