RCI-8719 & VCO
Whether or not the VCO can handle the additional 40 channels above and below the regular channels has nothing to do with the RCI-8719 PLL IC itself. It is a function of the synthesizer design that was done around the chip for the particular radio. To be precise it is a function of the VCO design.
a VCO is basically a free running oscillator equipped with a variable capacitance diode (a varactor) to keep it on a particular frequency. The varactor diode is coupled to the VCO's oscillator LC tank by means of a small coupling capacitor. The more tight the varactor is coupled to the oscillator LC tank, the larger the VCO range is over the full control voltage range. (the type of varactor used is also a parameter which determines the ultimate VCO range.)
The good news is that from the above it follows that the VCO range can be expanded if it turns out not to be enough by default, by simply increasing the value of the coupling capacitor, or by replacing the varactor with a different type having a larger capacitance range, or both.
Keep in mind that when doing this the VCO frequency range will increase, but the VCO center frequency will shift downward, which will have to be compensated for by reducing the oscillator tank inductor (usually by readjusting the ferrite core but if this is not enough a turn will have to be removed from the oscillator tank coil)
As a general remark, after studying the RCI-8719 chip data sheet, I have come to the conclusion that the chip is basically in the Motorola MC145106 family of PLL chips (of which the PLL02A is probably the most well-known family member). Although a reliable performer, this kind of chip design is more than 30 years old and I was stunned to see that companies like Ranger and Galaxy still use this ancient chip. The disadvantage of this PLL chip over newer versions is that it cannot handle frequencies much higher than 5 MHz on the input of the programmable divider, which as a result always requires the VCO frequency to be downmixed first with the help of a fixed crystal frequency before the programmable divider can digest it.
In this day and age of cellphones operating in the 1.8GHz band, and all kind of other consumer products operating much above 1 GHz (like satellite tuners, 802.11 wireless LAN and bluetooth devices), PLL synthesizers exist in modern CMOS technology that can divide-down these frequencies without the use of any mixers and even prescalers. So using these modern chips would eliminate the PLL crystal oscillator used in the CB radio's VCO downmixer. Cost can not be the reason either, as these modern CMOS PLLs are probably cheaper than the RCI-8719 chip itself. It is like time has been frozen to a standstill in the CB world...:sad:
