VFO Frequency Expansion    



Back in the early days of the CB hobby, when an avid CB enthusiast looked to break away from the limited capabilities that a stock 23 channel radio offered, one of the most sought after modifications was the expansion of frequency coverage beyond the "legal" channels.  Previously I've outlined the simple bonus Channel 22"A" modification, as well as adding or swapping crystals to gain additional channels, and SSB clarifier expansion. These mods were relatively simple and fairly inexpensive and provided a little more elbow room, and with them, some prospects for escaping the general chaos that often pervaded the regular CB channels.  But for many people, these simple mods were not enough.  Indeed, it would seem that many of us appear to be driven, by some invisible force, to push the limits of whatever activity we are currently engaged in (Extreme CB?). Then there's the "King of the Hill" mentality. That irresistible drive to be the fastest, or the strongest, the guy with the most power, or the guy with the most channels.

When those other channel mods just didn't do it for you anymore, you were left with few alternatives. Some people, who had deep pockets, opted to run modified Ham gear on the CB band. There were both advantages and disadvantages to going this route. A ham rig will give superior performance on SSB, but most contemporary ham gear at that time, was not designed with AM operation in mind (The Yaesu FT-101 series being a notable exception). They also required re-tuning when large frequency changes were made, which was intimidating for those operators who were not all that technically inclined.  So for many people, the second alternative, adding a VFO to an existing CB radio, was a more attractive option. You maintain the ease of operation and familiarity of your favorite CB radio, while allowing a huge increase in frequency coverage. Plus there was the illusion of piece of mind should "Uncle Charlie" come poking around. Just unplug the VFO and stash it away in some hidden location and you were back to a "stock" radio.  Of course most of the people who thought this way didn't realize that FCC agents were trained engineers, and would not likely get fooled so easily. But that's another story.

A VFO, or Variable Frequency Oscillator, is a device which generates a frequency which could then be varied within a specified range.  The theory of a VFO was nothing new, at least not from a receiver standpoint.  Many older CB radios had built-in "tunable" or variable receivers, which tuned through the CB channels much like you would tune an AM/FM broadcast radio.  The difference here is that variable frequency receive is legal, while variable frequency transmit is not. At least one company (Hy-Gain) offered an accessory VFO which mated to a few of its CB models through a rear panel jack. To meet FCC requirements, the unit only operated on receive.  But it was a simple matter to modify it to work on the transmit side as well.  There were also a few companies who offered accessory VFO's which could be mated with most CB's (Most notably Siltronix).  When adding a VFO to an existing CB radio, the new variable oscillator is usually substituted for the main crystal oscillator in the radio, so you had to order the specific model VFO, which matched the oscillator frequencies of your radio. A technically competent technician would then install the unit such that when the switch was thrown, the channel selector no longer controlled your frequency, that job now becoming the responsibility of the VFO dial.  Depending on the range of the VFO and bandwidth capability of your radio, you could now have up to hundreds of Khz of new channels on tap.

Running with a VFO was not all peaches and cream however, and there were some potential problems.  Most of the problems were the result of poor installation.  But there were some VFO's, which were not of the best quality and consequently suffered from frequency drifting.  Frequency drift is not something that you want, especially on SSB. Also, some radios just did not have the necessary bandwidth to transmit and receive well over the entire range of the VFO. Many people became disappointed after purchasing a VFO which promised to give range all the way up to the 10 meter ham band, but finding that their radio stopped transmitting any usable power above 27.500.  Correcting this condition was not always easy or possible in some cases. You could gain some additional range by "stagger" tuning the transmit and receive strips, but usually at the cost of losing maximum sensitivity and transmit power.  Other, more serious engineering solutions could also be applied as well.  But the end result was usually a radio which was less stable, more prone to generating RFI, and receiving interference.  Often the cost to properly correct such design shortcomings would approach the price of a decent ham radio.

When the digital Phase Locked Loop (PLL) radios came on the scene in 1976, a new era in frequency expansion dawned, and the VFO was soon relegated to the back shelf with the old tube radios.