PLL
Frequency Expansion
In previous articles, I've reviewed some of the most popular methods for frequency expansion which were used when an avid CB hobbyist wanted more from their radios than the legal 23 (and then later 40) channels would allow. The traditional methods of adding additional frequencies; Channel 22A, Adding crystals, Clarifier expansion, and adding a VFO, have all been covered. Each one had its own unique appeal, and each had their associated drawbacks. But that was what we had to deal with if we wanted to travel the path of the high performance CB crowd. But starting in 1975 our limited options would begin to improve as technology would give us yet another, and much more robust and interesting, method for frequency expansion.
In the early days of CB radio, frequencies were generated by mixing a number of crystals together to create the 23 channels. Originally this method of crystal synthesis was a technical and economic boon to radio manufacturers, who could provide the full 23 channels with as few as 12 crystals. This method remained pretty much unchanged for over 10 years. But as talk of the FCC considering a CB band frequency expansion started gaining in popularity (And the original proposals called for as many as 99 channels), the radio manufacturers realized that the number of crystals needed to support these expanded channels would become cost prohibitive and technically cumbersome. There had to be a better way.....
In 1975 there emerged a new frequency synthesizer method. This technologically advanced synthesizer was called the Phase Locked Loop (PLL) and it appeared initially under some clever marketing names, such as Royce's "Gyro Lock". How it worked was conceptually simple: A single voltage controlled oscillator (VCO) generated a frequency. This frequency was then digitally divided, through a series of programmable counter chips, down to 10 Khz. This 10 Khz frequency was then phase compared against a 10 Khz reference frequency, derived from a single master crystal oscillator. Any difference in the phase between the two 10 khz frequencies resulted in an error voltage being generated, which adjusted the VCO up or down until the two frequencies were "phase locked". You changed channels by changing the programmable divider count. Since the divider output needed to be 10 Khz, the master frequency would have to jump to whatever frequency, divided by the counter value, would equal 10 Khz. This synthesizer could be made with as few as 1 crystal (usually 2 or 3 were more common). Receiver offsets and SSB operation would require a few additional mixing crystals. But this was still a major savings over the old method. Not to mention that many more frequencies could be generated. But I'm getting ahead of myself........
The original PLL designs used individual discrete chips to perform the necessary functions, but less than a year later, fully integrated purpose-built custom PLL chips emerged on the market. Radio manufacturers slowly started replacing (The RS TRC-152 replaced the TRC-52, and the Midland 13-882C replaced the '882B etc.) or augmenting their current lineup of 23 channel crystal controlled radios with the new PLL designs. The FCC had not yet decided on a final plan for band expansion, but the radio manufacturers were slowly getting ready. The real beauty of this synthesizer scheme was that you selected frequencies simply by encoding one of a series of binary "words" created by the channel selector. A radio could be made as a 23, a 40, or a 99 channel unit simply by changing the channel selector switch.
Now we'll take a minute to talk a little bit more about the fun part. As I said before, the method for controlling frequency division is determined by digital logic (divider) circuits within the chip. These counters normally used binary numbers (The Uniden UPD-858 PLL used BCD) to program the channels. The total amount of frequencies available in a given PLL chip could therefore be determined by taking the number 2 and raising it to the power of the number of programming pins the particular chip had. For example, a chip with 6 control pins will have 64 total frequency combinations available (2^6=64). The current 23 channel band plan required a chip which supported at least a 32 channel capability to be able to cover all the current channels. But the radio manufacturers were looking forward. They were gearing up for that ever looming frequency expansion, so most radios contained chips which supported at least a 64 frequency capability. The PLL's frequency capability was limited only by the programming of the selector switch (And the lock range of the VCO). In theory (and also many times in practice), you could expand a 23 channel radio into a 30, 40 or 80 channel radio by simply swapping out the channel selector for a different one (Provided the radio's bandwidth could support the wider array of channels), once the final expanded band plan was approved. At least one company (CPI) produced a radio with 2 selector dials which went from 00 to 99. They then sold the radio with the promise that it would never become obsolete, and additional frequencies could be enabled by a factory authorized technician when they became legal. A clever reader should already see where this is going.......
While this scheme made for an easier transition over to whatever new band plan the FCC would finally approve, while not stranding manufacturers with obsolete stock, it also opened the door for technical "hackers" to start experimenting with the expansion of channels without waiting for authorization. Initially, many of us looked with disdain at these newfangled PLL radios. We didn't yet understand them, and they didn't respond to the usual expansion mods that we all grew accustomed to for crystal controlled radios. But while trying to put Channel 22"A" into a PLL radio, it was discovered that while channel 22"A" was not available in the usual place, there seemed to be a few other frequencies found instead. The earliest 23 channel PLL radios allowed the channel dial to be "wiggled" between the various channels, which resulted in unpredictable binary words being created, and new channels along with it. Many people found several "extra" channels that way, and it didn't even require the radio to be opened or modified in any way. The next generation radios would be equipped with a defeat mechanism which would disable the synthesizer or the transmitter if the channel selector moved out of the individual channel detent positions, which rendered the wiggle method ineffective. But by then the genie had already been left out of the bottle.....
I learned the secret of the PLL in 1976 while visiting with Windbreaker, a member of the Channel 6 group, and who was also also a physicist. He explained binary counting to me, and things suddenly became much clearer. I then sat down with a Midland 13-830 (Courtesy of a very trusting LIM), which contained a 23 channel Cybernet chassis and the venerable PLL-02a chip. I counted 6 active control lines connected to the selector switch (There are actually 9 total), and I got out my trusty VOM and started measuring the voltage on each of those pins starting at Channel 1 all the way up to Channel 23, and made a truth chart showing the 6 pins and whether it had 5 volts on it (logic 1) or 0volts (logic 0) on each channel. When I was finished, I noticed several things. The first being that the pattern skipped every 4th channel (The RC channels). Supplying the missing binary word for those skipped patterns revealed that the radio would now operate on the RC channels (Neat!). I also noticed that there were more binary numbers to go after Channel 23. Unfortunately the highest number (63) was only 2 channels above 23 (27.275). But I also noticed that there were a great deal of binary numbers below Channel 1. Thus began the practice of adding an external switchbox with those Radio Shack 3 position center-off DPDT mini-toggle switches to connect to significant PLL programming pins. Used in conjunction with the channel selector, you now had access to many more channels than were authorized.
When the FCC finally decided on the 40 channel band plan, and the new radios became legal in 1977, further expansion of the PLL continued in earnest. It was like being a kid in a candy store. With nothing more than a few dollars in parts (switches), you could add 25, 50 or 100 or more channels. And unlike many of the old 23 channel crystal radios, the bandwidth on the new 40 channel radios was considerably wider, allowing a typical coverage from -15 channels below Channel 1 all the way up to 28.045 with little loss in transmit power or receive sensitivity depending on the model of the radio. Then there was the stability factor. Previously, the only way to gain this many channels on an old 23 channel crystal set, was to use a VFO. But when using the old VFO expansion technique, there was usually the issue of drift to contend with. In sharp contrast, PLL radios were as stable as the reference crystal on each and every channel. With all this potential, it didn't take long for this popular mod to spread like wildfire among the local CB'ers. At one point, it seemed that more people had extra channels than those who didn't. The competitive "King Of The Hill" mentality drove many people to try and squeeze as many channels as they possibly could out of the radio, (even if they never used most of them) in order to earn bragging rights as to who had the most channels. There were all sorts of switching arrangements. Some, (like me) preferred to use externally mounted switch boxes, to control the PLL. Other people preferred the "stealth" mode of reassigning existing feature switches (Noise Blanker, PA etc.). It seemed that every tech had a "signature" method of adding the extra channels. In 1979, I went totally crazy and designed a digital frequency readout/sequencer box, which I used to replace the channel selector completely. This was accomplished by using an external box which contained 7 segment LED's as a frequency readout, and an UP/DOWN button set to move up and down in frequency using CMOS counters and adder chips.
Naturally, with the proliferation of this easy to perform (but still highly illegal) modification, the FCC eventually caught wind of it. They were rightfully concerned with the number of CB'ers who were moving further and further out of band. As a means of throwing some cold water on the situation, the FCC would revise the technical standards for CB radios and mandate that radio manufacturers utilize newer design PLL chips which were designed exclusively for the CB market. These new chips isolated the binary counters from the outside world through the use of a ROM decoder. The internal ROM decoder would only recognize the pin combinations which produced the legal 40 channels as valid. Any other combinations would disable the synthesizer. This effectively slowed the tide of frequency modifications. But they did not go away. Eventually, some clever radio hacker types found new ways to "trick" the PLL, and add some extra frequencies. While these frequency expansion tricks were not as easy as the older PLL radios, they did provide a solution for those seeking that ever nagging desire for more, more, more....... Eventually, export radios and the so-called "10 meter" (Freeband CB in disguise) radios hit the market, making the desire to modify legal radios much less a priority.