The wonder machine Adding the versatility of parallel-port switching to a digital phase-locked-loop (PLL) makes the basis of a complete receiver or transmitter with software-controlled selection of any frequency.
Having successfully converted the old IBM (or compatible) computer into a multimedia device, industry leaders in the computing field have started to loudly proclaim their “wonder-machines.” These devices will consist of everything from telephone “video-conferencing” through some hyperactive communications and data bus in the computer, to downloading Dow Jones information through “Windows” and plotting data to various charting software. Simultaneously, considerable action is taking place in the microprocessor field as the “work horse” is being trained to perform faster and more efficiently at lower voltages. In all, the world of the computer is becoming even more interesting. Although a great deal of “hype” can be observed in the computer industry, there is a notable lack of similar activity in the RF industry. We still are dealing with RF carriers, frequencies and modulation techniques that are easily traced back several generations. In some cases, however, we have learned to “co-exist” with microprocessors. Trunked radio and cellular have demonstrated agility in their compatibility with rapid switching techniques. Spread-spectrum media are finding favor with “smart” controller logic, and RF modems are in common use. But what about “plain Jane” communications?
Icandeau Several years ago, I was experimenting with the old Commodore C-64 computer as a means of controlling several devices simultaneously. I could either dial a telephone or conduct a telephone conversation, control any of four independent ac line-operated devices, translate “dit-dah” spoken into a microphone into the letter “A” or control the main menu to run several programs through the use of a sound-activated switch. I dubbed the thing “Icandeau” and tried to develop useful routines for the physically handicapped. Somewhere in the offices of United Cerebral Palsy in Sioux Falls, SD, copies of my various routines are still floating around. What my experiences using an 8-bit parallel port taught me was the remarkable agility of combinations of software and hardware. Routing various commands into specific bit locations allowed the software to control an output from other bit locations. On an eight-bit bus, I could use two bits as inputs and use the other six bits to control various pieces of hardware. The software provided the decision-making as it translated a telephone number, such as 123-4567, into corresponding relay impulses to control a dialing function. By running the software through a relatively simple filter, I could translate telegraphic ones and zeroes into screen-readable alphanumeric messages. Software control would place the user into either a telephone or telegraph mode, allowing use of the single input from a switched input line to yield the desired results. Although the original Icandeau controller box took about 21/2 cubic feet of volume, later versions of this prototype were envisioned as simply plugging in to the parallel port at the rear of the C-64. Time and circumstances have long since brought this device to its grave.
Port + loop = radio Even so, the memory of that parallel port’s capability has stayed with me. Adding the versatility of parallel-port switching to a digital phase-locked-loop (PLL) makes the basis of a complete receiver or transmitter with software- controlled selection of any frequency within the PLL’s locking characteristics. As the parallel port widens, encompassing more bits of input or output control, it becomes possible to perform both frequency and transmit-receive transitions. Not only can you receive a signal at, say, 150MHz, but you could offset the PLL to transmit at the same frequency. Another bit in the parallel port could easily control AM or FM modulation characteristics of both transmitted and received signals. Now, let’s assign the space-bar on the keyboard to control the “push-to-talk” function. Here, the software runs through a list of available options and finds that the space-bar will toggle the PLL and receive-transmit functions from one to the other when the space-bar is activated. This might not sound like much, but it is the same technique used with almost all of the existing two-way communications equipment. Let’s take a further look at what could be. A typical PLL radio transmitter-receiver combination would have the potential ability to: operate at any frequency within the band and step characteristics of the PLL. set frequency offsets to accommodate various IF frequencies. choose modulation techniques for both receiving and transmitting, or to use cross-modulated signaling (AM receive, FM transmit, for example). operate as a software-driven scanner when in receive mode. operate as a software-driven “spread-spectrum” communications system. use its software to generate encoded signals to open a remote receiver upon command. exhibit extreme versatility when used to selectively call individual receivers. The list continues. Let’s combine the frequency agility with the use of tone decoding and make a software-driven “trunker.” Then let’s take the binary ones and zeroes that modulate the carrier and/or audio tone and use them as an ASCII modem combination so the keyboard can communicate via alphanumerics. Next, let’s take the software and do a POCSAG decoding routine . . . . You get my point. The actual hardware in this case is little more than what you could obtain from Radio Shack in a PRO-series scanner. Selectively bit-flipping the oscillators in the scanner would allow a carrier to be generated at virtually any frequency the PLL is capable of generating, so you have the “exciter” portion of your transmitter. It all looks so good on paper!
Square-wave noise Then you realize the computer is generating a lot of square-wave noise (which ruins the effectiveness of the receiver) and the limited speed with which a PLL will respond to a frequency vs. bit pattern transition. Among the costs of shielding, filtering and software operating speed, there are some definite limitations to the hardware. There are, however, others who share the dream of a complete communications system residing (peacefully) within the confines of a computer. Motorola has been using such a technique successfully in mobilized bar-code reading equipment (as I outlined in the article “Designing for Power: Using Switchmode Supplies” in the February 1996 issue). Motorola’s answer is to use a crystal-controlled transmitter and receiver, which is suitable for such an application. But when it comes to the requirements of the Federal Aviation Administration for channel selection in aviation communications and the similar requirements for marine communications, the next big step in communications equipment probably will be found in “comp-unications” similar to what is described above.
Expansion port unit Such equipment will be built into a unit that will slide into an expansion port in a standard computer. On-board will be an extremely versatile, PLL-operated, frequency-generating block that probably will demand the use of some 12 bits of the available 32 bits in the operating system. Remaining bits will control transmit-receive functions, activate on-board volume controls to various output levels, determine modulation type (receive and transmit), decide the use of voice or data communications, and allow frequency-spread or hop techniques for spread-spectrum generation. Software (which could be modified at the point-of-sale) would severely restrict the end-user to a fraction of the available capability, but it would certainly cause amateur radio operators to become avid programmers! Power for such a device probably would take the form of one of my favorite devices–the switchmode power supply– which would be used to charge an external 12V battery. Regardless of the application, the battery would receive a charge as long as the computer is turned on. When turned off, the logic to the transceiver board also would be inactive.
Decoding capability Other enhancements probably would include software-driven logging capability and identifying “signatures” of received stations. The decoding capability of the computer certainly is going to be used when a fleet of vehicles is being monitored. Conversely, an individual within the fleet could be signalled without “bringing-up” the entire fleet by the use of a “universal” and “individual” signature assigned each unit. In all, I’m looking forward to such devices. They keep the gray matter from growing rusty and will challenge and intrigue the innovative minds among us.