Figure 1. Basic radio Teletype carrier frequency shift transmit system.
Figure 2. One complete loop with the electronic flow from the negative side to the positive side of the power supply.
Figure 3. The frequency of the carrier wave increases and decreases corresponding to the mark and space signals
Figure 4. The direct current mark and space signals are changed by the keyer unit into frequency shift intervals (nominal carrier plus and minus).
Figure 5. Radio frequency signal is shown leaving master oscillator in transmitter and going into keyer unit where carrier frequency is modulated with mark and space signals.
Figure 6. Carrier frequency shift system transmit showing how the direct current mark and space signals are changed by the keyer unit in frequency shift intervals (carrier plus and minus).
This is the fourth in a series of articles that describe the Navy's radio Teletype systems and how they operate. The previous articles are in the Bureau of Ships Journals of November 1955 and April and September 1956.
The Navy's radio Teletype systems afloat are designed to provide fast, reliable, printed communications over a great variety of distances. A complete installation includes a tone modulated system for short-range signals, and a carrier frequency shift system for long-range communications.
This article on the transmitting components of the latter system is primarily about the keyer-transmitter combination and the nature of the signals it sends out. The keyer may be built into the transmitter or it may be a separate component. In this article it is shown as a separate component.
Figure 1 shows the basic components used for transmission of the carrier frequency shift system. These components are: Power supply, Teletype panel, switching control, and teleprinter, as well as the Keyer and transmitter.
The direct current furnished by the power supply is known as the looping current. The term "looping current" originated from the series or loop path that the current followed through the equipment components.
In figure 2 the current can be traced from the negative terminal of the power supply through these components back to the positive terminal of the power supply.
The teleprinter may be visualized as a switch that opens and closes the looping circuit. This feature causes on-and-off pulses of the direct current. These pulses are known as "marks" and "spaces."
The teleprinter's direct current signal of "marks" and "spaces" moves through the switching control and one channel, or loop, in the Teletype panel, and then on to the keyer at the transmitter. In the keyer, the signal is changed into a corresponding sequence of shifted frequencies that are impressed on a radio carrier wave for transmission.
The same message is in two types of signalsódirect current and frequency modulated radio waves. The relationship is readily seen when the radio signal is paralleled with the corresponding Teletype signal (figure 3). Basically the transmitted signal is similar to the signal transmitted by the familiar FM radio stations.
The marks and spaces are formed by frequency shifts either higher or lower than the carrier frequency which is never transmitted.
The mark is the carrier frequency plus the modulating frequency - called a marking shift. The space is the carrier frequency minus the modulating frequency - known as a spacing shift. Thus the frequencies actually being transmitted are always higher or lower than the carrier frequency from which the shift is reckoned.
The carrier frequency itself never reaches the antenna because the keyer receives the carrier frequency from the transmitter and the direct current on-and-off pulses from the teleprinter. These pulses automatically shift the basic frequency to "plus" for marks and "minus" for spaces. Then the frequency shifted carrier wave goes back to the transmitter.
The changes take place in the following manner:
To summarize, in the transmitting function of a carrier frequency shift system for teletype communication,
direct current pulses from the teleprinter pass through the switching control unit and Teletype panel to
the keyer, where the basic oscillating radio frequency from the transmitter is changed to a
frequency shifted carrier wave, and fed back into the transmitter, amplified, and
then fed to the antenna where the power is radiated into the ether (figure 6).
The same message has two types of signals in the system. The frequencies transmitted are always increases or decreases of the basic carrier frequency that is never transmitted as such.
This is the complete basic pattern of operation that applies to this group of equipment. In this type of transmitter, the keyer is a built-in component, but it may be a separate unit.
This is the complete Navy carrier frequency shift transmitting system for reliable long-range radio-teletype operations.