Quantcast Frequency Multiplication oscillator frequency to the required output frequency by passing it through one or more frequency multipliers. ">

Share on Google+Share on FacebookShare on LinkedInShare on TwitterShare on DiggShare on Stumble Upon
Custom Search
 
  

Frequency Multiplication

Designing and building a stable crystal oscillator is difficult. As operating frequencies increase, the crystal must be ground so thin that it often cracks while vibrating. You will find that you can get around this problem by operating the oscillators in most transmitters at comparatively low frequencies, sometimes as low as 1/100 (.01) of the output frequency. You raise the oscillator frequency to the required output frequency by passing it through one or more frequency multipliers. Frequency multipliers are special power amplifiers that multiply the input frequency. Stages that multiply the frequency by 2 are called doublers; those that multiply by 3 are triplers; and those multiplying by 4 are quadruplers.

You will find the main difference between low-frequency and high-frequency transmitters is the number of frequency-multiplying stages used. Figure 2-5 shows the block diagram of the frequency-multiplying stages of a typical Navy uhf/vhf transmitter. The oscillator in this transmitter is tunable from 18 megahertz to 32 megahertz. You have multiplier stages that increase the oscillator frequency by a factor of 12 through successive multiplications of 2, 2, and 3.

Figure 2-5. - Frequency multiplying stages of a typical vhf/uhf transmitter.

Figure 2-6 is a block diagram of an fm transmitter showing waveforms found at various test points. In high-power applications you often find one or more intermediate amplifiers added between the second doubler and the final power amplifier.

Figure 2-6. - Block diagram of an fm transmitter and waveforms.

SINGLE-SIDEBAND TRANSMITTER

You should remember the properties of modulation envelopes from your study of NEETS, Module 12, Modulation Principles. A carrier that has been modulated by voice or music is accompanied by two identical sidebands, each carrying the same intelligence. In amplitude-modulated (AM) transmitters, the carrier and both sidebands are transmitted. In a single-sideband transmitter (ssb), only one of the sidebands, the upper or the lower, is transmitted while the remaining sideband and the carrier are suppressed. SUPPRESSION is the elimination of the undesired portions of the signal.

Figure 2-7 is the block diagram of a single-sideband transmitter. You can see the audio amplifier increases the amplitude of the incoming signal to a level adequate to operate the ssb generator. Usually the audio amplifier is just a voltage amplifier.

Figure 2-7. - Ssb transmitter block diagram.

The ssb generator (modulator) combines its audio input and its carrier input to produce the two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and suppresses the other one. By eliminating the carrier and one of the sidebands, intelligence is transmitted at a savings in power and frequency bandwidth.

In most cases ssb generators operate at very low frequencies when compared with the normally transmitted frequencies. For that reason, we must convert (or translate) the filter output to the desired frequency. This is the purpose of the mixer stage. A second output is obtained from the frequency generator and fed to a frequency multiplier to obtain a higher carrier frequency for the mixer stage. The output from the mixer is fed to a linear power amplifier to build up the level of the signal for transmission.

Suppressed Carrier

In ssb the carrier is suppressed (or eliminated) at the transmitter, and the sideband frequencies produced by the carrier are reduced to a minimum. You will probably find this reduction (or elimination) is the most difficult aspect in the understanding of ssb. In a single-sideband suppressed carrier, no carrier is present in the transmitted signal. It is eliminated after modulation is accomplished and is reinserted at the receiver during the demodulation process. All rf energy appearing at the transmitter output is concentrated in the sideband energy as "talk power."

After the carrier is eliminated, the upper and lower sidebands remain. If one of the two sidebands is filtered out before it reaches the power amplifier stage of the transmitter, the same intelligence can be transmitted on the remaining sideband. All power is then transmitted in one sideband, instead of being divided between the carrier and both sidebands, as it is in conventional AM. This provision gives you an increase in power for the wanted sideband. You should note in figure 2-8 that the bandwidth required for the ssb suppressed carrier, view B, is approximately half that needed for conventional AM, view A. This enables us to place more signals in a smaller portion of the frequency spectrum and permits a narrower receiver bandpass.

Figure 2-8. - Comparison of bandwidths of conventional AM and ssb voice channels.

Applications

Single-sideband transmission is the most common communications mode used today. Some of the ssb applications used in naval communications are described for you in the following paragraphs.

SSB VOICE CIRCUITS. - The high command (HICOM) network uses ssb as a means of communications between fleet commanders; and fleet commanders use it for communications between their subordinates and adjacent commands.

Ssb is generally used whenever special voice communications circuits are necessary between shore activities or between ships and shore activities because it is less susceptible to atmospheric interference than amplitude modulation.

SSB TELETYPEWRITER CIRCUITS. - With few exceptions, you will find ssb used on all long-haul (great distance) teletypewriter circuits, which includes ship-to-shore. Most of these systems are covered circuits; that is, an electronic cryptographic device on both ends of the circuit automatically encrypts and decrypts message traffic. These devices are used on point-to-point, ship-to-shore, ship-to-ship, and broadcast circuits.

Point-to-Point. - Most point-to-point, long-haul circuits between naval communications stations quickly use up the available frequency spectrum that ssb provides. Independent sideband (isb) transmission is normally used to compensate for the deficiency. Isb is used extensively in naval communications to expand our traffic capabilities. You will find there is a similarity between ssb and isb. Isb uses outputs from two sideband generators; it suppresses both carriers and then filters out an upper sideband from one and a lower sideband from the other. We then combine the two remaining sidebands and transmit an envelope with upper and lower sidebands that contain different intelligence. Isb can be used with MULTIPLEXING (a method for simultaneous transmission of two or more signals over a common carrier wave) to transmit a lot of intelligence on one circuit. Independent sideband and multiplexing will be discussed in more detail in chapter 3.

Ship-to-Shore. - Many ships handle enough message traffic to justify ship-to-shore teletypewriter circuits. Depending on traffic load, these circuits may contain from one to four (minimum) teletypewriter circuits on one sideband circuit. If the traffic load warrants more than one teletypewriter circuit, we usually use time division multiplex or frequency division multiplex (mux) equipment. This equipment is capable of handling many incoming and outgoing circuits. One circuit normally is used as an ORDERWIRE CIRCUIT for operator-to-operator service messages and for making frequency changes when necessary. The remaining circuits are available for handling official message traffic.

Q.8 Why are frequency multipliers used? answer.gif (214 bytes)
Q.9 What are two advantages of ssb transmission? answer.gif (214 bytes)
Q.10 What is the purpose of an order-wire circuit?answer.gif (214 bytes)




Privacy Statement - Copyright Information. - Contact Us

Integrated Publishing, Inc.