SINGLE-SIDEBAND
SINGLE-SIDEBAND
You know from studying the single-sideband transmitter material in this chapter you may transmit only one sideband of an AM signal and retain the information transmitted. Now you will see how a single-sideband signal is received.
Advantages
Figure 2-11 illustrates the transmitted signal for both AM and ssb. Ssb communications has several advantages. When you eliminate the carrier and one sideband, all of the transmitted power is concentrated in the other sideband. Also, an ssb signal occupies a smaller portion of the frequency spectrum in comparison to the AM signal. This gives us two advantages, narrower receiver bandpass and the ability to place more signals in a small portion of the frequency spectrum.
Figure 2-11.—Comparison of AM and SSB transmitted signals.
SSB communications systems have some drawbacks. The process of producing an ssb signal is somewhat more complicated than simple amplitude modulation, and frequency stability is much more critical in ssb communication. While we don’t have the annoyance of heterodyning from adjacent signals, a weak ssb signal is sometimes completely masked or hidden from the receiving station by a stronger signal. Also, a carrier of proper frequency and amplitude must be reinserted at the receiver because of the direct relationship between the carrier and sidebands.
Figure 2-12 is a block diagram of a basic ssb receiver. It is not significantly different from a conventional superheterodyne AM receiver. However, a special type of detector and a carrier reinsertion oscillator must be used. The carrier reinsertion oscillator must furnish a carrier to the detector circuit. The carrier must be at a frequency which corresponds almost exactly to the position of the carrier used in producing the original signal.
Figure 2-12.—Basic ssb receiver.
Rf amplifier sections of ssb receivers serve several purposes. Ssb signals may exist in a small portion of the frequency spectrum; therefore, filters are used to supply the selectivity necessary to adequately receive only one of them. These filters help you to reject noise and other interference.
Ssb receiver oscillators must be extremely stable. In some types of ssb data transmission, a frequency stability of ±2 hertz is required. For simple voice communications, a deviation of ±50 hertz may be tolerable.
These receivers often employ additional circuits that enhance frequency stability, improve image rejection, and provide automatic gain control (agc). However, the circuits contained in this block diagram are in all single-sideband receivers.
Carrier Reinsertion
The need for frequency stability in ssb operations is extremely critical. Even a small deviation from the correct value in local oscillator frequency will cause the IF produced by the mixer to be displaced from its correct value. In AM reception this is not too damaging, since the carrier and sidebands are all present and will all be displaced an equal amount. Therefore, the relative positions of carrier and sidebands will be retained. However, in ssb reception there is no carrier, and only one sideband is present in the incoming signal.
The carrier reinsertion oscillator frequency is set to the IF frequency that would have resulted had the carrier been present. For example, assume that a transmitter with a suppressed carrier frequency of 3 megahertz is radiating an upper sideband signal. Also assume that the intelligence consists of a 1-kilohertz tone. The transmitted sideband frequency will be 3,001 kilohertz. If the receiver has a 500-kilohertz IF, the correct local oscillator frequency is 3,500 kilohertz. The output of the mixer to the IF stages will be the difference frequency, 499 kilohertz. Therefore, the carrier reinsertion oscillator frequency will be 500 kilohertz, which will maintain the frequency relationship of the carrier to the sideband at 1 kilohertz.
Recall that 1 kilohertz is the modulating signal. If the local oscillator frequency should drift to 3,500.5 kilohertz, the IF output of the mixer will become 499.5 kilohertz. The carrier reinsertion oscillator, however, will still be operating at 500 kilohertz. This will result in an incorrect audio output of 500 hertz rather than the correct original 1-kilohertz tone. Suppose the intelligence transmitted was a complex signal, such as speech. You would then find the signal unintelligible because of the displacement of the side frequencies caused by the local oscillator deviation. The local oscillator and carrier reinsertion oscillator must be extremely stable.
Q16. What two components give a ssb receiver its advantages over an AM superheterodyne receiver?
댓글
댓글 쓰기