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.

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.

Q.16 What two components give a ssb receiver its advantages over an AM superheterodyne receiver?

RECEIVER CONTROL CIRCUITS

This section deals with circuits that control receiver functions. We will explain how some of the basic manual and automatic receiver control functions work.

Manual Gain Control (mgc)

You learned previously that high sensitivity is one of the desirable characteristics of a good receiver. In some cases high sensitivity may be undesirable. For example, let's suppose the signals received from a nearby station are strong enough to overload the rf sections of your receiver. This may cause the audio output to become distorted to the point of complete loss of intelligibility. To overcome this problem, you can use manual gain control of the rf section. By using the manual gain control, you can adjust the receiver for maximum sensitivity and amplify weak input signals. When you receive a strong input signal, the rf gain may be reduced to prevent overloading. A typical manual gain control circuit for a receiver is illustrated in figure 2-13. Let's go through the basic circuitry.

Figure 2-13. - Typical rf gain control.

C1 is an emitter bypass capacitor. Resistors R1 and R2 develop emitter bias for the amplifier. C2 provides dc isolation between the tank and the base of transistor Q1. You should recall from your studies of NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, and Module 8, Introduction to Amplifiers , that amplifier gain may be varied by changing bias. Potentiometer R2, the rf gain control, is nothing more than a manual bias adjustment. When the wiper arm of R2 is set at point B, minimum forward bias is applied to the transistor. This causes the amplifier to operate closer to cutoff and reduces gain. When you move the control toward point A, the opposite occurs. R1 limits the maximum conduction of Q1 when R2 is short circuited. You may run into an alternate biasing method when the transistor is operated near saturation. In that case, a large change in gain would again be a function of bias.

Manual Volume Control (mvc)

Figure 2-14 shows the circuitry for a common method of controlling volume in a superheterodyne receiver. C1 and R1 form an input signal coupling circuit and are also the means of controlling the level applied to the audio amplifier. R1, R2, and R3 develop forward bias and set the operating point for the transistor amplifier. R4 is the collector load resistor for Q1, and C3 is the output coupling capacitor. Potentiometer R1 in the circuit shown causes the input impedance of the stage to remain fairly constant. The signal from the preceding stage is felt across R1. By adjusting R1, you can change the input level to Q1 and vary the output amplitude.

Figure 2-14. - Typical manual gain/volume control.