RECEIVER CONTROL CIRCUITS

Figure 2-11.Comparison of AM and ssb transmitted signals

Neets Module 17-Radio-Frequency Communications Principles
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2-16 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? 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,

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