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,
