SINGLE-SIDEBAND (SSB)
Figure 1-9 is a block diagram of a basic ssb re-
ceiver. Though the ssb receiver is not significantly dif-
ferent from a conventional AM superheterodyne
receiver, it must use a special type of detector and a car-
rier reinsertion oscillator. The oscillators in a ssb re-
ceiver must be extremely stable. In some cases, a
frequency stability of plus or minus 2 hertz is required.
You can see that frequency stability is the most impor-
tant factor of ssb equipment.
Ssb receivers may use additional circuits that en-
hance frequency stability, improve image rejection, or
provide automatic gain control (age). However, the
circuits shown in figure 1-5 will be found in all single-
sideband receivers.
AMPLIFICATION
Because the incoming signal may be weak and be-
cause a certain minimum voltage level is required for
the auxiliary equipment to operate, considerable am-
plification must take place before the receiver output is
used to drive speakers, headphones, or terminal equip-
ment. This is usually called the gain of the receiver.
Gain is a term used to describe an increase in current,
voltage, or power. For example, if the detector, which
removes the desired intelligence, requires 1 volt to op-
erate and if the input to the receiver is 1 microvolt, a to-
tal amplification of 1 million is required before
detection. If the loudspeaker requires 10 volts, another
voltage amplification of 10 is necessary between the
detector and the loudspeaker.
The gain of an amplifier is expressed in decibels
(dB). The decibel is a means of measuring relative lev-
els of current, voltage, or power. Most often it is used to
show the ratio between input power and output power.
This ratio is expressed as gains and losses, where a mi-
nus (–) sign placed before dB indicates a loss and a plus
(+)(or no sign at all) indicates a gain. The number of
decibels change between two power values can be com-
puted by the formula:
The comparison of dB’s to power ratio is shown in
table 1-3. You can see instantly the reason behind us-
ing the decibel system. It is much easier to say the sig-
nal level has increased 40 dB than to say it has
increased 10,000 times.
Examining table 1-3 again, you can see that an in-
crease of 3 dB indicates a doubling of power. The re-
verse is also true. If a signal decreases by 3 dB, half the
power is lost. For example, a 100-watt signal de-
creased by 3 dB will equal 50 watts, while the same
100-watt signal increased by 3 dB will equal 200
watts. It’s important to understand that no matter how
much power is involved, a loss or gain of 3 dB always
represents a halving or doubling of the output power.
Technically, the dB level of a signal is a logarith-
mic comparison between the input and output signals.
Table 1-4 shows the common logarithms used to calcu-
late dB. Normally the input signal is used as a refer-
ence. However, sometimes a standard reference signal
is used. The most widely used reference level is a 1
milliwatt signal. Decibels measured in reference to 1
milliwatt are abbreviated dBm. A signal level of 3
dBm is 3 dB above 1 milliwatt and a level of-3dBm is
3 dB below 1 milliwatt. The formula for dBm is a varia-
tion of the dB power formula:
As a Navy technician, you will use the dBm system
of measurement often to perform receiver sensitivity
tests. For example, a receiver rated at -110 dBm will
detect a signal 110 dB below 1 milliwatt. Suppose the
Figure 1-9.—Basic ssb receiver.
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