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. 1-10