2-18
Q-24. Debunching of the electron bunches in the higher modes of a reflex klystron has what effect on
output power?
Q-25. What limits the tuning range around the center frequency of a reflex klystron in a particular mode
of operation?
The Decibel Measurement System
Because of the use of the decibel measurement system in the following paragraphs, you will be
introduced to it at this point. Technicians who deal with communications and radar equipment most often
speak of the gain of an amplifier or a system in terms of units called DECIBELS (dB). Throughout your
Navy career you will use decibels as an indicator of equipment performance; therefore, you need to have
a basic understanding of the decibel system of measurement. Because the actual calculation of decibel
measurements is seldom required in practical applications, the explanation given in this module is
somewhat simplified. Most modern test equipment is designed to measure and indicate decibels directly
which eliminates the need for complicated mathematical calculations. Nevertheless, a basic explanation of
the decibel measurement system is necessary for you to understand the significance of dB readings and
equipment gain ratings which are expressed in decibels.
The basic unit of measurement in the system is not the decibel, but the bel, named in honor of the
American inventor, Alexander Graham Bell. The bel is a unit that expresses the logarithmic ratio between
the input and output of any given component, circuit, or system and may be expressed in terms of voltage,
current, or power. Most often it is used to show the ratio between input and output power. The formula is
as follows:
The gain of an amplifier can be expressed in bels by dividing the output (P1) by the input (P2) and
taking the base 10 logarithm of the resulting quotient. Thus, if an amplifier doubles the power, the
quotient will be 2. If you consult a logarithm table, you will find that the base 10 logarithm of 2 is 0.3; so
the power gain of the amplifier is 0.3 bel. Experience has taught that because the bel is a rather large unit,
it is difficult to apply. A more practical unit that can be applied more easily is the decibel (1/10 bel). Any
figure expressed in bels can easily be converted to decibels by multiplying the figure by 10 or simply by
moving the decimal one place to the right. The previously found ratio of 0.3 is therefore equal to 3
decibels.
The reason for using the decibel system when expressing signal strength may be seen in the power
ratios in table 2-1. For example, to say that a reference signal has increased 50 dB is much easier than to
say the output has increased 100,000 times. The amount of increase or decrease from a chosen reference
level is the basis of the decibel measurement system, not the reference level itself. Whether the input
power is increased from 1 watt to 100 watts or from 1,000 watts to 100,000 watts, the amount of increase
is still 20 decibels.
