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Use these simple rules to determine power in a pulsed-wave shape:
Peak power is the maximum power reached by the transmitter during the pulse.
Average power equals the peak power averaged over one cycle.
Duty Cycle
In pulse modulation you will need to know the percentage of time the system is actually producing
rf. For example, let’s say that a pulse system is transmitting 25 percent of the time. This would mean that
the pw is 1/4 the prt. For every 60 minutes we operate the pulse system, we actually transmit a total of
only 15 minutes.
The DUTY CYCLE is the ratio of working time to total time for intermittently operated devices.
Thus, duty cycle represents a ratio of actual transmitting time to transmitting time plus rest time. To
establish the duty cycle, divide the pw by the prt of the system. This yields the duty cycle and is
expressed as a decimal figure. With this information, we can figure percentage of transmitting time by
multiplying the duty cycle by 100.
Applications of Pulse Modulation
Pulse modulation has many applications in the transmission of intelligence information. In telemetry,
for example, the width of successive pulses may tell us humidity; the changing of the rest time may tell us
pressure. In other applications, as you will see later in this text, the changing of the average power can
provide us with intelligence information.
In radar a pulse is transmitted and travels some distance to a target where it is then reflected back to
the system. The amount of time it takes provides us with information that can be converted to distance.
Telemetry and radar systems use the principles of pulse modulation described in this section. Let’s
quickly review what has been presented:
Pulse width (pw) — the duration of time rf frequency is transmitted
Rest time (rt) — the time the transmitter is resting (not transmitting)
Carrier frequency — the frequency of the rf wave generated in the oscillator of the transmitter
Pulse-repetition time (prt) — the total time of 1 complete pulse cycle of operation (rest time
plus pulse width)
Pulse-repetition frequency (prf) — the rate, in pulses per second, that the pulse occurs
Power peak — the maximum power contained in the pulse
Average power — the peak power averaged over 1 complete operating cycle
Duty cycle — a decimal number that expresses a ratio in a pulse modulation system of transmit
time to total time
