modulation systems modulate a carrier in this manner. Others produce no rf until pulsed; that is, rf occurs only during the actual pulse as shown in view (A) of figure 2-30. For example, let's start with an rf carrier frequency of 1 megahertz. Each cycle of the rf requires a certain amount of time to complete. If we allow oscillations to occur for a given period of time only during selected intervals, as in view (B), we are PULSING the system.">
Thus far, we have established a carrier and have caused its peaks to increase and decrease as a modulating square wave is applied. Some pulse-modulation systems modulate a carrier in this manner. Others produce no rf until pulsed; that is, rf occurs only during the actual pulse as shown in view (A) of figure 2-30. For example, let's start with an rf carrier frequency of 1 megahertz. Each cycle of the rf requires a certain amount of time to complete. If we allow oscillations to occur for a given period of time only during selected intervals, as in view (B), we are PULSING the system. Note that the pulse transmitter does not produce an rf signal until one of the positive-going modulating pulses is applied. The transmitter then produces the rf carrier until the positive input pulse ends and the input waveform again becomes a negative potential.
Figure 2-30A. - Pulse transmission.
Figure 2-30B. - Pulse transmission.
Refer back to figure 1-41 and the over-modulation discussion in chapter 1. You will notice that the overmodulation wave shape of view (D) in figure 2-29 and the pulse-modulation wave shape of figure 2-30, view (B), are very similar to figure 1-41.
Actually, both figure 1-41 and view (D) of figure 2-29 result from overmodulation. Even though the output of the pulse transmitter in figure 2-30 looks like overmodulation, it is not; rather, it is pulsed. However, the frequency spectrums are similar. Sideband distributions are similar, but not identical, since the pulse transmitter in figure 2-30 is gated on and off instead of being modulated by a square wave as was the case in view (D) of figure 2-29.
Remember, in pulse modulation the sidebands produced to accompany the carrier during transmission are directly related to the harmonic content of the modulating wave shape. In figure 2-31, (view A, view B and view C), observe the square and rectangular wave shapes used to pulse modulate the same carrier frequency in each of the three views.
Figure 2-31A. - Varying pulse-modulating waves.
Figure 2-31B. - Varying pulse-modulating waves.
Figure 2-31C. - Varying pulse-modulating waves.
Let's take note of some timing relationships in the three modulating sequences in figure 2-31:
In figure 2-32, observe the relative time for individual rf cycles.
The time for each cycle is the same in views (A) and (B). Since this time is the same, we can assume that the carrier frequency is the same. But in view (C) the time for each cycle is about half that in views (A) and (B). Therefore, the frequency of the carrier in view (C) is nearly twice that of the other two. This illustration shows that carrier frequencies in pulse systems can vary.
Figure 2-32A. - Carrier frequency.
Figure 2-32B. - Carrier frequency.
Figure 2-32C. - Carrier frequency.
The carrier frequency is not the only frequency we must concern ourselves with in pulse systems. We must also be concerned with the frequency that is associated with the repetition rate of groups of pulses. Figure 2-33 shows that a specific time period exists between each group of rf pulses. This time is the same for each repetition of the pulse and is called the PULSE-REPETITION TIME (prt). To find out how often these groups of pulses occur, compute PULSE-REPETITION FREQUENCY (prf) using the formula:
Figure 2-33. - Pulse-repetition time (prt).
Just remember that the pulse-repetition time is the time it takes for a pulse to recur, as shown in figure 2-34. The duration of time of the pulse (a) plus the time when no pulse occurs (b) equals the total pulse-repetition time.
Figure 2-34. - Pulse cycles.
The time during which the pulse is occurring is called PULSE DURATION (pd) or PULSE WIDTH (pw), as shown in figure 2-35. As you will soon see, pulse width is important in pulse modulation.
Figure 2-35. - Pulse width (pw).
The time we have been referring to as the time of no pulse, or nonpulse time, is referred to as REST TIME (rt). The duration of this rest time will determine certain capabilities of the pulse-modulation system. The pulse width is the time that the transmitter produces rf oscillations and is the actual pulse transmission time. During the nonpulse time, shown in figure 2-36, the transmitter produces no oscillations and the oscillator is cut off.
Figure 2-36. - Rest time (rt).
Power in a Pulse System
When discussing power in a pulse-modulation system, we have to consider PEAK POWER and AVERAGE POWER. Peak power is the maximum value of the transmitted pulse; average power is the peak power value averaged over the pulse-repetition time. Peak power is very easy to see in a pulse system. In figure 2-37, all pulsed wave shapes have a peak power of 100 watts. Also note that in views (A), (B), and (C) the pulse width is the same, even though the carrier frequency is different. In these three cases average power would be the same. This is because average power is actually equal to the peak power of a pulse averaged over 1 operating cycle. However, the pulse width is increased in view (D) and we have a greater average power with the same prt. In view (E) the decreased pulse width has decreased average power over the same prt.
Figure 2-37. - Peak and average power.
Use these simple rules to determine power in a pulsed-wave shape:
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)
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
Pulse modulation will play a major part in your electronics career.
In one way or another, you will encounter it in some form. The function of the particular system may involve many variations of the characteristics presented here. We will now look at some specific applications of pulse modulation in radar and communications systems.
Q.14 Overmodulating an rf carrier in amplitude modulation produces a waveform which is
similar to what modulated waveform?
|Integrated Publishing, Inc.|