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DUTY CYCLE
Duty cycle (or duty factor) is a measure of the fraction of the time a radar is transmitting. It is important because it relates to peak
and average power in the determination of total energy output. This, in turn, ultimately effects the strength of the reflected signal as well
as the required power supply capacity and cooling requirements of the transmitter.
Although there are exceptions, most radio frequency
(RF) measurements are either continuous wave (CW) or pulsed RF. CW RF is uninterrupted RF such as from an oscillator. Amplitude modulated (AM),
frequency modulated (FM), and phase modulated (PM) RF are considered CW since the RF is continuously present. The power may vary with time due
to modulation, but RF is always present. Pulsed RF, on the other hand, is bursts (pulses) of RF with no RF present between bursts. The most
general case of pulsed RF consists of pulses of a fixed pulse width (PW) which come at a fixed time interval, or period, (T). For clarity and
ease of this discussion, it is assumed that all RF pulses in a pulse train have the same amplitude. Pulses at a fixed interval of time arrive
at a rate or frequency referred to as the pulse repetition frequency (PRF) of so many pulse per second. Pulse repetition interval (PRI) and
PRF are reciprocals of each other.
[1] PRF =
1/T = 1/PRI
Power measurements are classified as either peak pulse power, P
_{p}, or average power, P
_{ave}. The
actual power in pulsed RF occurs during the pulses, but most power measurement methods measure the heating effects of the RF energy to obtain
an average value of the power. It is correct to use either value for reference so long as one or the other is consistently used. Frequently
it is necessary to convert from P
_{p} to P
_{ave}, or vice versa; therefore the relationship between the two must be understood.
Figure 1 shows the comparison between P
_{p} and P
_{ave}.
Figure 1. RF Pulse Train
The average value is defined as that level where the pulse area above the average is equal to area below average between pulses. If the
pulses are evened off in such a way as to fill in the area between pulses, the level obtained is the average value, as shown in Figure 1 where
the shaded area of the pulse is used to fill in the area between pulses. The area of the pulse is the pulse width multiplied by the peak pulse
power. The average area is equal to the average value of power multiplied by the pulse period.
Since
the two values are equal:
[2] P
_{ave} x T = P
_{p}
x PW
or
[3] P
_{ave}/P
_{p} = PW/T
Using [1]
[4] P
_{ave}/P
_{p} = PW/T =
PW x PRF = PW/PRI = duty cycle
(note that the symbol
τ represents pulse width (PW) in most reference books)
The ratio of the average power to the peak pulse power is the duty cycle and represents the percentage of time the power is present. In the
case of a square wave the duty cycle is 0.5 (50%) since the pulses are present 1/2 the time, the definition of a square wave.
For
Figure 1, the pulse width is 1 unit of time and the period is 10 units. In this case the duty cycle is:
PW/T = 1/10 = 0.1 (10%).
A more typical case would be a PRF of 1,000 and a pulse width of 1.0 microseconds. Using [4], the duty cycle is 0.000001 x 1,000 = 0.001. The
RF power is present one-thousandth of the time and the average power is 0.001 times the peak power. Conversely, if the power were measured with
a power meter which responds to average power, the peak power would be 1,000 time the average reading.
Besides expressing duty cycle
as a ratio as obtained in equation [4], it is commonly expressed as either a percentage or in decibels (dB). To express the duty cycle of equation
[4] as a percentage, multiply the value obtained by 100 and add the percent symbol. Thus a duty cycle of 0.001 is also 0.1%.
The duty
cycle can be expressed logarithmically (dB) so it can be added to or subtracted from power measured in dBm/dBW rather than converting to, and
using absolute units.
[5] Duty cycle (dB) = 10 log(duty cycle
ratio)
For the example of the 0.001 duty cycle, this would be 10 log(0.001) = -30 dB. Thus the average power would be 30 dB less than
the peak power. Conversely, the peak power is 30 dB higher than the average power.
For pulse radars operating in the PRF range of 0.25-10
kHz and PD radars operating in the PRF range of 10-500 kHz, typical duty cycles would be:
Pulse :
0.1 - 3% = 0.001 - .03 = -30 to -15 dB
Pulse Doppler :
5 - 50% = 0.05 - .5 = -13 to -3 dB
Continuous Wave :
100% = 1
= 0 dB
Intermediate Frequency Bandwidths of typical signals are:
Pulse
1 to 10 MHz
Chirp or Phase coded pulse 0.1 to 10 MHz
CW or PD
0.1 to 5 kHz
PRF is usually subdivided into the following categories: Low 0.25-4 kHz; Medium 8-40 kHz; High 50-300 kHz.
Table of Contents for Electronics Warfare and Radar Engineering Handbook
Introduction |
Abbreviations | Decibel | Duty
Cycle | Doppler Shift | Radar Horizon / Line
of Sight | Propagation Time / Resolution | Modulation
| Transforms / Wavelets | Antenna Introduction
/ Basics | Polarization | Radiation Patterns |
Frequency / Phase Effects of Antennas |
Antenna Near Field | Radiation Hazards |
Power Density | One-Way Radar Equation / RF Propagation
| Two-Way Radar Equation (Monostatic) |
Alternate Two-Way Radar Equation |
Two-Way Radar Equation (Bistatic) |
Jamming to Signal (J/S) Ratio - Constant Power [Saturated] Jamming
| Support Jamming | Radar Cross Section (RCS) |
Emission Control (EMCON) | RF Atmospheric
Absorption / Ducting | Receiver Sensitivity / Noise |
Receiver Types and Characteristics |
General Radar Display Types |
IFF - Identification - Friend or Foe | Receiver
Tests | Signal Sorting Methods and Direction Finding |
Voltage Standing Wave Ratio (VSWR) / Reflection Coefficient / Return
Loss / Mismatch Loss | Microwave Coaxial Connectors |
Power Dividers/Combiner and Directional Couplers |
Attenuators / Filters / DC Blocks |
Terminations / Dummy Loads | Circulators
and Diplexers | Mixers and Frequency Discriminators |
Detectors | Microwave Measurements |
Microwave Waveguides and Coaxial Cable |
Electro-Optics | Laser Safety |
Mach Number and Airspeed vs. Altitude Mach Number |
EMP/ Aircraft Dimensions | Data Busses | RS-232 Interface
| RS-422 Balanced Voltage Interface | RS-485 Interface |
IEEE-488 Interface Bus (HP-IB/GP-IB) | MIL-STD-1553 &
1773 Data Bus |
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