Transmitter Waveform Generation Power Amplifier: Propagation

Block Diagram of Radar System
Transmitter
Power Waveform
Propagation
Amplifier Generation
Medium
Target
Radar T/R
Cross Switch
Section
Antenna
Signal Processor Computer
A/D Pulse Clutter Rejection

Receiver
Converter Compression (Doppler Filtering)
User Displays and Radar Control
General Purpose Computer
Parameter
Tracking Thresholding Detection
Estimation
Data
Recording
CW Pulse, Its Frequency Spectrum, and
Range Resolution
1 μsec pulse Frequency spectrum of pulse
3 20
Bandwidth
Pulsewidth
Power (dB)
Amplitude
2 10
1
T T
1 0
0 -20
1 2 3 4 0 1 2 3 4 5
Time (μsec)
Frequency (MHz)
cT
• Range Resolution (Δ r ) Δr= 2
– Proportional to pulse width ( T ) c
– Inversely proportional to bandwidth (B = 1 / T) Δr = 2 B
1 MHz Bandwidth => 150 m of range resolution
Motivation for Pulse Compression
• High range resolution is important for most radars
– Target characterization / identification
– Measurement accuracy
• High range resolution may be obtained with short pulses

– Bandwidth is inversely proportional to pulsewidth
• Limitations of short pulse radars

– High peak power is required for large pulse energy
– Arcing occurs at high peak power , especially at higher
frequencies
Example: Typical aircraft surveillance radar
1 megawatt peak power, 1 microsecond pulse, 150 m range resolution,
energy in 1 pulse = 1 joule
To obtain 15 cm resolution and constrain energy per pulse to 1 joule implies 1
nanosecond pulse and 1 gigawatt of peak power
– Airborne radars experience breakdown at lower voltages than
ground based radars
Motivation for Pulse Compression
• Radars with solid state transmitters are unable to operate at
high peak powers
– The energy comes from long pulses with moderate peak
power (20-25% maximum duty cycle)
– Usually, long pulses, using standard pulsed CW waveforms,
result in relatively poor range resolution
• A long pulse can have the same bandwidth (resolution) as a

short pulse if it is modulated in frequency or phase
• Pulse compression, using frequency or phase modulation,

allows a radar to simultaneously achieve the energy of a
long pulse and the resolution of a short pulse
• Two most important classes of pulse compression

waveforms
– Linear frequency modulated (FM) pulses
– Binary phase coded pulses
Pulse Width, Bandwidth and Resolution
for a Square Pulse
Resolution: Pulse Length is Larger than Target Length
Cannot Resolve Features Along the Target
Δr =c T
2
c
Δ r =2 B
Pulse Length is Smaller than Target Length

Can Resolve Features Along the Target
0
RCS (dB)
High Bandwidth
Relative
Metaphorical Δr = .1 x Δ r
Example : -20
BW = 10 x BW
-40 Low Bandwidth
Relative Range (m)
Shorter Pulses have Higher Bandwidth and Better Resolution

Frequency and Phase Modulation of Pulses
• Resolution of a short pulse can be achieved by modulating

a long pulse, increasing the time-bandwidth product
• Signal must be processed on return to “pulse compress”
Binary Phase Linear Frequency

Square Pulse Coded Waveform Modulated Waveform
Pulse Width, T Pulse Width, T Pulse Width, T
τ
Frequency F1 Frequency F2
Bandwidth = 1/T Bandwidth = 1/τ Bandwidth = ΔF = 2F -F1
Time × Bandwidth = 1 Time × Bandwidth = T/τ Time × Bandwidth = TΔF
Linear FM Pulse Compression
Linear FM waveform Increasing Frequency

T
Amplitude
Output of Pulse
Time Compression Filter
Amplitude
Frequency of transmitted pulse as
a function of time
f2
Frequency
B= 2f -f1 2 Time
Time B
f1 Bandwidth
Product = BT
T Time
Linear FM Pulse Compression
Linear FM waveform Decreasing Frequency

Output of Pulse
T Compression Filter
Amplitude
Time
Amplitude
Frequency of transmitted pulse as
2 Time
a function of time B
f1
B= 1f –f2
Frequency
Time Because range is measured by a

Bandwidth shift in Doppler frequency, there
f2 Product = BT is a coupling of the range and
Doppler velocity measurement
T Time
Range Doppler Coupling with FM Waveforms
Frequency vs. Time
Frequency
B
Transmitted Waveform Waveform Slope = T
Time
Frequency
t Received Waveform
from a stationary target at
range R = c t / 2
Time
Frequency
Is the Received Waveform from a stationary

t+T target at range R 1 = c(t + T) / 2 or from a moving
target atR = c t / 2 , with Doppler frequency,
f D = Bt / T
Time
Range and Doppler measurements are coupled with

Frequency modulated waveforms
Linear FM Pulse Compression Filters
• Linear FM pulse compression filters are usually

implemented digitally
– A / D converters can often provide the very wide bandwidths
required of high resolution digital pulse compression radar
• Two classes of Linear FM waveforms

– Narrowband Pulse Compression
– High Bandwidth Pulse Compression (aka “Stretch
Processing”)
Linear FM Pulse Compression by Digital
Processing
• Linear FM pulse compression waveforms can be processed

and generated at low power levels by digital methods, when
A / D converters are available with the required bandwidth
and number of bits
• Digital methods are stable and can handle long duration
waveforms
• The same basic digital implementation can be used with :
– multiple bandwidths
– multiple pulse durations
– different types of pulse compression modulation
– good phase repeatability
– low time sidelobes
– when flexibility is desired in waveform selection
Implementation Methods for LFM Pulse
Compression
• Direct Convolution in Time Domain
Uncompressed Convolution Compressed

Received Echo Pulse
Transmitted
(Reference)
Signal
• Frequency Domain Implementation
Uncompressed Discrete Fourier Inverse

Received Echo Transform Discrete Fourier
Transform
Transmitted Discrete Fourier

(Reference) Transform
Signal
Binary Phase Coded Waveforms
Binary Phase
Coded Waveform • Changes in phase can be used
to increase the signal
Pulse Width, T bandwidth of a long pulse
• A pulse of duration T is divided
into N sub-pulses of duration τ
• The phase of each sub-pulse is
changed or not changed,
according to a binary phase
code
• Phase changes 0 or π radians
τ (+ or -)
• Pulse compression filter output
Bandwidth = 1/τ will be a compressed pulse of
width τ and a peak N times that
Pulse Compression Ratio = T/τ of the uncompressed pulse

Transmitter Waveform Generation Power Amplifier: Propagation

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Block Diagram of Radar System

Signal Processor Computer

A/D Pulse Clutter Rejection

General Purpose Computer

• High range resolution may be obtained with short pulses

• Limitations of short pulse radars

• A long pulse can have the same bandwidth (resolution) as a

• Pulse compression, using frequency or phase modulation,

• Two most important classes of pulse compression

Pulse Length is Smaller than Target Length

Relative Range (m)

Shorter Pulses have Higher Bandwidth and Better Resolution

• Resolution of a short pulse can be achieved by modulating

Binary Phase Linear Frequency

Linear FM waveform Increasing Frequency

Linear FM waveform Decreasing Frequency

Time Because range is measured by a

Is the Received Waveform from a stationary

Range and Doppler measurements are coupled with

• Linear FM pulse compression filters are usually

• Two classes of Linear FM waveforms

• Linear FM pulse compression waveforms can be processed

• Direct Convolution in Time Domain

Uncompressed Convolution Compressed

• Frequency Domain Implementation

Uncompressed Discrete Fourier Inverse

Transmitted Discrete Fourier

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