January 1965 Electronics World
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
 December 1971. All copyrights hereby acknowledged.

Pulse compression (aka 'chirp')
radar was invented in the 1950s by Sperry and a couple other
defense contractors. It was new enough by the time the radar
I worked on as a technician in the USAF that it was not incorporated.
Our MPN13 and
MPN14
radar systems used simple singlefrequency pulses. Pulse
compression employs a swept frequency within a fairly narrow
bandwidth to exploit the benefits outlined in this 1965
Electronics World article. If you were to listen to the
signal used to sweep the RF pulse in frequency, it would sound
a lot like a bird's chirp, hence the name. Treatment by author
Donald Lancaster is fairly heavy in that it fearlessly presents
the mathematical concepts of sin(x)/x waveforms, Fourier transforms,
and weighting. He references Skolnik, Ridenour, et al. Even
so, it is a quick read that provides a good introduction to
chirp radar.
"Chirp" A New Radar Technique
By Donald Lancaster
Using a sweptfrequency approach, this new radar has
greatly improved range and target resolution over conventional
pulse methods and is also less susceptible to presentday jamming
techniques.
The dark veil of military secrecy has been lifted on a most
amazing and powerful radar technique called pulse compression
 nicknamed "chirp." By applying mathematical techniques to
a conventional pulsed radar, range and resolution can simultaneously
be increased while at the same time using less peak input power.
The radar also beĀcomes significantly harder to jam, and much
more immune to certain forms of noise. Further, the requirements
for very high powersupply voltages can be reduced.
Chirp is perhaps the most significant radar advance of recent
years. Its improvement upon radar performance can be as high
as several hundred times the capability of conventional techniques.
There is no apparent limit to the ultimate attainable improvement.
Fig. 1. A fourfrequency radar will have
four times the resolution of a conventional radar of equal range
if the target returns are processed as shown. This is the basis
of "chirp."
The majority of presently used radars are of the pulse type,
i.e., systems that transmit a very large burst of radio frequency
energy for a very brief interval and then wait for echo returns
from any targets within range. From these echoes, the position,
size, type, and movement of a target may be determined. Pulse
radar uses extend from weather observation, airport flight control,
travel aids for the blind, and vehicle collision devices, to
the military longrange mapping radars, battlefield radars,
and other detection systems.
In a conventional pulsed radar, a narrow, highvoltage pulse
is applied to an r.f. tube such as a magnetron which briefly
oscillates at a fixed microwave frequency. (Microwaves are used
for radar because physically small antennas with good directional
properties can be obtained only at very high frequencies.) This
microwave pulse is first transmitted. A receiving antenna and
detector then monitor for target echoes, and these returns are
displayed on a cathoderay tube whose sweep is synchronized
with the transmitted pulse. The time it takes an echo to get
back to the receiver is directly related to the distance between
radar and target. The radar burst travels at the speed of light,
or roughly 1000 feet in a microsecond. Since the transmitted
signal has to make a round trip, each microsecond of delay accounts
for a targettoradar distance of 500 feet. Another pulse is
transmitted after all the initial echoes have had a chance to
return. This process is repeated again and again, thus producing
a continuous plot of targets.
The bigger a target is, the more energy it will return and
the brighter it will appear on the display. Further signal processing,
based upon the Doppler effect, can determine whether the target
is moving or stationary, and if moving, in what direction and
how fast.
Chirp becomes valuable only when the ultimate limits of the
conventional pulse radar fall short of the required performance
in range and resolution. Range of a radar is the maximum target
distance at which reliable echo returns can be expected, while
resolution is its ultimate ability to discriminate between two
closely spaced targets while still giving two distinct return
echoes.
Range is determined by the amount of r.f. energy being transmitted.
This is equal to the pulse height (power) multiplied by the
pulse width (time). (Power X time = energy.) This is equal to
the area of the transmitted pulse. The range of a radar is proportional
to the fourth root of the transmitted energy because both radar
and target transmit energy in a squarelaw manner. To double
the range of a radar, the transmitted energy must be increased
by a factor of 2^{4} or 16 times.
Resolution of a radar is determined solely by transmitter pulse
width. Two targets separated by less than the pulse width will
give a single echo return because the end of the transmitted
pulse will be reflected by the near target at the same time
the beginning of the transmitted pulse is being reflected from
the far target.
Range and resolution, therefore, are two radar system requirements
in opposition to each other. To obtain resolution, the transmitted
pulse must be as narrow as possible to obtain range, the area
of the transmitted pulse must be a large as possible. These
two taken together result in a very narrow, extremely highpower
r.f. pulse. System and component capabilities then enter the
picture. The transmitter tubes are asked to provide very brief
pulses of extreme power, sometimes as high as several megawatts.
The narrow duty cycles used in very brief pulses are inefficient.
There is also an upper limit to the maximum voltage power supply
that is practical in an airborne application, due to arcing
problems. Voltages in excess of 40 kV. become quite troublesome.
Highercurrent transmitting tubes can be used, but there is
a limitation here also. The resonant cavities of the tubes must
be of a small size if they are to produce microwaves; there
is also a limit to the maximum currentproduced heat that will
not melt the tube structure.
This was the problem before chirp. What was needed was a
method of increasing the transmitted pulse length, thus increasing
power, yet not degrading the resolution.
How Chirp Works
Fig. 2. If a linearly swept frequency is
fed to a linear display vs frequency network, the different
portions of the signal will be delayed long enough so that all
frequencies will pile up into a narrow output pulse. This network
can also be used in reverse.
To explain chirp, consider the imaginary system of Fig. 1.
Instead of transmitting a single frequency pulse, the radar
now transmits, in turn, four discrete frequencies forming the
overall transmitted pulse. The first frequency (f_{1})
is transmitted for a time T, then frequency f_{2} for
a time T, then f_{3} for time T, and finally f_{4}
for time T. The time length T (in microseconds) of each frequency
of transmission is identical. The receiver uses four separate
filters and detectors for the targetreturned frequencies f_{1},
f_{2}, f_{3}, and f_{4}. The outputs
of the four detectors are then timedelayed in such a manner
that the outputs all "pile up" or coincide in time. Thus, f_{1}
is delayed for 3T seconds, f_{2} for 2T seconds, f_{3}
for T seconds, and f_{4} is not delayed. The summed
output pulse width is T seconds. However, the original transmitted
pulse was 4T seconds long; therefore, the resolution has been
increased by a factor of four with no decrease in transmitted
energy.
Resolution is determined by what each individual detector
receives, which is a pulse only T microseconds wide. With a
conventional radar, the return pulse would have to be 4T microseconds
wide.
This 4:1 improvement does not have to mean heightened resolution.
It can just as well be a 4:1 increase in transmitted energy
resulting in increased range with no change in resolution. By
the fourthroot law, this would extend radar range by a factor
of 1.20. Or, if both the conventional range and resolution were
satisfactory, the fourfrequency modulation technique reduces
the peak power required by a factor of four, thus greatly simplifying
system power supplies.
The more frequencies that are used and the less time spent
at each frequency, the better will be the result. The limit
of more and more frequencies is a linearly swept signal. The
delay required at the receiver would then be a linearly increasing
delay vs frequency device. This is the foundation of chirp.
A chirp radar is one that transmits a sweptfrequency signal,
receives it from a target, and then delays the signal in such
a manner that the return signal is compressed in time to give
a short, intense return signal. The swept signal is called the
chirp signal. The final narrow pulse is called the dechirped,
collapsed, or compressed signal.
When a linearly swept or chirp signal is run through a linear
delay vs frequency network, as in Fig. 2, the various frequencies
are delayed so that they pile up in time at the output. This
piling up does not result in a perfectly rectangular pulse,
but instead the signal assumes the shape of the pulse shown
in Fig. 2. This pulse is called a (sin x/x) pulse because this
is its mathematical shape. (A mathematician at this point might
correctly point out that chirp radar signal processing is nothing
but a means of taking the Fourier transform of the rectangular
energy spectrum of the transmitted signal. This is where the
(sin x/x) pulse comes from.) If the sidelobes of this waveform
are eliminated, a very good approximation to a conventional
rectangular pulse results.
Fig. 3. A "chirp" radar system requires much
less power than a pulsed radar to produce equal range and target
resolution.
A linear delay vs frequency network is a reciprocal device.
This means that a (sin x/x) pulse can be fed through the network
to produce a sweptfrequency signal or a linearly swept signal
can produce a (sin x/x) pulse.
In a chirp radar, a (sin x/x) pulse of the desired resolution
is generated and passed through the network to produce a sweptfrequency
signal. This signal is then transmitted at microwave frequency
at the required highpower level. The echo returns are then
received and passed through a second delay network to obtain
return echoes the same shape and resolution as the initial (sin
x/x) pulse. In the process, a significant improvement in range,
resolution, and peakpower requirement is obtained.
The ratio of lengths between the swept signal and the (sin
x/x) pulse is called the chirp ratio and is a figure of merit
of the expected improvement of a chirp system over a conventional
system. The chirp ratio can be as high as several hundred although
the minimum chirp ratio meeting system requirements is always
chosen, since the wide receiver bandwidths needed add greatly
to system cost and complexity.
A chirped radar is compared to a conventional radar in Fig.
3. Here the chirp ratio is about five. Although both radars
have equal range and resolution, only 1/5 the peak power is
required using the chirp system.
There are a number of ways of generating the swept signal.
There is a distinct advantage to the method of starting with
a (sin x/x) pulse and passing it through a delay network. If
the same network is used for both chirping and dechirping, any
system nonlinearities or distortions cancel, giving a cleaner
signal than would otherwise be possible. This is called a matchedfilter
technique, a tremendously significant radar tool. It is possible
also to actively generate a linearly swept frequency without
using a delay network. This method is simpler but requires very
careful control of system linearity and sweep rate.
Fig. 4. The sidelobes of the collapsed chirp
pulse can be significantly reduced by the "weighting" technique.
There are likewise a number of dechirp, or pulse compression,
methods. Certain ultrasonic aluminum delay lines, as well as
special quartz delay lines, can directly produce the required
delay vs frequency characteristic. A delay line that delays
various frequencies different lengths of time is called a dispersive
line. A second method uses a bridgedT network. By carefully
"stacking" the right number of bridgedT's, with properly chosen
delay widths and center frequencies, a linear delay vs frequency
can be very closely approximated. The complexity of this method
is offset by the wide bandwidth and adjustability attainable.
There are some more subtle but equally significant advantages
of chirp. Note that a chirp system has to have a much wider
receiver bandwidth than a conventional pulsed radar of equal
peak power. Also note that the energy transmitted is distributed
over a much wider range of frequencies. This makes the radar
relatively immune to jamming.
There is another significant advantage of chirp. The delay
network will only pile up, or compress, one particular swept
frequency whose slope and bandwidth exactly match the network.
Random signals fed into the delay network will not pile up and
will come out of the network with the same amplitude they had
when they went in (assuming a lossless network). However, the
real signal will pile up by the chirp ratio and increase in
amplitude by the same factor. This means that the signaltonoise
ratio of the radar echo returns is considerably improved going
through the delay network.
One thing remains  the sidelobes on the (sin x/x) pulse.
By controlling, or shaping the amplitude of the sweptfrequency
signal before or after it is transmitted, the side lobes can
be very greatly reduced in magnitude (Fig. 4). This increases
the radar dynamic range.
For further reading in chirp and chirped radars, the references
listed below might be of interest. Most of the references assume
an extensive knowledge of advanced mathematics and, with the
exception of reference 1, might make fairly difficult reading.
Sources 1 and 2 are excellent general radar texts, while the
others deal specifically with chirp and pulse compression.
References
1. Ridenour, L. N.: "Radar System
Engineering," McGrawHill Book Co.,New York, 1945, or Boston
Technical Lithographers,
Lexington, Mass., 1963.
2. Skolnik, M. I.: "Introduction
to Radar Systems," McGrawHill Book Co., New York,1962.
3. Klauder, J. R. et al.: "The
Theory and Design of Chirp Radars," Bell System Technical Journal,
July 1960.
4. "IRE Transactions on Military
Electronics," Vol MIL6, April 1962. Special issue on signal
processing radar.
5. Omeara, T. R.: "The Synthesis
of BandPass AllPass TimeDelay Networks Using Graphical Approximation
Techniques,"
Hughes
Research Report No. 114, Hughes Research Labs., Malibu, California.
Posted February 23, 2015