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Design and Development of a Low Cost Chirp Generator for
Airborne Synthetic Aperture Radar
Y. K. Chan and S. Y. Lim
Faculty of Engineering & Technology, Multimedia University
Jalan Ayer Keroh Lama, Bukit Beruang, Melaka 75450, Malaysia
Abstract— An airborne C-band Synthetic Aperture Radar (SAR) has been designed and developed over the past few years as a key geometric data source for environmental monitoring
by the Centre for Applied Electromagnetic of Multimedia University (MMU), Malaysia. Several
modifications and enhancement are underway on top of the successful construction of the current SAR prototype. The highlight of this continuation work is the hardware implementation
of the airborne SAR, specifically on the modifications of the existing SAR sensor microwave
transceiver, such as the design and development of one high-speed dual-channel chirp generator
using a digital approach. The digital approach is selected over an equivalent analogue solution for
its stability, repeatability and flexibility following breakthrough and advancement in the world
of digital electronics. This paper focuses on the design and development issues concerning the
chirp generator.
1. INTRODUCTION
Synthetic Aperture Radar (SAR) is fast becoming a favourite among researches in the field of remote
sensing ever since the nineties, overriding real aperture Side-Looking Airborne Radar (SLAR) as
the sole imaging radar on a textbook case while attracting worldwide participation in the research
and development of itself. The advantages of SAR have been detailed in many books and journals,
which record the concrete proof and support behind the blossoming of SAR systems not only
nationwide as in Malaysia but worldwide. Among them includes fine resolution achievable that
made headline when the technique first came to light, often credited to Carl Wiley of Goodyear
Aerospace in 1951 [1]. The onset of SAR has since significantly done away with many limitations
revolving SLAR [2], like offering much longer apertures to improve the along-track resolution. Also,
it permits round the clock operation as it is capable of providing its own illumination and therefore
not dependent on light from the sun. Besides, being an active system, it can work under any
weather conditions as clouds, fog and precipitation have no remarkable effect on microwaves. All
these merits of SAR can be collectively phrased into one sentence that reads “SAR is an all-weather
imaging tool that achieves fine along-track resolution by taking the advantage of radar motion to
synthesize a large antenna aperture”.
Nearly forty years since the emergence of SAR, Multimedia University, Malaysia has also stepped
into this research field, developing theoretical modelling and image processing techniques on SAR
images. In the meantime, a radar system of C-band with single polarization and linear FM has been
designed and developed to serve as a test-bed for demonstrating SAR technology and acquiring data
for the development of radar processing techniques and applications [3–5]. The current transmitter
consists of an exciter, a high power amplifier (HPA), a STALO, a modulation circuitry, a chirp
mode gate and some RF accessories such as RF cables and isolators; whereas the receiver consists
of radio frequency (RF) section, intermediate frequency (IF) section, and data acquisition unit
(DAU).
One breakthrough in technology made up the primary motivation of this research work, which is
to take a digital approach towards designing and developing the exciter of SAR. The idea of taking
the digital approach is rooted in the belief that, as for most contemporary radar systems designs,
digital electronics offer better stability, repeatability, and flexibility over an equivalent analogue
upshot. This revolution in radar designs has over the decades justified the common perception of
many radar designers about radar system, that unlike early radar systems that consisted entirely
of analogue circuits, digital techniques can now be employed too for optimization purpose [6].
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2. DESIGN AND SYSTEM IMPLEMENTATION
2.1. SAR Design Consideration
Having surveyed the state-of-the-art in both SAR transmitter and digital chirp generator, it is
determined that RF electronics is favoured while RAM/ROM based method is taken. RF electronics
method is chosen over all other methods because it is deemed the most practical yet most economical
approach. On the other hand, RAM/ROM based method by comparison outshines all others in
that it offers high bandwidth and linear ramps yet puts no threat to the coherency of SAR system.
2.2. Chirp Simulations
Typically, to construct a chirp generator there are two aspects involved, namely, software simulation
and hardware implementation. For software simulation, an original source code is written on
MATLAB to generate chirp signal prior to the actual implementation of hardware. Data is then
obtained from the simulation and converted from the numerical data in decimal into machine code
readable by the components using an assembler. Variation of the chirp rate and the data format,
which is the bit of DAC, allows for viewing of chirp signals by simulation before decision is made
as to which set of data to be used. After software simulation, direct hardware implementation is
headed.
In ramp sweep, the output sine wave frequency is increased from a start frequency to a stop
frequency. This produces a linear frequency versus time plot. Its accuracy, sweep time, and
frequency resolution of the source are usually specified. The chirp generation can be described as
a Cosine function with quadratic time sample multiply with the number of sample generated by
the DAC. By interchanging between the Cosine and Sine function, depending on the application
for dual channels operation, with In-Phase and Quadrature (IQ) Sampling being the driving force
behind the dual-channel design. Table 1 lists down the specifications for running simulation of
chirp signal on MATLAB.
Table 1: The specifications for running simulation of chirp signal on MATLAB.
Parameters
Start Frequency, fs
Stop Frequency, f1
Start Time, ts
Stop Time, t1
Bandwidth, B
Chirp Pulse Duration, τP
Chirp Rate, K (K=B/τP )
Data format of DAC, m
Number of Time Samples, nt
Interval between Time Samples
Functions
Values
0 Hz
40 MHz
0s
20 µs
40 MHz
20 µs
2 × 1012
8-bit
1000
2 × 10−8
Sine
Cosine
2.3. Hardware Construction
The chirp generator is built on the concept of Direct Digital Synthesis (DDS), first introduced
by Barry and Fenwick in year 1965, which sparked the reports of a variety of digital generation
methods in the literature with varying degree of success. Figure 1 shows the construction of the
chirp generator. It consists of a microcontroller, a counter, a crystal oscillator, 2 UV-PROMs, and
2 DACs.
2.4. Output Chirp Analysis
Since the output chirp signal appears differently in the time domain and in the frequency domain,
both results are presented. An Analogue-to-Digital Card (ADC) is used to digitise the analogue
chirp signals into digital data for viewing and processing of the chirp signals. Figure 2 shows
the In-Phase and Quadrature chirp signals of 40 MHz bandwidth (20 MHz on the left and right
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STALO
10 MHz
XTAL
50 MHz
Trig In
Microcontroller
271
I
UVPROM I
DAC
I
UVPROM Q
DAC
Q
Q
Counter
1 KHz
Figure 1: Basic hardware construction of a high-speed chirp generator.
Figure 2: The captured In-Phase and Quadrature chirp signals with 40 MHz bandwidth.
Figure 3: The captured In-Phase and Quadrature chirp signals with 20 MHz bandwidth.
each) captured by the ADC. They are all together 1000 number of time samples and with each
number constituting 20 ns, a concept illustrated by uniform time sampling, the obtained chirp pulse
duration is 20 µs. It is observed that as the frequency of the chirp signal increases towards its left
and right, the amplitude of the chirp signal reduces slightly. Since this phenomenon does not occur
on the oscilloscope, its occurrence on the analysis in this part can be explained by the insufficient
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sampling rate and the limited bandwidth of the ADC. The ADC has 50 MHz of bandwidth and its
sampling rate is 100 MS/s (max) for single channel and 50 MS/s (max) for dual channels. Because
dual-channel is used concurrently for I and Q channels, the sampling rate is limited to 50 MS/s
only. In Figure 3, when the bandwidth is reduced to 20 MHz (10 MHz on the left and right each),
this phenomenon disappears.
Figure 4 is obtained by modulating the In-Phase and Quadrature chirp signals together on
MATLAB. A straight vertical line at the centre is seen in Figure 4 as a result of negative number
elimination. On Matlab simulation, when chirp signal spreads from −128 to 128, no such vertical
line appears but when it is pushed up by 128 that give 0 to 256, the vertical line appears on
its spectrum. For software modulation, a resultant total bandwidth of 40 MHz is presented as one
contiguous spectrum after applying fftshift on MATLAB. For hardware modulation of the In-Phase
and Quadrature chirp signals, an I/Q modulator is required.
Figure 4: The spectrum of the complex chirp signal features a total bandwidth of 40 MHz.
3. CONCLUSION
This paper incorporates the simulation, development, and measurement of the output chirp signals
of a newly constructed digital chirp generator, the name of which has been personalized from a
common term “exciter” to the chirp generator. The primary function of the chirp generator is to
generate a coded pulse waveform from a crystal oscillator that allows the input of reference signal
from continuous tone STALO output. The pre-stored waveform approach taken to develop the
chirp generator is proven at the end of the research to give maximum performance for short pulses.
REFERENCES
1. Wiley, C. A., “Synthetic aperture radar — A paradigm for technology evolution,” IEEE Trans.
Aerosp. Electron. Syst., Vol. 21, 440–443, 1985.
2. Curlander, J. C. and R. N. McDounough, Synthetic Aperture Radar, Systems and Signal Processing, John Wiley & Sons, New York, 1991.
3. Chan, Y. K., M. K. Azlindawaty, V. Gobi, B. K. Chung, and H. T. Chuah, “The design and
development of airborne synthetic aperture radar,” Proc. Igarss’00, Vol. 2, 518–520, 2002.
4. Chan, Y. K., B. K. Chung, and H. T. Chuah, “Transmitter and receiver design of an experimental airborne synthetic aperture radar sensor,” Progress In Electromagnetic Research,
PIER 49, 203–218, 2004.
5. Koo, V. C., Y. K. Chan, V. Gobi, T. S. Lim, B. K. Chung, and H. T. Chuah, “The MASAR
project: Design and development,” Progress In Electromagnetic Research, PIER 50, 279–298,
2005.
6. Brandon, D. and J. Kornblum, “Synchronized synthesizers aid multichannel systems,” Microwaves & RF, 57–68, 2005.