Principle of FMCW radar

Remote Sensing of the Environment

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DelftUniversity ofTechnology

Principle of FMCW RadarsTobias Otto


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Contents

I. Principle of FMCW radarII. FMCW radar signal processingIII. Block diagram of an FMCW radar

for precipitation measurements


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Principle of FMCW radar

frequency-modulated continuous-wave

A radar transmitting a continuous carrier modulated by a periodic function such as a sinusoid or sawtooth wave to provide range data (IEEE Std. 686-2008).Modulation is the keyword, since this adds the ranging capability to FMCW radars with respect to unmodulated CW radars.We will concentrate in this talk on linear FMCW radar (LFMCW).

time

amplitude

time

frequency

f0

up-chirp


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time

amplitude

time

frequency

f0

down-chirp


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time

amplitude

time

frequency

f0

triangular


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Single target

time

frequency

Rada

r

range R

sweep time Ts

frequency excursion,sweep bandwidth

Bsweep


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Single target

time

frequency

Rada

r

range R

sweep time Ts

frequency excursion,sweep bandwidth

Bsweep

beat frequency fb

cRtd

2

d b

s sweep

t fT B

2s b

sweep

cT fR

B

receiveroutput

time

modulus ofthe spectrum

fb frequency

Fouriertransformation range


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Moving single target

time

frequency

Rada

r

range R

sweep time Ts

Dff

beat frequency

A moving target induces a Doppler frequency shift2 r

Dvf

with the radar wavelength λ.

radial velocity rvfre

quen

cy e

xcur

sion,

swee

p ba

ndwi

dth

B swe

ep

The beat frequency is not only related to the range of the target, but also to its relative radial velocity with respect to the radar.

fD


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Moving single target

time

frequency

Rada

r

range R

Dff radial velocity rv

time

beat frequency

fbu fbd fbu fbd

bu b df f f

2sweepb

s

B RfT c

2 rD

vf

bd b df f f

Beat frequency components due to range and Doppler frequency shift:

that are superimposed as

4

sbd bu

sweep

cTR f fB

4r bd buv f f

so range and radial velocity can be obtained as


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Atmospheric FMCW radar

Rada

rrange R

When the expected Doppler frequency shift of the target has a negligible effect on the range extraction from the beat frequency, it can be estimated by comparing the phase of the echoes of successive sweeps, e.g. for meteorological applications.the phase of the received signal is

22Rtr

the change of the phase of the received signal with time is given by4 4r

rd dR vdt dt

and the change of the phase of the received signal from sweep to sweep is given as 4r

rs

vT

4r

rs

vT


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Contents




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FMCW radar signal processing

time

frequency

FFT FFT FFT FFT

FFT .. fast Fourier transformation

time

rang

e

FFT

Dopplerfrequency

rang

e


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FMCW radar signal processing

time

frequency

Data: IDRA, TU Delft

sam

ples

sweeps

sam

ples

sweeps

in-phasecomponent

quadraturecomponent

wind

ow fu

nctio

n

2D FFTra

nge

Doppler frequency

spectrogram of the received power


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Contents




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General block diagram of an FMCW radar

high-powermicrowave amplifier

low-noise amplifier

and filtering

powerdivide

r

mixerradar control andsignal processing

modulatedoscillator

amplifier andlow-pass filter

beat frequency fb


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IDRA – TU Delft IRCTR Drizzle radar

IDRA is mounted on top of the 213 m high meteorological tower.

CESA

R – C

abau

w Ex

perim

enta

l Site

for A

tmos

pher

ic Re

sear

ch

Specifications• 9.475 GHz central frequency• FMCW with sawtooth modulation• transmitting alternately horizontal and

vertical polarisation, receiving simultaneously the co- and the cross-polarised component

• 20 W transmission power• 102.4 µs – 3276.8 µs sweep time• 2.5 MHz – 50 MHz Tx bandwidth• 60 m – 3 m range resolution• 1.8° antenna half-power beamwidth

ReferenceJ. Figueras i Ventura: “Design of a High Resolution X-band Doppler Polarimetric Weather Radar”, PhD Thesis, TU Delft, 2009. (online available at http://repository.tudelft.nl)

Near real-time display:http://ftp.tudelft.nl/TUDelft/irctr-rse/idra

Processed and raw data available at:http://data.3tu.nl/repository/collection:cabauw


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IDRA - IRCTR Drizzle radar

transmitterreceiver


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IDRA - IRCTR Drizzle radar (transmitter)

transmitter

- GPS stabilised 10 MHz oscillator, for synchronisation of the whole system and data timestamp

- direct digital synthesizer (DDS) that generates the sawtooth modulation(other waveforms can be easily programmed)

- first up-conversion to the 350-400 MHz band, filtering and amplification /a power splitter provides the signal reference for the down-conversion in the receiver

- second up-conversion to the radar frequency 9.45 – 9.5 GHz (X-band)- switch for transmitting either horizontal or vertical polarisation,

and high-power solid-state microwave amplifiers


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IDRA - IRCTR Drizzle radar (transmitter)

receivertransmitter

- GPS stabilised 10 MHz oscillator, for synchronisation of the whole system and data timestamp

- direct digital synthesizer (DDS) that generates the sawtooth modulation, other waveforms can be easily programmed

- first up-conversion to the 350-400 MHz band, filtering and amplification /a power splitter provides the signal reference for the down-conversion in the receiver

- second up-conversion to the radar frequency 9.45 – 9.5 GHz (X-band)- switch for transmitting either horizontal or vertical polarisation,

and high-power solid-state microwave amplifier


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IDRA - IRCTR Drizzle radar (receiver)

receiver

- two-channel receiver to receive simultaneously the horizontal and vertical polarised echoes,that first undergo the low noise amplification and first filtering stage

- first down-conversion to the 350-400 MHz band followed by filtering and amplification

- I/Q receiver, i.e. the received signal is splitted and mixed with 90° phase difference realisations of the transmitted signal at 400 MHz in order to obtain the in-phase and the quadrature-phase components of the received signal

- after the analog-to-digital conversion, the received signal is sent to theradar control computer for signal processing

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Principles and Applications of FMCW RadarsTobias Otto

e-mail [email protected]

web http://atmos.weblog.tudelft.nl