GENERAL
INTEREST
19
11/2002
Elektor Electronics
sure the speed of vehicles, we’ll take a look
at the commercial applications that are found
at the side of the road.
The basis of every speed camera (let’s call
it a radar) is a SHF generator, which can
transmit a beam in a specific direction. From
the previous section we have learnt that the
sensitivity of the device is directly propor-
tional to the frequency of the beam. The exact
frequency used depends on the manufacturer,
but is generally between 2 GHz and 15 GHz.
The device can either have a SHF oscillator
based on a Gunn diode and a resonant cav-
ity, or a transistor oscillator followed by a
power amplifier. The power of these oscilla-
tors is not very high (usually less than 10
mW), but the effective power output is
increased through the use of a directional aer-
ial.
The receiver for the reflected signal is
often based on a Schottky diode, situated at
the focal point of the aerial (usually the same
aerial is used for transmission and reception),
which functions as a mixer of the transmitted
and reflected signals.
The output signal of the receiver is ampli-
fied, conditioned by an analogue circuit and
then passed on to the measurement section,
which is nothing more than a frequency
counter.
the aerial assembly on top of a
turntable (the cathode-ray tubes
were now given deflections in two
directions).
Introducing the Doppler
effect
The device just described is not
capable of determining the speed of
the detected object; this was limited
to measuring the movement of the
echo on the screen, which gave a
rather inaccurate result.
As an example, consider a car
that makes a sound with a fixed fre-
quency (a car that is driven with
fixed revs for example). When you
are in the car you won’t notice any
variation in the frequency of the
engine sound.
If however you stand at the side
of the road and listen to the car
when it drives past under identical
conditions you will notice that the
frequency of the engine sound
increases as the car comes nearer
and then decreases as the car trav-
els past you. (This effect is also
noticeable in F1 Grand Prix races
when the cars roar past the camera.)
This phenomenon works the other
way round as well: when you drive
your car past somebody who is
shouting on the pavement, you
should notice that the frequency of
the shouting increases as you go
towards the person, and then
decreases as you move away.
When the distance between the
sound source and the receiver
remains constant then the frequency
of the received signal won’t vary
either.
The Doppler effect (named after
the physicist who discovered it) is
nothing more than that described by
the formula in
Figure 2
:
f
M
= 2
v f
E
cos (
α
/
c
)
where
f
M
is the frequency of the received
signal.
v is the speed of the vehicle.
f
E
is the frequency of the transmitted
signal.
α
is the angle between the transmit-
ter and the path along which the
vehicle travels.
c
is the propagation speed of the sig-
nal in air (300,000 km/s for radio
waves, 340 m/s for sound
waves).
From this we can deduce that
sending a fixed frequency signal
towards the car and then measuring
the frequency of the returning signal
will give you the speed of a car.
This is the principle used for
radars in speed cameras, although
they have little in common with the
systems described in the first part of
this article.
It should be mentioned that the
sensitivity of the radar increases as
the angle between the beam and the
path of the vehicle decreases. For
this reason the aerials of speed cam-
eras are positioned parallel to the
roads rather than across them! This
is also the reason why few types of
radar can work along bends, since
the angle between the beam and the
vehicle continually changes, creating
errors in the measurements.
From theory to practice!
Now that we’ve seen how the
Doppler effect can be used to mea-
frequency
generator
controlled
SHF
oscillator
synchronised
sawtooth
generator
switch
TX/RX antenna
transmitted
burst
received
burst
020165 - 11
return
signal
amplifier
horizontal
amplifier
vertical
amplifier
Figure 1: Principle of operation of first generation radars (1940).
Figure 2: The Doppler effect.
020165 - 12
V
f
E
f
M
direction of vehicle M
Speed check
TX/RX
M
α
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