The signal from the frequency counter
goes to a microprocessor that calculates the
speed and sends it to a display. It also checks
if the measured speed exceeds a preset value
and warns the police officers who are nearby
that an offender has just passed, or it acti-
vates a camera and flashgun.
In short, the basic principles behind a high
frequency speed detector (
Figure 3
) are not
very complex. (Note that we’re talking about
the principles, a practical implementation is
something else altogether, especially the high
frequency section.)
How well does it work?
Now that we know how it all works we may
wonder how reliable the measurements made
by these devices are. Let’s keep one thing
clear: we have no intention of encouraging
any of our readers to break the speed limit or
behave irresponsibly. We just want to look at
the problem from a technical viewpoint to dis-
cover what the limits are of SHF speed cam-
eras. In this way we can distinguish between
proven facts and ‘rumours’ that are doing the
rounds, made by people with little knowledge
of electronics. Instead of straying into difficult
technical considerations we’ll just answer the
most common questions.
Operation during rain or mist:
in contrast to
widespread opinion, radar works perfectly
well during rain or mist (after all, radar is
used extensively to help with the landing of
aeroplanes in bad weather!). In general,
when it rains it comes down vertically (at
least it does here!), which is at right angles
to the radar beam, bringing about a Doppler
effect of zero (cos 90° = 0, so
f
M
= 0). Heavy
rain that comes down at an angle due to
strong gusts of wind can affect the signal-to-
noise ratio of the receiver and prevent its cor-
rect operation. In this case the processor will
simply reject the measurements.
Since mist doesn’t move with respect to
the radar beam (or only very slowly) it will be
practically invisible to the receiver and the
measurements are completely unaffected.
Measurement range
: the distance from which
a radar can measure the speed of a vehicle
depends on two factors: the power of the SHF
oscillator and the sensitivity of the detector.
We already knew that the oscillator power is
generally low and that the use of a directional
aerial increased the transmitted power. The
biggest problem for the detector is the signal-
to-noise ratio, which doesn’t get any better
with Schottky diodes. In this section the sen-
sitivity can also be improved through the use
of aerials. Whilst the first radars could only
take measurements up to twenty
meters, the newer models with their
ultra-sensitive detectors are capable
of taking measurements over several
hundred meters, so well before they
can be seen from the car!
Reaction time
: just as with any other
equipment that use frequency coun-
ters, these speed cameras also
require a certain time to take a mea-
surement. Furthermore, most
devices now take several measure-
ments rapidly, making it possible to
reject any possibly erroneous mea-
surements. Older models required
about half a second to take a reliable
measurement. Current models react
within a tenth of a second, so any
motorist who ignores the speed limit
will have very little chance of avoid-
ing a fine after noticing a speed
camera. Sometimes the radar equip-
ment also contains a DSP (Digital
Signal Processor), which uses a spe-
cial algorithm with a very short pull-
in time, making extremely fast read-
ings possible.
Continuous transmission
: in contrast
to what you may have thought after
reading the theoretical part of this
article, a radar does not need to have
its oscillator functioning continu-
ously. It only needs to be active long
enough to stabilise and take a mea-
surement. Actual radar equipment
works on a random basis or is acti-
vated only when a vehicle comes
nearby.
Discrimination
: when several vehi-
cles travelling at different speeds
encounter a radar beam, the result-
ing Doppler signal contains a mix-
ture of signals at different frequen-
cies. The majority of current devices
can’t separate these components
and reject the measurement as
faulty. There are however newer sys-
tems that contain a DSP (the author
has worked on one of these sys-
tems), which can measure the speed
of several cars simultaneously. So
now only those cars that happen to
be in the ‘shadow’ of others can
escape the speed cameras.
The long and short of it is that
speed cameras have become so
accurate and reliable (which can be
confirmed by drivers who don’t keep
to the speed limit), that it has
become extremely difficult to evade
them!
On the wrong side
of the law
Mankind, and especially homo auto-
mobilis, behaves in such a way that
when he comes across an obstacle
he will try everything to get round
GENERAL
INTEREST
20
Elektor Electronics
11/2002
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