Continuous Wave Radar
Figure 1: Continuous- Wave Radar uses two antennas often
CW radar sets transmit a high-frequency signal continuously. The echo signal is received and processed permanently. One has to resolve two problems with this principle:
- prevent a direct connection of the transmitted energy into the receiver (feedback connection),
- assign the received echoes to a time system to be able to do run time measurements.
A direct connection of the transmitted energy into the receiver can be prevented by:
- spatial separation of the transmitting antenna and the receiving antenna,
e.g. the aim is illuminated by a strong transmitter and the receiver is located in the
missile flying direction towards the aim;
- frequency dependent separation by the Doppler-frequency during the measurement of speeds.
A run time measurement isn't necessary for speed gauges, the actual range of the delinquent car doesn't have a consequence. If you need range information, then the time measurement can be realized by a frequency modulation or phase keying of the transmitted power.
A CW-radar transmitting a unmodulated power can measure the speed only by using the Doppler- effect. It cannot measure a range and it cannot differ between two reflecting objects.
Figure: TRAFFIPAX SpeedoPhot
Speed gauges are very specialized CW-radars. A speed gauge uses the Doppler- frequency for measurement of the speed. Since the value of the Doppler- frequency depends on the wavelength, these radar sets use a very high frequency band. The figure shows the speed gauge „Traffipax Speedophot” produced by ROBOT Visual Systems GmbH. This radar uses a frequency of 24.125 gigahertz.
It can measure the speed of the incoming and the outgoing traffic, from the right or left border of the street. The radar can be mounted in a car or on a tripod. The traffic offence can be circumstantiated by a photo camera with high resolution.
Block Diagram of an CW-Radar
Simple CW Doppler- Radar sets have the following design:
Figure 3: schematic diagram of CW Doppler- Radar
Figure 3: schematic diagram of CW Doppler- Radar (interactive picture)
A mixer stage with a following narrowband filter generates the stable local-oscillator-frequency as sum of the transmitter's frequency and a generated IF-frequency.