User Manual

You will recall in the example of the automobile horn that the frequency of the horn tone and its

rate of travel through the air were assumed to be constant, so that the only factor affecting the tone

from the observer's standpoint was the change in position of the automobile. With radio waves, we

are able to assume this with much greater confidence. For a source of radio waves, MPH has

selected a sophisticated solid-state device called a Gunn oscillator that generates radio energy in the

microwave region. Specifically, a K-band radar transmits at a frequency of 24,150 MHz, and a

Ka-band radar transmits at a frequency of 33,800 MHz. This high frequency radio energy is

focused into a narrow beam and directed at the target vehicle and travels at the speed of light. A

small portion of the beam is reflected back to a second solid-state device called a mixer diode. The

mixer diode compares the frequency of the reflected beam to the transmitted frequency. The

difference between these two frequencies is called the Doppler frequency. Furthermore, the

Doppler frequency is directly proportional to the sum of the transmitter (patrol) and target

velocities. It can be shown mathematically that for a transmitted K-band frequency of 24,150

MHz, a Doppler frequency of 72.0 Hz will be produced for each mile per hour that the target is

moving. Similarly, a transmitted Ka-band frequency of 33,800 MHz will cause a Doppler

frequency of 100.8 Hz to be produced for each mile per hour. For example:

K-band: 72.02 Hz x 60 mph = 4321.0 Hz Doppler frequency

Ka-band: 100.8 Hz x 60 mph = 6048.0 Hz Doppler frequency

Knowing this relationship, we are able, by means of modern electronic circuitry, to convert the

Doppler frequency as determined by the mixer diode into a digital presentation of the target's speed

in miles per hour.

Some appreciation of the accuracy required of the complete system may be gained by looking at

the very small numerical value of the Doppler frequency as compared to the transmitted and

received frequencies.

K-Band Vehicle Approaching at 60 mph

Reflected Frequency 24,150,004,321 cycles per sec.

Transmitted Frequency 24,150,000,000 cycles per sec.

+ 4,321 cycles per sec.

Vehicle Receding at 60 mph

Reflected Frequency 24,149,995,679 cycles per sec.

Transmitted Frequency 24,150,000,000 cycles per sec.

– 4,321 cycles per sec.

Note again how the reflected frequency is greater than the transmitted as the vehicle approaches

and less than the transmitted as it recedes, yet the difference, the Doppler shift, remains constant

for this particular vehicle speed.

Stationary radar theory and Automatic Same Direction

™

(ASD

™)

technology

Most stationary radars cannot detect what direction a target is moving. In both of the previous

examples, a “normal” stationary radar would detect a Doppler shift of 4321 cycles per second and

convert that to 60 mph. They cannot tell if the true Doppler shift was +4321 or - 4321 cycles per

second.