Ramsey Electronics BS1 Instrukcja Obsługi Pobierz pdf (Strona 6)

SG7• 6

A Doppler radar measures the velocity of a moving object by doing

something useful with the detected shift in carrier frequency of the returned

or "echo" signal. The shift is proportionate to the speed of the object as it

approaches or moves away.

An easy way to understand what a Doppler radar actually does is to think

about how one of our other kits actually works- the much simpler but fun and

practical MD-3 Microwave Motion Detector or "intrusion Alarm". Like the

SG7, the MD-3 employs a microwave oscillator radiating a microwave signal

from a tiny wire antenna. (The MD-3 runs at 960MHz, rather than in the

2.6GHz spectrum of the SG7.)

Picture what is really going on with the MD-3 turned on in a room where

every object is still. The receiver portion of the circuit hears all the echoes,

and they all stay the same. If someone enters the space, the receiver

detects the returned or echo signal at a frequency different enough to trigger

its switching or alarm circuitry. While it does not measure or count, the

receiver has detected the Doppler shift! For each mile per hour that a target

is traveling toward the radar speed gun, the echo signal will be shifted 7.76

Hertz higher. If an oncoming car is traveling at 50 mph, the echo or reflected

signal will be 388 Hz higher than the original, and it is this difference which

the frequency counter converts to a miles-per-hour reading. If the oncoming

target's speed varies, the SG7 counter can update the readout every 1/7

second. Other readouts such as Kilometers/Hour are possible by changing

R21 in series with the Calibration potentiometer.

The earphone jack is much more than a novelty. This audio output is the FM

modulated Doppler shift difference in Hertz between the transmitted signal

and the reflected signal. In addition to "just listening" to the output

frequency, you can connect this audio sampling of the Doppler shift to an

interesting variety of other devices including standard Ramsey Frequency

Counters, or (for higher speeds) audio tone decoders such as our

inexpensive TD-1 Tone Decoder Kit. For example, if the TD-1 is adjusted to

trigger with a 621 Hz input tone, it might be used with the SG7 to sound an

alarm if a target hits 80 Mph! (80 X 7.76 = 620.8 Hz). Pretty neat, huh?

The World Beyond 99 MPH?

The SG7's two-digit LED display directly counts to 99 MPH or other

calibrated unit of speed/time. At 100 MPH and beyond, the counter "rolls

over" just like the gas pumps (and drivers) that never dreamed that fuel

would go above a dollar a gallon. We do not have hard data on how far

beyond 100 MPH you can continue to count accurately. However, you can

find out by correlating the monitored Doppler shift frequency with the

indicated SG7 two-digit readings and known high-speed targets such as

airplanes on approach.

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Ramsey Electronics BS1 Instrukcja Obsługi Strona 6