Ultrasonic intrusion detection system

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG.
1, there is shown circuitry suitable for providing the transmitter output signal applied to the transmitting transducers of the present alarm system.
Typically, this circuitry is located within the alarm system master control, and the transmitter excitation signal is applied via a cable to each of the plurality of transmitters which are included in the system.
The transmitter circuitry includes sources of two frequencies which are alternately applied to each of the transmitters.
These frequencies are denoted as f.
sub.
1 and f.
sub.
2 and are equally spaced by an amount .
DELTA.
f from a center or reference frequency, which will be denoted as f.
sub.
c.
The reference frequency f.
sub.
c is not applied to the transmitters, but is required by the receiver processing circuit.
In the preferred embodiment described herein the reference frequency is 26.
5 kHz.
The offset frequency .
DELTA.
f is 100 Hz and therefore frequency f.
sub.
1 equals 26.
4 kHz and frequency f.
sub.
2 equals 26.
6 kHz.
In the embodiment shown in FIG.
1, f.
sub.
1 and f.
sub.
2 are shown as being produced by two oscillators 12 and 14.
The carrier frequency, f.
sub.
c is produced by a third oscillator 16.
It should be appreciated that use of three oscillators as shown in FIG.
1 is not necessary, and the required frequencies may be produced by other means.
The f.
sub.
1 and f.
sub.
2 frequency outputs from oscillators 12 and 14 are applied to an electronic switch 18.
In response to a digital signal applied on line 20 to switch 18, switch 18 alternately selects f.
sub.
1 or f.
sub.
2 for application to a modulator 22.
The signal on line 20 is typically a square wave signal which controls the state of switch 18, and the frequency of this signal is the modulation frequency which will be denoted as f.
sub.
m.
In the embodiment described herein, f.
sub.
m is typically on the order of 3 Hz.
A square wave oscillator 24 provides a square wave output signal having a frequency of 2f.
sub.
m.
This signal is applied to a flip-flop 26 which divides the signal from oscillator 24 by two to produce the f