Signal detector and method for detecting signals having selected frequency characteristics

A signal detector and signal detection methodology is provided to monitor oscillatory signals, such as sound signals, to produce detector output in response to the presence of target signals having a frequency within a selected frequency range, such as ultrasonic sound signals. The detector has input processing circuitry that receives the oscillatory signals and filters out those which do not fall within the selected frequency range to produce an input signal. The input signal is split into two components, a primary input component and a control component. The primary input component is amplified proportionally to the amplitude of the control component. Preferably, the primary input component is processed to produce a clean, stable intermediate signal. Here, the signal detector provides processing circuitry that passes primary input components that have amplitudes exceeding a threshold, to eliminate low level noise; the primary input components are variably amplified inversely to the amplitude thereof to produce an initial intermediate signal having a substantially uniform amplitude at the frequency of the input signal. This initial intermediate signal is processed by a Schmitt-Trigger to create a square-wave signal. The processing circuit includes a scaler sub-circuit to divide the square-wave signal by an integer, and an integrating sub-circuit integrates the resulting scaled signal to produce the intermediate signal. An amplification circuit has variable gain controlled by averaging the amplitude of the input signal by way of the control component so that the intermediate signal is amplified according to the original amplitude to produce a scaled replica of the filtered input signal. Sensitivity adjustment and visual and audible displays are used for the detector output.

FIELD OF INVENTION 
The present invention generally relates to procedures and apparatus for 
monitoring incoming signals to detect the presence of selected target 
signals contained within the universe of signals received. More 
specifically, however, the present invention is directed to monitoring 
ambient audio signals to detect selected ultrasonic signals in an ambient 
environment. Thus, the present invention is even more specifically 
directed to ultrasonic leak detectors and the methods for detecting 
ultrasonic signals generated by the passage of pressurized gas through 
small openings. 
BACKGROUND OF THE INVENTION 
A variety of different signal detectors have been developed in the past, 
and, indeed, many electronic circuits include tuning circuitry adapted to 
receive, filter and process signals within a selected frequency range. 
With respect to signal detectors constructed specifically to monitor a 
broad range of oscillatory signals, certain problems arise where those 
circuits seek signals having certain selected frequency characteristics. 
For example, many of the existing detectors exhibit a high susceptibility 
to noise and produce faulty readings where random noise signals fall 
within the target frequency range. Further, these systems often encounter 
problems with signal to noise ratio for the target signals versus the 
background noise. In order to solve these problems, many existing signal 
detectors resort to superheterodyning in order to increase sensitivity, 
eliminate noise and shift frequency bands. 
As noted above, the present invention especially concerns the detection of 
ultrasonic signals in an ambient sound environment. The desirability of 
these devices has been recently increasing due to the recognition that 
ultrasonic detectors may readily be implemented as leak detectors to 
detect ultrasonic signals which, for example, are created by the escape of 
pressurized gasses through small openings. This is useful, for example, in 
detecting leakage from pipelines as well as in detecting air flow paths, 
for example, through insulation of houses and commercial buildings and 
through automobile doors and panels. Other analytical values of such 
ultrasonic detectors are being discovered as well. 
Two types of ultrasonic detectors currently dominate the market. A first 
type employs a crystal system to mechanically couple an ultrasonic input 
signal to a local oscillator in order to convert the frequency of the 
input ultrasonic signal to a resultant signal that has a frequency within 
the audible range. While being relatively inexpensive, crystal-based 
systems exhibit limited performance and have significant problems of 
sensitivity. These crystal-based systems are susceptible to noise, have 
problems with signal to noise ratio. In addition, though, crystal based 
systems are susceptible to mechanical vibrations and are susceptible to 
temperature changes which can effect their sensitivity and yield false 
readings. Further, crystal-based systems often and undesirably respond to 
infra-sonic and sonic signals that modulate the system so that again 
faulty readings occur. These crystal-based systems further usually have a 
very limited frequency range for target signals unless there is an ability 
to adjust the frequency of the local oscillator within the system. 
A second system commonly used employs signal mixers that heterodyne a local 
oscillator with the input signal. Again, these systems are susceptible to 
noise, have problem with signal to noise ratio and have a limited 
frequency range unless the oscillator frequency can be adjusted. While 
these systems do not exhibit problems due to sonic or mechanical 
vibrations, they are nonetheless susceptible to temperature changes that 
can yield faulty readings. Further, systems that employ the heterodyne 
technique require multi-offset settings and are thus difficult to adjust 
and maintain over an extended period of use. 
Accordingly, there remains a long felt need for a signal detector that is 
both sensitive in operation and which can operate to detect the large 
range of target frequencies. Further, there is a need for a signal 
detector and methodology which is less susceptible to mechanical vibration 
and temperature change. There is a further need for signal detectors and 
methodology that are more effective at filtering noise signals in order to 
get a cleaner target signal detection. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a new and useful signal 
detector and signal detection methodology for the detection of target 
signals over a wide target frequency range. 
It is a further object of the present invention to provide a signal 
detector and detection methodology effective to detect target signals 
while eliminating low level noise even where such noise has frequency 
components within the target frequency range and to otherwise 
substantially eliminate entrained noise from a detected target signal. 
Yet another object of the present invention is to provide a signal detector 
and detection methodology wherein a target signal, once present, is 
compressed to have an amplitude that is within a selected amplitude band 
in order to create a relatively noise free and stable signal after which 
the compressed target signal is frequency translated to another band and 
is amplitude modulated directly proportionally to the amplitude of the 
incoming signal so that the resultant output is a scale model amplitude 
replica of the original signal. 
Still a further object of the present invention is to provide a signal 
detector that has reduced susceptibility to both mechanical vibration and 
temperature change. 
Yet a further object of the present invention is to provide an inexpensive 
ultrasonic detector that is not modulated by sonic or sub-sonic signals 
and which is inexpensive to produce and yet which will display target 
signals both visually and audibly with the audible display portion being 
such that the frequency and amplitude of the ultrasonic signal is 
represented. 
The present invention, therefore, is generally directed to a signal 
detector and a signal detection method for monitoring a range of 
oscillatory signals that define a signal universe in order to detect the 
presence therein of one or more target signals having a frequency 
characteristic within a selected frequency range. In its broad form, the 
signal detector includes an input processor that receives oscillatory 
signals and removes those oscillatory signals which do not have the 
frequency characteristics of the selected range and which produces an 
input signal corresponding to target signals within the selected frequency 
range. An amplifier then receives the input signal, either directly or 
after further processing and amplifies the input signal at a gain that 
increases proportionally to the increasing input amplitude of the original 
input signal in order to produce a detector output signal. Preferably, 
this signal detector includes an input signal cut-off that inhibits input 
signals which have an amplitude that is less than the selected threshold 
and further includes adjustment circuitry for adjusting the magnitude of 
amplitude of the input signal before being introduced into the amplifier. 
Where the input signal is further processed prior to being amplified, the 
signal detector includes means for splitting the input signal into a 
control signal that is operative to control the gain of the amplifier and 
into a primary input component. This detector then includes amplitude 
compressor means for establishing a ceiling amplitude for the primary 
input component and for compressing or clipping to said ceiling amplitude 
for all amplitudes of the primary input component that exceeds the 
ceiling. The resultant detector output signal can be displayed either 
visually, audibly or both. 
In a more specific embodiment, the signal detector includes a first control 
circuit that is electrically coupled to the input signal and is operative 
to produce a first control signal proportionally to the amplitude of the 
input signal. Processing circuitry is provided to then receive the input 
signal and which in turn produces an intermediate signal having an 
intermediate frequency proportional to the frequency of the input signal. 
This processing circuitry includes a plurality of stages including a 
variable amplification circuit that converts the input signal in to an 
initial intermediate signal that has an intermediate frequency and a 
compressed, substantially uniform amplitude. The processing circuitry then 
converts this initial intermediate signal into a square-wave intermediate 
signal with the square-wave generator including frequency response 
circuitry so that the responsiveness of the square-wave generator is 
increased at higher frequencies. The square-wave signal may then be 
further processed by scaler circuitry to produce a scaled intermediate 
signal having a scaled frequency that is proportional to the frequency of 
the square-wave intermediate signal. The processing circuitry then 
includes an integrator which integrates the scaled intermediate signal to 
produce a resultant intermediate signal that provides an input for the 
first amplifier. This resultant intermediate signal is then amplified, as 
described above, at a gain that is proportional to the amplitude of the 
intial input signal. In the preferred embodiment of the present invention, 
the signal detector is directed to detection of ultrasonic signals which 
may be processed and scaled into an audible frequency. 
In the broad form of methodology, the present invention provides an method 
for the detection of oscillatory signals and for producing an output 
corresponding to the presence of target signals within a selected 
frequency range. Broadly, this method includes a first step of receiving 
oscillatory signals and filtering those oscillatory signals to remove 
therefrom substantially all oscillatory signals which do not have a 
frequency within the selected frequency range and thereafter producing an 
input signal when at least one of the target signals is present. This 
input signal has an input amplitude and input frequency representative of 
the frequency and amplitude of all these selected signals which are then 
present. Next, the broad methodology produces a detector output by 
amplifying the input signal at an amplification gain that is a function of 
the input amplitude. This detector output signal is then displayed. 
This broad method can include the step of deriving an intermediate signal 
from the input signal so that the step of amplifying the input signal is 
accomplished by amplifying the intermediate signal. The input signal may 
first be processed to block out all input signals having initial 
amplitudes which are smaller than a given threshold. Further, the input 
signal may be expanded and compressed according to the present methodology 
such that input signals having smaller initial amplitudes are amplified 
with higher gain and such that input signals having greater amplitudes are 
amplified with a lesser gain to produce an initial intermediate signal of 
substantially uniform amplitude. The initial intermediate signal may also 
be scaled to alter the frequency thereof and, preferably this by an 
intregal division of the frequency. The detector output is displayed 
either audible, visually, or both. 
In a more specific form of the methodology according to the present 
invention, a method for monitoring ultrasonic sound signals is described. 
Here, the method indicates the presence of ultrasonic signals within a 
selected range, and includes a first step of receiving sound signals and 
producing a first electrical signal having a composite frequency and 
amplitude corresponding to the sound signals. Next, the first electrical 
signal is filtered to remove all components thereof which do not have a 
frequency within the selected frequency range so that an input signal is 
produced corresponding to all sound components having a frequency within 
the selected range. A primary input component and an input control 
components are then produced from the input signal. The primary input 
component is processed to produce an intermediate signal having an 
intermediate frequency that is scaled into an audible frequency range and 
which has a substantially uniform amplitude. The intermediate signal is 
amplified proportionally to the amplitude of the input control signal to 
produce a detector output signal as an amplitude modulated scaled replica 
of the input signal. Again, those primary input components which have 
amplitudes less than a selected threshold are inhibited and the primary 
input components are amplitude compressed as an initial intermediate 
signal having a frequency corresponding to the input signal and a 
substantially uniform amplitude. The initial intermediate signal is then 
translated into a square-wave signal, and, thereafter, the square-wave 
signal may be processed into a scaled signal having a frequency that is an 
intregal division of the frequency of the square-wave signal. The scaled 
signal may then be integrated into an integrated signal to define the 
intermediate signal. Again, the detector output may be displayed. 
These and other objects of the present invention will become more readily 
appreciated and understood from a consideration of the following detailed 
description of the preferred embodiment when taken together with the 
accompanying drawings, in which:

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT 
The present invention is directed to a signal detector and a signal 
detection methodology directed to monitoring a range of signals that 
define a signal universe in order to detect within that universe the 
presence of one or more target signals having frequency characteristics 
that are within a selected frequency range. It should be understood that 
the signal detector and method according to the present invention for 
purposes of an exemplary embodiment, are described with respect to an 
ultrasonic detector. However, it should further be appreciated and 
understood that the detector circuitry and methodology described herein 
has a broader application to the general monitoring of signals so that the 
techniques described herein can be employed by the ordinarily skilled 
person in the field to monitor other signals, for example, electromagnetic 
signals, as well as sound signals. Thus, virtually any type of detectable 
oscillatory signal to which a transducer may respond to produce an 
electrical signal can be monitored by the present system and method. 
With respect to the exemplary embodiment, therefore, it may be seen with 
reference to FIG. 1(a) that signal detector 10, in its broadest form, is 
shown to include input processing circuitry 14 operated to receive 
oscillatory signals from input 12 to produce an input signal at 16 that is 
split into a control component 17 and a primary input component 17'. Input 
signal 16 is fed into both intermediate processing circuitry 18 and 
expander/modulator circuitry 20. Intermediate processing circuitry 18 
produces a resultant intermediate signal 22 that is fed for amplification 
into expander/modulator circuitry and thereafter into amplifier circuitry 
24. Alternately, FIG. 1(b) shows the most general form of the present 
invention wherein primary input component 17' is looped directly into 
expander/modulator circuitry 20 as is shown in at 26. Here, intermediate 
processing circuitry 18 is eliminated. However, it is far preferable, as 
described below, to process the primary input component 17' prior to 
amplification thereof. Expander/modulator circuitry 20 amplifies 
intermediate signal 22 directly proportionally to the amplitude of the 
input signal 16. That is, where input signal 16 has a low amplitude, the 
gain is low; whereas, when the amplitude of input signal 16 is high, the 
gain is also high. It is preferable but not required that this direct 
proportional relationship is a linear function. Detector output signal 28 
may be displayed directly at output 32 and/or may be further amplified by 
amplifier circuitry 24 prior to display. It should be appreciated that 
input oscillatory signals which are fed into input processing circuitry 14 
typically will be a mixed signal containing a variety of signal components 
of different frequencies and amplitudes. Thus, for example only, FIG. 3 
shows three representative signals including a low frequency signal 34, a 
high frequency signal 36 and an intermediate frequency signal 38 which may 
be mixed together to form a mixed signal. A mixed signal 40 is shown in 
FIG. 2 as representative of the type of oscillatory signal received at 
input 12. It may be noted that the various oscillatory signals are 
typically corrupted with noise and that the resulting mixed signal 40 is 
likewise corrupted with noise components. 
The exemplary embodiment of the signal detector according to the present 
invention may better be seen with reference to FIG. 4 which is a block 
diagram of the signal processing circuitry according to the present 
apparatus and which implements the preferred method of the present 
invention. Here, a universe of ambient signals is presented by input 12 
and fed into input processing circuitry 14 to generate an input signal 16 
as described above. Input processing circuitry 14 includes a transducer 
50, a dual stage pre-amplification circuitry 52, bandpass filter circuitry 
54, amplification circuitry 56 and sensitivity adjust circuitry 58. 
Transducer 50 may be a piezoelectric ultrasonic microphone or any other 
appropriate transducer operative to convert sound signals from input 12 
into a transducer electrical signal 51 having a composite amplitude and 
frequency that corresponds to that of the sound signals from input 12, 
which, for explanatory purposes, is similar to FIG. 2. Bandpass filter 54 
operates to establish a selected frequency characteristic in the form of a 
selected frequency window for target signals which are sought within input 
12. Thus, components of the oscillatory signals from input 12 which do not 
have the frequency characteristic within the band of bandpass filter 
circuitry 54 are removed to produce a filtered signal 55 that is fed into 
amplifier 56. Thus, the resulting input signal 16 only has present therein 
target signals having the frequency within the range established by 
bandpass circuitry 54. It should be appreciated, though, that filter 
circuitry could be implemented which provides a threshold and/or a ceiling 
target frequency as opposed to both. Input signal 16, after being 
processed by sensitivity adjustment circuitry 58 has an input amplitude 
which is controllably adjusted by sensitivity adjustment circuitry 58 
which may simply be a variable resistance, as described below. 
Input signal 16 is split into two components. A control component 17 is 
electrically coupled to expander/modulator circuitry 20 which includes a 
first linear averaging rectifier 60 which processes control component 17 
into a first control signal 61 which controls the gain of a variable gain 
cell 62 having output 64 fed into first operational amplifier 70. 
Primary input component 17' of input signal 16 defines the input for 
intermediate processing circuitry 18 that is processed thereby into 
resultant intermediate signal 22. More specifically, primary input 
component 17' enters intermediate processing circuitry 18 through input 
cut-off circuitry 80 which acts to inhibit all portions of primary input 
component 17' which have an amplitude less than established minimum 
threshold. Thus, only input signals having amplitude in excess of the 
threshold are allowed to be processed by intermediate processing circuitry 
18. This accordingly acts to eliminate low level noise even where such 
noise has a frequency characteristic within the target frequency range. 
Input cut-off circuitry 80 feeds a second linear averaging rectifier 82, a 
second variable gain cell 84 and a second operational amplifier 86. 
Rectifier 82, variable gain cell 84 and operational amplifier 86 act to 
establish a ceiling amplitude for an initial intermediate signal 87. To 
this end, linear averaging rectifier 82 converts the signal from input 
cut-off circuitry 80 into a DC current which varies linearly in magnitude 
as a function of the average value of the input signal. This DC current 
then controls variable gain cell 84, which may be a temperature 
compensated transconductance amplifier. As the DC current output of linear 
averaging rectifier 82 changes, the gain of the variable gain cell 84 
changes. Thus, a high input signal from the linear averaging rectifier 
translates into a low impedance across the variable gain cell 84; 
likewise, a low input signal from the linear averaging rectifier 
translates into a high impedance across the variable gain cell. 
Accordingly, as the current from linear averaging rectifier 82 increases 
in a linear fashion, the feedback impedance across the variable gain cell 
reduces to cause operational amplifier 86 to reduce its gain. Similarly, 
as the input level is decreased the DC current from the linear averaging 
rectifier decreases at a linear fashion to increase the feedback impedance 
across the variable gain cell thus causing operational amplifier 86 to 
increase its gain. The result that initial intermediate signal 87, as 
described below, becomes a stable signal derived by having the input 
signal's low input amplitudes modified with higher gain than high input 
amplitudes. Therefore, the input signal is compressed within a selected 
amplitude band. 
Initial intermediate signal 87 is then passed through buffer circuitry 88 
which is a unity gain buffer in order to isolate operational amplifier 86 
and the square wave generator 90. The output of buffer 88 is fed into the 
square-wave generator 90, which is preferably in the form of a modified 
Schmitt-Trigger that is modified as described more thoroughly below. The 
output of square-wave generator 90 is in the form of a square-wave signal 
91 that is then processed by scaler circuitry 92 to produce a scaled 
signal 93 which has a frequency that is proportional to the frequency of 
primary input component 17' and thus proportional to the frequency of 
input signal 16. Preferably, scaler 92 acts to reduce the frequency of the 
intermediate square-wave signal by an intregal divisor. The scaled signal 
93 is then integrated by integrator circuitry 94 which produces a 
triangle-wave signal as resultant intermediate signal 22 that has a 
substantially uniform amplitude and a frequency equal to the frequency of 
scaled signal 93. 
Resultant intermediate signal 22 feeds expander/modulator circuitry 20 so 
that it is amplified by first variable gain cell 62 and first operational 
amplifiers 70 a detector output signal 28. As noted above, the gain in 
amplification of intermediate signal 22 is controlled by control signal 61 
generated by linear averaging rectifier 60. As the amplitude of input 
signal 16, and thus control component 17 increases, the DC output of 
linear averaging rectifier 60 increases. This increase is linear in 
magnitude as a function of the average value of that input signal. 
Variable gain cell 62 is again a temperature compensated transconductance 
amplifier so that signal 64 varies in amplitude proportionally to the 
average amplitude of the input signal 16. Thus, the intermediate signal 22 
is a modulated manner directly proportionally to the average amplitude of 
input signal 16. Accordingly, the initial detector output signal 28 is 
amplitude modulated as a scaled replica of the original input signal; 
however, the resultant initial detector output signal 28 has been 
"cleaned" of noise and has been shifted in frequency. The initial detector 
output signal 28 may be displayed at video output 100 which, for example, 
may be a voltage meter. Initial detector output signal 28 can further be 
amplified by power amplifier 72 to form a final detector output signal 30 
that can drive an audio output 102 which, for example, may be a set of 
earphones, a speaker, and the like. 
An understanding of the shapes of the signals at different stages in 
processing can be further understood by the representative examples shown 
in FIGS. 5-11. In these Figures, it may be appreciated that FIG. 5 shows a 
representative input signal 16 which has a frequency within the selected 
frequency range passing through bandpass filter 54. Maximum gain block 80 
provides the input cut-off to establish a threshold T so that signals 
within the range "a" to "b" are not amplified; likewise, signals within 
the range "c" to "d" are not amplified. For those signals in the range 
from "b" to "c" as is shown in FIG. 6, are amplified with a variable gain 
to expand the lower amplitude signals which are above threshold T such as 
components 101, 102, 103 and 107. For those components having amplitudes 
greater than ceiling C, such as components 104 and 105, such amplitudes 
are clipped, again as shown in FIG. 6. Thus, an initial intermediate 
signal is represented by FIG. 6 and this initial intermediate signal 87 is 
further processed by square-wave generator 90 to produce square-wave 
signal 91, as shown in FIG. 7. Here, the pulses of the signal in FIG. 6 
are translated into a square-wave signal having a relatively uniform 
amplitude A, with the same frequency as the pulses of initial intermediate 
signal 87. Thus, square-wave signal 91 has pulse components 101'-107'. 
After square-wave signal 91 is produced, the scaler 92 further processes 
the square-wave signal to divide the frequency of the square-wave signal 
to a frequency within the audible range. Preferably, this division for the 
ultrasonic detector described by this invention is by an intregal factor 
of n=16, but it should be understood that other scalers including both 
divisors and multipliers, are within the scope of this invention. With 
respect to FIG. 8, however, a division of n=4 is shown for representative 
purposes; thus pulses 101'-104' create a single pulse 108. Assuming the 
signal pattern in FIGS. 5, 6, and 7, for a continuous wave train, FIG. 8 
shows a plurality of pulses, such as pulses 108 and 109, which have a 
frequency that is an n=4 intregal division of square-wave pulse 91. This 
scaled pulse 93 is then intregated into integrated intermediate signal 22, 
as shown in FIG. 9. Here, integrator 94 takes each square-wave pulse to 
produce a triangular-wave pulse such as pulse 108' and 109'. 
As noted above, intermediate signal 22 is then amplified in a variable 
manner according to the amplitude of the original input signal. Input 
control signal 17 is introduced through first linear averaging rectifier 
60 and first variable gain cell in order to vary the gain of 
expander/modulator circuitry 20. FIG. 10 shows the gain level of 
expander/modulator circuitry 20 as a function of the input signal 16 so 
that it may be seen that the gain of the expander/modulator circuitry is a 
function of the amplitude of the signal shown in FIG. 5. 
Expander/modulator 20 has a maximum output level G.sub.1 so that portions 
of the input signal that would generate a gain in excess of maximum output 
level G, such as that shown at 110, are clipped at gain G. The 
intermediate signal 22 from FIG. 9 is thus amplfied according to the 
corresponding gain shown in FIG. 10 gain for a series of pulses such as 
108" and 109" and so on to produce an amplitude modulated signal that 
forms the scaled replica of the original signal. Thus, it may be seen that 
triangular-wave 108' is amplified by the corresponding gain at region 98 
while pulse 109' is amplified by the corresponding gain at region 99. This 
continues for the stream of pulses to create, for example purposes only, 
the detector output signal 28 shown in FIG. 11. This signal may be audibly 
displayed to produce an audible signal having a tone that is proportional 
to the frequency of the ultrasonic sound signal received by transducer 50 
and which has an amplitude that is a model of the amplitude of the 
original input signal. However, it is important to note that the 
intermediate processing circuitry has cleaned this signal and made it into 
an extremely stable signal for processing. Further, since the gain control 
has a maximum output level G, as shown in FIG. 10 the audible signal can 
not increase excessively so as to damage the human ears when the detector 
is being utilized. Further, a visual output, such as a voltage meter, a 
set of diodes, or the like, may be employed to register the varying 
amplitudes of the output signal 30. 
FIG. 12 shows a circuit diagram of the exemplary embodiment of the present 
invention employed as an ultrasonic signal detector. As noted above, this 
but one possible implementation of the invention described herein and 
represents a particularly suitable adaptation of the signal detector and 
methodology described. In FIG. 12, then, piezoelectric transducer 50 is 
connected between ground 110 and input 120 of a first pre-amplifier 116. 
Input 118 of pre-amplifier 116 is connected to ground through a resistor 
114 and input 120 of pre-amplifier 116 is connected to ground through 
resistor 115. Resistor 122 is connected in parallel to pre-amplifier 116 
and therefore across input 118 and output 119 in order to set the gain of 
the pre-amplifier 116. Output 119 of pre-amplifier 116, at point A, feeds 
the input 128 of a second pre-amplifier 126 through a capacitor 123 and a 
resistor 124 connected in series with one another. Input 130 of 
pre-amplifier 126 is connected to ground. Resistor 132 and capacitor 133 
are connected in parallel to one another across input 128 and output 129 
of pre-amplifier 126 again to set gain. The result of connecting 
pre-amplifiers 116 and 126 in series as opposed to the utilization of a 
single pre-amplifier establishes a configuration operative to improve the 
signal to noise ratio of the pre-amplification of mixed composite signal 
113 generated by transducer 112. 
The signal existing pre-amplifier 126, at point B, is passed through an 
operational amplifier 140 which, with its associated components is 
configured as a bandpass filter. Specifically, the signal from 
pre-amplifier 126 is passed through resistor 134 and capacitor 136 which 
are connected in series with one another and then fed into input 141 of 
operational amplifier 140. Resistor 134 and capacitor 136 have a 
connecting point 137, and a resistor 138 interconnects point 137 to ground 
110. Capacitor 139 is wired from point 137 to output 143 of pre-amplifier 
140. A resistor 144 is connected across pre-amplifier 140 from input 141 
to output 143. This configuration allows for the elimination of low 
frequency signals such as those which would be caused by infrasonic and 
sonic signals as well as mechanical vibrations of the device. 
The resultant signal, at point C, is then capacitively coupled to a final 
pre-amplifier 150 by means of capacitor 146 and resistor 147 connected in 
series to one another to the input 151 of pre-amplifier 150. Input 152 of 
pre-amplifier 150 is connected to ground 110, and a capacitor 148 and a 
resistor 149 are connected in parallel to one another and across input 151 
and output 153 of pre-amplifier 150 to set its gain. A variable resistor 
160 interconnects output 153 of pre-amplifier 150 to ground 110 and 
includes a wiper arm 162 which carries the resultant input signal. Thus, 
variable resistor 160 defines a sensitivity adjustment for the system 
since it alters the amplitude of the input signal on electrical lead 164, 
at point D. 
The input signal from point D is connected to the expander/modulator 
circuitry and to the intermediate processing circuitry. First, with 
respect to the intermediate processing circuitry, the input signal feeds a 
linear averaging rectifier 170 through a capacitor 166 and a resistor 168 
connected in series with one another. Capacitor 166 is a coupling 
capacitor included to minimize DC off-set errors prior to inputting the 
signal to rectifier 170. Capacitor 166 and resistor 168 are connected 
together at point 167, and this point 167 is connected to negative rail 
V.sup.- through resistor 169 which forms part of the maximum gain block 
that sets a minimum amplitude threshold for a signal to be processed. That 
is, resistor 169 acts to bleed off any signal not having sufficient 
amplitude so that, unless some signal having minimum amplitude is present, 
there is no resultant signal for which rectifier 170 will average the 
amplitude thereof. Capacitor 172 acts as a damper on rectifier 170 by 
interconnecting rectifier 170 to ground. Thus, capacitor 172 sets the 
speed at which the output of linear averaging rectifier 170 changes at 
output 174 thereof. Lead 176 thus carries a gain control signal for 
variable gain control cell 200 whose operation is described below. 
The input signal from point D is also fed to an operational amplifier 190 
through resistor 178, capacitor 179 and resistor 180 connected in series 
with one another to the input 191 of amplifier 190. Resistor 178 acts also 
to set the minimum threshold for a signal to be processed so that, 
together with resistor 169, resistor 178 forms a maximum gain block for 
the intermediate processing circuitry. Capacitor 179 is, again, a coupling 
capacitor to minimize DC off-set errors for the input pre-amplifier 190. 
Input 191 of pre-amplifier 190 is connected to negative rail V.sup.- 
through resistor 182 and input 192 of operational amplifier 190 is 
connected to a reference voltage source 184. 
Variable gain control cell 200 has an input 202 connected to input 191 of 
operational amplifier 190 so that variable gain control cell 200 also 
receives the signal from resistors 178, 180 and capacitor 179. Capacitor 
204 is connected to terminal 201 of the variable gain control cell 200 and 
acts to bypass the variable gain control cell in order to correct for 
harmonic distortion. Output 203 of variable gain control cell 200 is then 
connected to output 193 of operational amplifier 190 through a resistor 
206 and a capacitor 208 connected in series with one another. Capacitor 
208 is again a coupling capacitor whose function minimizes DC off-set 
errors. Input 191 of operational amplifier 190 is connected to the output 
193 of operational amplifier 190 through a pair of resistors 210 and 212 
which are connected in series with one another at point 211, and a 
capacitor 214 interconnects point 211 to ground. The network composed of 
resistors 210, 212 and capacitor 214 set the DC output quiescent voltage 
level for the variable gain control cell 200 and the operational amplifier 
190. 
In operation, it should be appreciated that the output of linear averaging 
rectifier 170 is presented to input 205 of variable gain control cell 200 
in order to vary the gain of operational amplifier 190. It may be seen 
from FIG. 12(b) that this control is not a closed loop system; thus, as 
the input level, that is the amplitude, of the input signal at point D 
increases, the current in rectifier 170 linearly increases to reduce the 
feedback impedance across variable gain control cell 200 thus causing the 
operational amplifier 190 to reduce its gain. A decrease in input 
amplitude decreases the current output of rectifier 170 thus causing an 
increase in the feedback impedance across variable gain control cell 200 
which, in turn causes operational amplifier 190 to increase its gain. 
Thus, the input signal at point D results in an initial intermediate 
signal at point E that is compressed within an amplitude band. That is, 
where the amplitude is not sufficient to pass through the maximum gain 
block established by resistors 169 and 178, no signal is present at point 
E. On the other hand, the amplitude of signals having excessive amplitude 
is compressed and clipped by operational amplifier 190. This output is in 
the form, therefore, of a relatively stable, noise-free signal. 
The initial intermediate signal present at point E is further processed by 
the intermediate processing circuitry. This initial intermediate signal is 
coupled to input 221 of unity gain buffer 220 through capacitor 218, and 
unity gain buffer 220 drives a modified Schmitt-Trigger 230. Input 221 of 
unit gain buffer 220 is connected to ground through resistor 224 and input 
222 of unit gain buffer is directly connected to the output 223 thereof. 
Output 223 of buffer 220 is connected to input 232 of Schmitt-Trigger 230 
while input 231 of Schmitt-Trigger 230 is connected to ground by means of 
a resistor 234 and a capacitor 236 connected in parallel to one another. 
Output 233 of the Schmitt-Trigger 230 is connected to input 231 thereof by 
resistor 246 and output 233 further clocks a scaler 240 at the input 241. 
The output of Schmitt-Trigger 230, at output 233, is a square-wave whose 
frequency is the frequency of the initial intermediate signal from 
location E. However, due to the modification of the classical 
Schmitt-Trigger achieved by connecting input 231 to ground through 
parallel resistor 234 and capacitor 236, the transfer function is changed 
from the simple equation: 
##EQU1## 
where: 
s=Laplace transform variable 
e.sub.o =Voltage at output 233 
e.sub.1 =Voltage at input 232 
R.sub.1 =Value of resistor 234 
R.sub.2 =Value of resistor 246 
into the equation: 
##EQU2## 
where: 
s=Laplace transform variable 
R.sub.1 =Value of resistor 234 
R.sub.2 =Value of resistor 246 
C.sub.1 =Value of capacitor 236 
This modification transforms Schmitt-Trigger 230 into a frequency dependent 
Schmitt-Trigger such that the higher the frequency of the input signal, 
the more sensitive the circuit is and the faster the system changes 
states. Thus, as the frequency of the signal being input into the 
Schmitt-Trigger increases, the Schmitt-Trigger responds faster thereby 
enabling the modified Schmitt-Trigger to produce discreet pulses at higher 
frequencies. 
Scaler 240 receives the square-wave signal from Schmitt-Trigger 230. Input 
243 of scaler 240 is connected to negative rail V.sup.-. Scaler 240 
divides the frequency of the square-wave signal to produce a scaled signal 
that is an integral division of the output of Schmitt-Trigger 230. In the 
exemplary embodiment, N=16 so that, for every sixteen pulses from the 
Schmitt-Trigger, a single pulse is produced by scaler 240. The scaled 
pulse from output 242 is then fed into integrator 250 by means of resistor 
247. The signal is presented to input 251 of integrator 250 while input 
252 is connected to ground. A capacitor 256 is connected across input 251 
of integrator 250 and output 253 thereof. Integrator 250 thus acts to 
integrate the scaled square-wave pulse into a stable triangular wave. This 
triangular wave is in the form of a resultant intermediate signal at 
location F, and is fed into the expander/modulator circuitry 200. 
Turning, then, to the expander/modulator circuitry, it may be seen that the 
original input signal, at location D, is fed into linear averaging 
rectifier 260 through a coupling capacitor 266 and a resistor 268 
connected in series with one another. An output 262 of linear averaging 
rectifier 260 is connected to negative rail V.sup.- through capacitor 264, 
and the output at 263 of rectifier 260 is connected to variable gain 
control cell 270. Rectifier 260 generates a gain control signal, in the 
form of a DC voltage, which causes variable gain control cell 270 to vary 
its gain in accordance to the average amplitude of the initial input 
signal from location D. Output 273 of variable gain control cell 270 is 
connected to negative rail V.sup.- through capacitor 274. 
As noted above, the resultant intermediate signal from location F is fed 
through coupling capacitor 258 and is inputted into variable gain control 
270 through input 271. This feed is accomplished through a resistor 259 
connected in series with capacitor 258. The output at output 272 of 
variable gain control cell 270 is provided to an operational amplifier 280 
and, specifically, is inputted into the input 281 thereof. Input 282 of 
operational amplifier 280 is connected to reference voltage 284 while the 
output of 272 of variable gain control cell 270 is connected to negative 
rail V.sup.- through a resistor 286. Resistor 287 is connected across 
input 281 of operational amplifier 280 and output 284 thereof. 
It should thus be appreciated that the resultant detector signal, at 
location G, is a scaled replica of the initial input signal having been 
"cleaned" and integrated by the intermediate processing signal. More 
particularly, it may be seen that the intermediate processing signal 
produces a very stable triangular wave as a resultant intermediate signal 
which has a frequency that is an integral division of the frequency of the 
original input signal that has been compressed and which has had various 
noise components removed. This stable triangular wave signal is amplitude 
modulated by the expander/modulator circuitry since the linear averaging 
rectifier 260 provides a modulating variable gain control signal for 
variable gain amplifier 270 so that the triangular wave amplitude varies 
as the average amplitude variation of the original input signal. 
The detector output at location G can then be displayed in a variety of 
well known means. For example, this output can be coupled to a visual 
read-out in the form of a meter, such as shown in FIG. 4 but can also be 
power amplified by amplifiers 290, 292 and 294 which are connected to ear 
phones 296 to provide an audible output. The detector output signal from 
location G is fed first to amplifier 290 by means of resistor 288 with the 
input of power amplifier 290 being connected to ground through resistor 
289. Amplifiers 292 and 294 provide power to the pair of speakers and 
earphones 296, as is well known in the art. 
Power for the signal detector described above is provided by power supply 
300 which preferable includes a power source in the form of battery 302 
which may be turned on and off by switch 304. Capacitor 306 is connected 
in parallel with battery 302 and a resistor 308 connects the positive 
terminal of battery 302 to input 311 of unity gain buffer 310. Input 311 
of unity gain buffer 310 is connected to the negative terminal of battery 
302 through resistor 316. Input 312 of unitary gain buffer 310 is 
connected to output 313 thereof and to ground. Output 313 of operational 
amplifier 310 is also connected, through capacitor 318, to negative rail 
V.sup.- which may be seen to also be the negative terminal of battery 302. 
It may thus be seen that resistors 308 and 316 act as a voltage divides so 
that the negative battery terminal defines negative rail V.sup.- which the 
positive battery terminal defines V.sub.cc. Further, division point 307, 
acting as the input to unity gain buffer 310, defines the ground. If 
desired, a battery monitor circuit (not shown) could be used to monitor 
battery 302 to warn the user of a low battery condition. These monitor 
circuits are of a type well known in the art. 
From the foregoing description, it is believed that the construction of the 
present invention; should be readily understood by the ordinary circuit 
designer. To further explain this invention as implemented in the form of 
an ultrasonic detector, though, it is perhaps helpful to provide the 
following Table I which lists component values for the circuit shown in 
FIG. 12: 
TABLE 1 
______________________________________ 
RESISTORS CAITORS 
Value Value 
Element # 
(in ohms) Element # (in microfareds) 
______________________________________ 
114 1000 123 0.1 uF 
115 10K 133 12 pF 
122 15K 136 1 uF 
124 1000 139 1 uF 
132 75K 146 0.1 uF 
134 10K 148 100 pF 
138 270 166 0.1 uF 
144 39K 172 0.1 uF 
147 1000 179 0.1 uF 
149 100K 204 0.1 uF 
160 0-10K (variable) 
208 0.1 uF 
168 10K 214 10 uF 
169 100K 218 0.1 uF 
178 27K 236 2.2 uF 
180 20K 256 330 pF 
182 30K 258 0.1 uF 
206 20K 264 2.2 uF 
210 24K 266 0.1 uF 
212 24K 274 0.1 uF 
224 1000 306 22 uF 
234 1000 309 0.1 uF 
246 15K 318 10 uF 
247 220K 
259 20K 
268 10K 
286 30K 
287 20K 
288 5600 
289 51 
308 100K 
316 100K 
______________________________________ 
Further, it should be understood that pre-amplifiers 120 and 126, bandpass 
filter 140 and pre-amplifier 150 are common circuits such as may be formed 
employing standard chip components. One example is a four stage LM837 
integrated circuit chip; here, pre-amplifiers 120, 126, 150 and filter 140 
are constructed from respective stages of the LM837 circuit chip. 
Similarly, Schmitt-Trigger 220, buffer 2330, integrator 250 and unity gain 
buffer 310 may be constructed by respective stages of a signal, four-stage 
LF444 integrated circuit chip. Scaler 240 may be constructed from a CD4060 
integrated circuit ship. Amplifiers 290, 292 and 294 may be fabricated 
from a single TDA2052 integrated circuit chip. With respect to the linear 
averaging rectifiers 170, 260, the variable gain cells 200, 270 and 
operational amplifiers 190, 280, the same may be standard type 
sub-circuits such as those described, for example, in the Linear 
Applications Handbook (National Semiconductor Corp. 1980), the disclosures 
of which are incorporated herein by reference, all be understood by the 
ordinarily skilled circuit designer. 
Based on the foregoing, it should be appreciated that the detector output, 
as presented by earphones 296 may present an audible signal that is a 
replica of the ultrasonic signals detected by transducer 250. Assuming an 
audible range of approximately 40-15,000 cycles per second, where scaler 
240 divides the frequency by 16, ultrasonic signals having a frequency of 
up to approximately 240,000 cps can be heard by the user. The frequency of 
the tone heard by the user is a replica of the frequency of the ultrasound 
detected by transducer 250 so that, for higher pitches, the ultrasonic 
signal is of a higher frequency. The loudness of the tone indicates of the 
amplitude of the original ultrasonic signal although excessive amplitudes 
are suppressed by the processing circuitry and the variable gain. Where 
the device is used to detect gas leaks, this information allows the user, 
for example, to estimate the size of the leak by both as to geometric size 
and escaping gas. 
Further, from the foregoing, it should be appreciated that the method 
according to the present invention may broadly be described to include 
certain processing steps. More particularly, the broad method of the 
present invention is directed to monitoring a universe of oscillatory 
signals and producing a detector output in response to the presence of one 
or more target signals having frequency characteristics within a selected 
frequency range. First, this method includes the step of receiving the 
oscillatory signals and then filtered them to remove substantially all 
oscillatory signals which do not have a frequency within the selected 
frequency range. An input signal is produced whenever at least one of the 
selected target signals is present within the received oscillatory 
signals; this input signal has an input amplitude and input frequency that 
is a composite of all target signals present within the monitored signals. 
The input signal is amplified at a gain to produce the detector output 
wherein the gain at the amplification gain of the input signal is 
adjustably varied as a function of the input amplitude. Finally, the 
detector output is displayed. 
Preferably, this broad method includes the step of deriving an intermediate 
signal from the input signal such that the step of amplifying the input 
signal is accomplished by amplifying the intermediate signal. Further, the 
step of deriving the intermediate signal preferably includes the step of 
producing an initial intermediate signal that is compressed by amplifying 
smaller initial input amplitudes at a higher gain then greater initial 
input amplitudes. Further, it is preferred to scale the initial 
intermediate signal to alter the frequency thereof and to include the step 
of inhibiting amplification of input signals having amplitudes less than a 
selected threshold. The step of displaying the detector output can either 
be visual, audible or both. 
In the exemplary form of the present invention, the preferred method is 
directed to the detection of ultrasonic signals which are present in a 
universe of sound signals. This method includes the first step of 
receiving the sound signals and producing, in response thereto, a first 
electrical signal having a composite frequency and amplitude corresponding 
to these sound signals being monitored. Next, this first electrical signal 
is filtered to remove all components thereof which do not have a frequency 
within the selected frequency range, and an input signal is produced 
corresponding to all sound components having a frequency within the 
selected frequency range which are present in the first electrical signal. 
Next, the method includes the step of deriving a primary input component 
and an input control component from the input signal. The primary input 
component is processed to produce an intermediate signal having an 
intermediate frequency that is scaled into an audible frequency range and 
which has a substantially uniform amplitude. The intermediate signal is 
then amplified proportional to the amplitude of the input control 
component of the input signal as an amplitude modulated scaled replica of 
the input signal. 
This exemplary method may further include the step of inhibiting the 
processing of primary input components having amplitude less than a 
selected threshold and, if desired, the step of compressing the amplitude 
of the primary input components to produce an initial intermediate signal 
having a frequency corresponding to the input signal and a substantially 
uniform amplitude. The initial intermediate signal may be translated into 
a square-wave signal, and the square-wave signal may then be translated 
into a scaled signal having a frequency that is an intregal division of 
the frequency of the square-wave signal. Also, this method may include the 
step of integrating the scaled signal into an integrated signal defining 
the intermediate signal. Again, the detector output signal may be 
displayed either visually, audibly or both. 
Accordingly, the present invention has been described with some degree of 
particularity directed to the preferred embodiment of the present 
invention. It should be appreciated, though, that the present invention is 
defined by the following claims construed in light of the prior art so 
that modifications or changes may be made to the preferred embodiment of 
the present invention without departing from the inventive concepts 
contained herein.