Note: Descriptions are shown in the official language in which they were submitted.
~lSS183
LOW AMPLITUDE SIGNAL DETECTOR
.
This invention relates to a system for detecting
signals which are received in noise environments which
result in a very low signal to noise ratio.
Where signals which are received have extremely
low amplitudes, or are received along with high noise levels,
a low signal to noise ratio results. Such signals are very
difficult to detect, particularly where the signal to noise
ratio is less than unity. Detection of such signals has
become particularly important in submarine detection, with
the advent of the quiet submarine, in order that a fix be
established without too much difficulty.
A number of techniques have been devised for
increasing the signal to noise ratio, and these have generally
fallen into the two categories of incoherent, and coherent
processing.
Incoherent techniques have generally utilized
amplification techniques, clipping, band width shaping, etc.
Coherent techniques utilize a prior knowledge of the shape
of the received wave in circuits which provide enhancement
of the recovered signal.
~he present invention is directed to a partly
coherent type of system, which time averages the multiplied
output of two independent sets of signals plus noise for a
relatively long period of time, while partly cancelling out
the noise. The longer the period of time averaging, the
closer the signal portion of the received signal plus noise
approximates direct current, and this resultant approximation
is representative of the received signal.
Time averaging circuits using resistor-capacitor
integration are not new. However, it has been found that
the use of a transformer as the basis of the time averaging
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circuit results in considerable impxovement over the resistor-
capacitor time averaging circuit in signal rise-time, fall-
time, and apparent resulting signal to noise ratio.
Time averagers are usually employed in a two input
broadband correlator circuit by means of a two input multiplier
followed by a time averager. In the described embodiment of
the invention the use of the semi-coherent time averager en-
hances the detection capability of a correlator.
The present semi-coherent time averager additionally
may be usefully employed in a semi-coherent spectrum analyzer,
which pre-processes and enhances the signal to noise ratio
prior to application to the time averager by removing noise
outside the frequency range of interest.
~ The inventive time averager circuit comprises a
first operational amplifier having a pair of-inputs, including
-means for applying a signal which includes the signal to be
detected to one of the inputs. A transformer, having a pair
of windings, has one winding connected to the output of the
operational amplifier. A second operational amplifier has a
pair of inputs, one of the inputs being electricall~ connected
through an amplitude control to the first winding of the
transformer. The second of the inputs to the second opera-
tional amplifier is connected to the secondary winding of
the transformer. Means are also provided for feeding back
the A.C. components from the secondary winding of the
transformer to the second input of the first operational
amplifier, to cancel a fraction of the alternating current
within the first operational amplifier, within the transformer
signal translation frequency range. The signal to be detected
contains a near D.C~ component, which is the signal to be
detected, and the component is obtained by preprocessing in
a multiplier.
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A better understanding of the invention will be
obtained by reference to the following description, and to
the appended drawings, in which:
Figure 1 is a schematic diagram of an
embodiment of the invention, connected in a spectrum analyzer
circuit, and
Figure 2 is a schematic diagram of a filter which
can be used in the spectrum analyzer.
Turning to Figure 1, a semi-coherent time averaging
circuit 1 is shown in the broken line block, and is
comprised of a first inverting operational amplifier 2,
which has unity gain. The operational amplifier is of the
well known type which will pass both A.C. and D.C. signals.
The noisy near D.C. signal to be detected is applied at a
first input 3.
Connected to the output of operational amplifier 2
is a first winding 4 of a metal core transformer 20, such as
General Radio Type 941A.
The output of first operational amplifier 2 is
also applied to a first lnput 5 of a second operational
amplifier 6 of similar type as the first operational amplifier,
preferably through an amplitude control such as potentiometer
7.
A secondary winding 8 of transformer 20 is connected
through an inverting third operational amplifier 9 to the
second input 10 of first operational amplifier 2. The
secondary winding 8 is also connected to the second input 11
of second operational amplifier 6. ~An output signal is
obtained from the output of operational amplifier 6, at
terminal 12.
The mode of operation for random noise is as
follows. Noise (A.C. having equally dispersed positive and
negative components) is applied at the first input 3 of
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operational amplifier 2. This is applied to the first
winding 4 of transformer 20, and is translated to the secondary
winding 8, from which it is applied through operational
amplifier 9 to the second input 10 of operational amplifier
2. Since an inversion occurs in both of operational amplifiers
2 and 9 and the transformer 20, an inverted fraction of the
noise signal which is applied at first input 3 is also
applied at second input 10 of operational amplifier 2. A
differential addition is made, and the result is a reduced
resultant A.C. output from operational amplifier 2.
However, where a low amplitude signal is multiplied
with noise, a skewing occurs in the positive or negative- j
going excursions of the noise. The result appears to be the
creation near D.C. or of D.C.-like components in the mixed
signal, of frequencies which are too low to be translated by
the transformer. While-the exact physical nature of what
happens in the transformer is only speculated, the use of
the transformer results in the signal appearing at the
output of the operational amplifier 2 having D.C. components
(in the presence of signal), while the signal applied at
input 10 of the same operational amplifier does not appear
to have these components to the same relative extent as the
signal applied to the input 3 due, apparently, to the inability
of the transformer to pass very low frequency components.
Since a differential addition occurs in amplifier 2, its output
maintains the D.C. components which are equivalent to the
original signal components.
The signal enhanced output ~f operational ampliier
2 is also applied through potentiometer to input 5 of second
operational amplifier 6. The A.C. components from the
secondary winding 8 of the transformer which are applied to
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operational amplifier 9 are also applied to a second input
11 of operational amplifier 6, where a differential addition
also occurs. Potentiometer 7 is adjusted to apply the A.C.
signal to input 5 to just cancel the A.C. slgnal input 11 of
operational amplifier 6. The output of amplifier 6 is sub-
stantially the D.C. like signal.
The major contribution of the transformer and opera-
tional amplifier 9 in the feedback circuit appears to be in
changing the time constant of the feedback circuit to input 10
of amplifier 2. It has been found that significant rise
time and decay time improvements are obtained by the use of
the above-described time averager over a resistor-capacitor
time averager. The reasons for the advantages are not fully
understood, but appear to relate to core saturation phenomena
of the transformer at frequencies well below the nominal
design limit of the transformer itself, which, in an experiment,
was below 5 Hz. It has been found that high input amplitudes
result in short rise and decay times, whereas low input
amplitudes yield long rise times and decay times. Given
this effect, there appears to be dependence of rise times on
feedback gain through operational amplifier 9, i.e., the
higher the feedback gain, the smaller is the amplitude fed
to the transformer 20, and hence the longer the time constant.
It can be analogised that the system behaves adaptively,
being able to respond rapidly to short periods of high
signal -to noise ratios, and also to reduce to a minimum A.C.
noise where the signal to noise ratio may be low for short
periods.
As one application, the above-described circuit is
usefully integrated as the detection circuitry of a spectrum
analyzer. Continuing in our description of Figure 1, an
input signal with accompanying noise is applied at input
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terminal 13, which is directly ~pplied to input 14 of multiplying
means 15. The same signal and noise from input terminal 13
is also applied to the input terminal of a bandpass filter
16, which has its output connected to input 17 of multiplying
means 15.
The circuit operates as follows. The complete
noisy input signal is filtered to the narrow band of the
signal of interest in filter 16. Consequently we may say
that the signal applied to the input 14 of multiplier 15
consists of Sl ~ Nl, and the signal applied to input 17 of
multiplying means 15 is composed of narrow band S2 + N2,
where Sl-= S2 = S represents the signal component and N the
noise component. These are multiplied in the multiplier 15,
the output signal of which is applied to input 3 of operational
amplifier 2 of the time averager 1. This signal consists of
the following resulting components: very narrow band signal
SlS2 + broad band signal S2Nl + narrow band signal SlN2 +
broad band signal NlN2.
If the filter is exactly tuned to the correct signal
frequency, SlS2 = S will be pure D.C. at the multiplier output
and hence will be optimumly detected by the time avera~er
circuit.
If narrow band filter 16 is detuned slightly from
the signal frequency of interest, the product SlS2 appears
to have its signal energy spread in a band between, for
example, 0 and 2.8 ~Iz; there will also be some energy contribution
from the SlN2 term. This signal frequency clearly is in the
region which the time averaging circuit 1 previously described
is best adapted to detect, and conse~uently to enhance the
signal to noise ratio. Detection of the signal is thus
facilitated. Furth~r detuning results in reducing the neax
D.C. component by increasing the bandwidth of SlS2 which then
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appears as a noise like A.C. component which is cancelled.
The presence of a low amplitude signal is thus detected by
tuning the filter past the unknown signal frequency in the
normal way. The filter 16 can be a standard series and
parallel tuned L-C filter network as shown in Figure 2, a
well known form. Alternatively, a General Radio Type 1568-A
one percent wave analyzer has been found to work satisfactorily,
and was used successfully in an experiment, with center
frequency tuned to about 280 Hz.
lq For a stable signal, a filter band width of 0.2 Hz
and a center frequency of 100 Hz, in a 100 Hz noise bandwidth
it has been calculated that the above equipment, using the
time averager just described provides an improvement in
output signal-to-noise ratio of about 16 dB better than the
resistor-capacitor time averaging. For the situation in
which an unstable signal is processedj an improvement using
the above-described time averager has been calculated to be
about 13 dB, better than resistor-capacitor time averaging.
The gain in signal to noise ratio of this invention over one
using resistor-capacitor techniques is thus evident.
As noted earller, each of the operational amplifiers
should have unity gain, be of inverting type, and be able to
process signals having frequencies down to direct current.
A useful operational amplifier may be obtained from Electronic
Associates Inc. as Model 8800, Hybrid Computer. The same
model computer can also be used as the multiplier 15. Resistor
7 can be about 10,000 ohms, and a resistor 18, connected as
a load across the secondary winding 8 of the transformer,
can be 600 ohms. The narrow bandpass filter 16, as noted
earlier, can be General Radio Type 1568-A wave analyzer.
A time averager having significant advantages over
a non-coherent resistor-capacitor time averager has been
l~SS~
deseribed, whieh ean be used in a reeeiver such as a submarine
deteetor with eonsiderable signal to noise ratio enhancement.
A person skilled in the art understanding this invention may
conceive of additional modifieations, improvements, applications
or faeilities incorporating the design. All are considered
to be within the seope of this invention as defined by the
appended elaims.
L0
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