Apparatus for visually aiding MSK detection and interference recognition and classification

An apparatus provides patterns for visually aiding an operator in determig the nature and noise environment of the incoming signal. A conventional signal receiver and an oscilloscope are coupled to a pair of phase detectors, a clock circuit, a reference circuit, and a timing circuit. The clock circuit synchronizes the operation of the receiver, reference circuit, and timing circuit. The phase detectors have pair of inputs and an output with the inputs of one detector being connected to the receiver and the reference circuit, and its output is connected to the Y deflection axis of the oscilloscope screen. The inputs of the other detector are connected to the receiver and the output of the reference circuit via a preselected analog time delay, and its output is connected to the X deflection axis of the oscilloscope screen. The output of the timing circuit is connected to the Z intensity axis of the oscilloscope screen for a preselected time duration to provide the capability to vary the intensity of a part of the screen such that patterns are generated on the oscilloscope screen representing the presence of some constituents and the inherent nature of the incoming signal which might include, the type of noise, the presence of doppler shift or CW interference, or drift of a component.

BACKGROUND OF THE INVENTION 
The detection of a signal of interest and the discernment of noise and its 
constituency have always been of high interest to those who must assure 
reliable communications. These considerations are foremost in the minds of 
operators of very low frequency (VLF) systems since systems of this type 
have been selected to convey information that is most important. Any 
delays in recognizing information and various interferences could have 
severe consequences. So the need continues for an apparatus for visually 
aiding a rapid analysis of incoming signals, such as VLF, and, more 
particularly, the detection, interference recognition and classification 
of minimum shift keying (MSK) signals in incoming VLF signals. 
A VLF receiving terminal operator sitting at a console must determine the 
nature and electromagnetic noise environment of the incoming signal 
usually without any previous knowledge about the signal. One prior art 
approach relies upon the use of trained operators who listen to audio 
outputs and develop an "ear" for judging the incoming signal traffic. A 
main drawback of this approach is that its success is contingent solely 
upon the level of operator skill which inherently varies from one 
individual to the next. Furthermore, the audio monitoring of signals for a 
period of time can be tedious and fatigue may set in to compromise 
performance. 
Another prior art approach to improve performance is to give an operator a 
visual indication by employing a spectrum analyzer to "see" the frequency 
characteristic of the environment. Drawbacks of this approach are that it 
has relatively slow response and requires considerable expensive 
equipment. 
In view of the aforementioned drawbacks of these prior art approaches, a 
continuing need exists in the state of the art for a cost effective visual 
monitoring of a signal of interest to aid a receiving terminal operator in 
the task of identifying signals of interest and of determining the nature 
and noise environment of incoming signals. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing a cost effective apparatus 
for visually aiding an operator in the detection of MSK signals and 
interference signal recognition and classification thereof in an incoming 
VLF signal. Low cost for the capability is assured since some of its 
expensive components, i.e. a VLF receiver, frequency time standard and 
oscilloscope, are already on board to function in their intended role. 
Advantageously, the apparatus is uncomplicated and shows quick response to 
changes. A pair of phase detectors, one of which receives a signal delayed 
90.degree. with respect to the other, has their respective outputs 
connected to the X and Y axes input terminals of the oscilloscope to 
provide representative waveforms. A timing circuit has its output 
connected to the Z axis of the oscilloscope and is actuated for 
preselected time durations to vary the intensity of the waveforms on the 
oscilloscope screen. The generated patterns on the screen visually 
represent the signal of interest, the presence of doppler shift in the 
signal of interest, the type of noise, or CW interference, and other 
information concerning the inherent nature of the incoming signal and, as 
such, the patterns give a real-time visual aid for an operator. 
Another object of the invention is to provide an apparatus which provides a 
visual indication of signals of interest. 
An object of the invention is to provide an apparatus which provides a 
visual output for aiding a receiver operator. 
Another object of the invention is to provide an apparatus for generating 
patterns on an oscilloscope screen which visually aid the operator in 
determining the nature and noise environment of an incoming signal. 
Still another object of the invention is to provide an apparatus which 
advantageously includes other components already available in the signal 
receiving system where the apparatus will be used. 
A further object of the invention is to disclose an apparatus which is 
uncomplicated to improve reliability, has a quick response to change and 
is cost effective. 
These and other objects, advantages and novel features of the invention 
will become more readily apparent from the ensuing specification and 
claims when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 of the drawings, an apparatus 10, is provided 
herein that visually aids a radio receiving terminal operator in 
determining the nature and noise environment of an incoming signal of 
interest. Its use to aid an operator receiving VLF signals, particularly 
in MSK detection and interference recognition and classification, will 
herein be elaborated on. It being understood, however, that in accordance 
with the principles of the present invention, many receivers of a variety 
of spectrums with other modulation schemes could advantageously employ 
this concept. 
Apparatus 10 is fabricated to rely on the utilization of some of the most 
costly components which are already included at a receiver station. These 
include a VLF signal receiver 12 for receiving an incoming signal via an 
antenna 13, and an oscilloscope 14. The receiver could be any one of a 
variety of commercially available models, such as, a Type Number R-1738/WR 
marketed by Rockwell Corporation of Richardson, Tex. 75018 and the antenna 
could be any of many designs. 
Oscilloscope 14 has a display screen 16 that appropriately displays the 
information fed to the oscilloscope on an X deflection input terminal 18, 
Y deflection input terminal 20 and Z deflection input terminal 22 for 
defining X, Y and Z axes thereon. The oscilloscope may be one selected 
from numerous suppliers but the unit selected must have the capability to 
provide the X and Y information on orthogonal axes and the Z axis 
information by being able to change its intensity. 
The apparatus has a clock circuit 23 for assuring proper operation. A 
frequency time standard unit marketed as a Model 0-1695/U by 
Hewlett-Packard Corporation, Palo Alto, Calif. 94303 was selected. It 
feeds a 5 Mhz. signal to a reference circuit 24 of a reference circuit 
means that also includes a conventional analog time delay 25. 
A typical reference circuit 24 used in accordance with the teachings of 
this inventive concept might be a Model 8660 by Hewlett-Packard 
Corporation, Palo Alto, Calif. 94303, which provides the necessary 
capability to appropriately down convert the 5 MHz. signal to a 7.5 KHz. 
signal A. The 7.5 KHz signal A from the reference circuit is fed to analog 
time delay 25 which, in this case, is an analog delay line to impart a 
33.3 microsecond time delay to accordingly create a 90.degree. phase 
shifted 7.5 KHz signal B. Other suitable time delays for other 
intermediate frequencies of interest can be created as needed by 
appropriate selection of components in accordance with the teachings of 
this inventive concept. 
Looking to FIGS. 1 and 2, the unshifted 7.5 KHz. signal A, shifted 7.5 KHz. 
signal B and the intermediate frequency, or signal of interest C from 
signal receiver 12 are coupled to phase detectors 26 and 28 which provide 
the Y and X inputs to terminals 20 and 18. Phase detectors 26 and 28 are 
implemented by standard exclusive-OR gates each having a pair of inputs 
26a and 26b or 28a and 28b, respectively, which are interconnected as 
depicted in FIG. 2. 
A timing circuit 32 is made up of a counter 36 coupled to receive the 5 
MHz. clock signal from clock 23. An interconnected logic gate circuit 38 
and a one shot multivibrator circuit 40 with a timing circuit 42 provides 
the appropriate Z axis signal for oscilloscope 14 in a manner to be 
elaborated on below. The circuit diagram of FIG. 2 sets forth the details 
of the timing circuit. 
Clock circuit 23 is connected to appropriately actuate receiver 12, 
reference circuit 24, and timing circuit 32 to assure the synchronized 
operation thereof. Inputs 28a of phase detector 28 and 26a of phase 
detector 26 are connected to receive the intermediate frequency, or signal 
of interest C from signal receiver 12. Input 28b of phase detector 28 is 
coupled to receive the in-phase component A of the 7.5 KHz. signal from 
reference circuit 24 and input 26b is coupled to receive the 90.degree. 
phase shifted component B from reference circuit 24 via time delay 25. As 
mentioned above, the output of phase detector 28 is connected to terminal 
20 for the Y axis deflection on oscilloscope display screen 16 and the 
output of phase detector 26 is connected to terminal 18 for the X axis 
deflection on oscilloscope display screen 16. 
By way of this example which is intended to be for the purposes of 
demonstration only and is not to be construed as being limiting, VLF 
signal receiver 12 produces an intermediate frequency or frequency of 
interest output signal C that may be at 7.5 KHz. Clock circuit 23 produces 
a 5 MHz. signal so that reference circuit 24 down converts the 5 MHz. 
signal from clock circuit 23 to the substantially the same frequency A as 
the receiver intermediate frequency C, e.g. 7.5 KHz., which is fed to 
input 28b of phase detector 28. The 90.degree. phase shifted component B 
at the output of time delay 25 which is fed to input 26b of phase detector 
26. 
The serially connected arrangement of timing circuit 32 interconnects clock 
circuit 23 and terminal 22 for display of the Z axis information on 
oscilloscope display screen 16. The adjustable device 42 of timing circuit 
32 may be a rheostat or variable resistor connected to one-shot 
multivibrator 40. Device 42 is adjustable for presetting the operation of 
one-shot multivibrator 40 of timing circuit 32 for a preselected time 
duration. 
In the embodiment thusly described, the count of the signal of clock 
circuit 23 that was applied to the input of counter 36 is reduced therein 
by 100,000. The logic gates in logic gate 38 pass only one logic change 
every 20 milliseconds (corresponding to the intensity modulation period 
during which the intermediate frequency signal C is fed from the signal 
receiver to the phase detectors) and the output from logic gate 38 is fed 
to one shot multivibrator 40. 
The output of one-shot multivibrator 40 and, thus, of timing circuit 32 is 
connected to terminal 22 for the Z axis information on the oscilloscope 
display screen 16 and is operable for the time duration when the one shot 
pulse is preselected by adjustment of variable resistor 42 to vary the 
intensity of a preselected part of the display screen. 
Representative patterns are generated on the oscilloscope screen 
representing the type of noise, the presence of doppler shift or CW 
interference, and the inherent nature of the incoming signal. Such 
patterns are capable of visually aiding the receiving terminal operator in 
determining the nature and noise environment of the incoming signal. 
FIGS. 3 through 5 show typical representative patterns P(1) to P(3) 
displayed on oscilloscope display screen 16. A so-called normal pattern 
P(1) of an intermediate frequency of interest C, such as shown in FIG. 3, 
and appears as the intensified sections S of the pattern. This pattern 
should remain in a constant position. If the pattern of the intensified 
sections S rotates, then a doppler shift is present. The greater the speed 
of rotation, the greater the doppler shift. If the intensity of the 
pattern of the intensified sections S constantly and repetitively varies, 
a frequency drift in the time standard of the distant transmitter clock is 
indicated. Frequency locked coherent signals would not show such drift. 
FIG. 4 is representative of a visual indication of a pattern P(2) of 
Gaussian noise that is much greater than the intermediate frequency signal 
of interest coming from signal receiver 12. This distinctive pattern 
appears as a number of haphazard loops that pass through the center of the 
oscilloscope screen. 
FIG. 5 is representative of a visual indication of the pattern P(3) which 
is produced by CW interference with a small offset frequency. The small 
cluster of a sub-pattern that resembles a smaller version of the pattern 
P(2) rotates in orbit around the center of the oscilloscope screen. The 
arrows point in the direction of positive offset frequency. The greater 
the offset, the greater the rate of revolution. 
The invention and many of its attendant advantages will be understood from 
the foregoing description and it will be apparent that various changes may 
be made in the form, construction and arrangement of the parts thereof 
without departing from the spirit and scope of the invention.