Radar warning receiver

A radar warning receiver includes means for distinguishing pulsed or continuous fixed frequency radar signals emanating from a radar transmitter from variable frequency signals such as those radiated from a nearby superhomodyne receiver. This invention includes a superheterodyne receiver having a local oscillator that is repeatedly varied in frequency over a range of frequencies sufficiently large and at a rate fast enough to cause any signals within an input range of frequencies to be detected by a sensitive, limited bandwidth detector. If an input signal is fixed in frequency, that signal will be detected at the same time during successive sweeps of the local oscillator, and a correlated output will be generated; otherwise, an uncorrelated output will be generated. If the number of correlated outputs equal or exceed the uncorrelated outputs detected, then an alarm is provided. Even when the receiver of this invention is receiving signals from a superhomodyne receiver, if a fixed frequency signal is detected for a predetermined number of consecutive sweeps of the local oscillator, then an alarm, indicating the presence of that fixed frequency signal, is again provided.

BACKGROUND OF THE INVENTION 
This invention relates to a radar receiver capable of discriminating 
between fixed frequency and variable frequency signals in the X- and 
K-band portions of the radio spectrum. 
This invention is particularly useful in connection with a receiver 
designed to detect signals emitted by police speed radar detectors, and to 
distinguish those fixed frequency signals from other signals emitted by 
nearby superheterodyne-type radar detectors. 
Some newly manufactured radar receivers have been introduced to the 
marketplace using the so-called superhomodyne scheme for detecting 
frequencies in the X- and K-bands. These receivers use a signal generator 
or local oscillator on the same frequency as the signal to be received, 
and this internal signal tends to be re-emitted by the antenna of the 
receiver. Although its power level is low, the proximity of a receiver of 
this type to a sensitive receiver could make it appear that a radar 
transmitter is in the vicinity, thus sounding an alarm. Since the local 
oscillator in a superhomodyne receiver is at the same frequency as the 
received signal, it is impossible to trap that signal and thus prevent it 
from being reradiated by the antenna. 
Thus, receivers of the superhomodyne type are continuously broadcasting X- 
and K-band signals. They do have one characteristic, however, that permits 
their signals to be distinguished from police radar signals, and that is 
the frequency of the superhomodyne emitted signal is constantly varying 
over the range of frequencies that it is designed to detect. 
Accordingly, this invention provides means for distinguishing between the 
fixed frequency emission of a police radar transmitter and the varable 
frequency emissions from a superhomodyne-type receiver. 
SUMMARY OF THE INVENTION 
In the present invention, a superheterodyne receiver includes a local 
oscillator which is varied or swept across a range of frequencies so as to 
cause a predetermined range of X- and K-band frequencies to be swept. The 
output of the local oscillator, when mixed with the input signal, produces 
an intermediate frequency (IF) that is applied to a highly sensitive, 
limited bandwidth detector circuit. The time when a signal is detected 
during each sweep of the local oscillator will be representative of the 
frequency of that signal, and since signals emitted by a transmitter are 
fixed in frequency, whereas those emissions from a superhomodyne receiver 
vary in frequency, it will be possible, through appropriate circuitry to 
be described, to distinguish between these two sources of radio frequency 
energy. 
More specifically, this invention includes means for correlating the output 
of the receiver during successive sweep cycles of the local oscillator, 
and to provide an alarm output whenever a correlation for a predetermined 
number of cycles is found. 
This is done by comparing the frequency of the detected radio frequency 
signal during each sweep of the local oscillator, and whenever the 
detected radio frequency signal occurs at the same frequency during 
multiple sweeps of the local oscillator, an alarm output is provided. 
Because a large number of uncorrelated signals can statistically produce 
the appearance of a correlated signal, this invention provides an alarm 
output when the number of correlated signals equals or exceeds the number 
of uncorrelated signals detected. 
Further, because the presence of a superhomodyne receiver adjacent a 
receiver incorporating this invention could possibly mask the presence of 
a single frequency signal emitted by a radar transmitter, this invention 
also provides means for generating an alarm output whenever the number of 
consecutive sweeps of the local oscillator, each including at least one 
correlated signal, exceeds a predetermined number. 
It is therefore an object of this invention to provide an apparatus for 
distinguishing pulsed or continuous signals emitted from a fixed frequency 
source from other signals generated by a variable frequency source, said 
apparatus comprising: a superheterodyne receiver including means for 
sweeping the frequency of the local oscillator over a predetermined range 
of frequencies thereby providing output pulses during each sweep of the 
local oscillator upon the detection of signals within the receiver 
bandpass, wherein the positions of the output pulses during said sweep are 
representative of the frequencies of the corresponding detected signals, 
correlation means for providing an alarm output each time an output pulse 
occurs at the same position during a predetermined number of sweeps of the 
local oscillator. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings which illustrate the preferred embodiment of 
the invention, and particularly to the block diagram of FIG. 1, a radar 
receiver 10 is illustrated wherein a radar signal occurring within a 
specified input range of frequencies is received by a conventional antenna 
system, not shown, and directed on line 15 to a first mixer 20. The 
antenna may be of conventional design, and as customary for radar 
receivers of this type, it may include filters for preventing signals at 
unwanted frequencies from being passed to the first mixer 20. The antenna 
may also include other filters for preventing outward radiation of 
internally generated radio frequency signals. The first mixer 20 may be 
and is generally built into and made a part of the antenna structure. 
A local oscillator 30 provides an output signal to the first mixer 20 such 
that the resulting intermediate frequency signal will fall within the 
limited response bandwidth of the detector circuit 40. In a preferred 
embodiment of the invention, two quasi-harmonically related radar 
frequencies can be detected by a single receiver. These radar frequencies 
are nominally at 10.525 GHz with a tolerance of .+-.0.025 GHz (X-band), 
and at 24.150 GHz with a tolerance of .+-.0.100 GHz (K-band). By selecting 
the frequency of the local oscillator 30 to be in the order of 11.5583 
GHz, the fundamental or first harmonic of the local oscillator, when mixed 
with the 10.525 X-band radar signal will result in a difference or 
intermediate frequency of 1.033 GHz. The second harmonic of the local 
oscillator 30, when mixed with a K-band radar signal at 24.150 GHz will 
also provide an intermediate frequence of 1.033 GHz. Thus, a single 
receiver may be designed to receive both X- and K-band radar signals 
simultaneously. 
Since the radar signals may be within a range of frequencies rather than on 
a single frequency, it will be necessary to vary or sweep the frequency of 
the local oscillator 30 through a range of approximately 0.120 GHz, or 
from 11.4983 to 11.6183 GHz. This sweeping of the local oscillator is 
accomplished by means of a sweep generator 50. 
The output of the sweep generator 50 on line 52 to the local oscillator 30 
is a variable voltage which changes in a generally sawtooth manner with 
respect to time. Also provided is a signal on line 54 from the system 
clock, and a signal on line 56 which is a start signal representing the 
start of each sweep cycle. 
The detector circuit 40 includes a first intermediate frequency (IF) 
amplifier 60 for receiving the intermediate frequency output of the first 
mixer 20 and applies the amplified signal to a second mixer 65. A second 
local oscillator 70 is preferably a fixed frequency device having an 
output at 1.03 GHz so that the difference between the output of the first 
IF amplifier 60 and the local oscillator 70 is a 10 MHz signal applied to 
a second intermediate frequency (IF) amplifier 75. A filter circuit 80 has 
a limited bandpass, in the order of 1.3 MHz, and its output is applied 
through a limiting second IF amplifier 85 to a frequency discriminator 
circuit shown at 90. The output of the frequency discriminator is 
processed by lowpass filter 95 and applied to a voltage comparator circuit 
100 which compares the filter output to a reference voltage provided by 
source 105. As a result, the comparator circuit 100 will provide a detect 
pulse output on line 110 for each radar signal detected within the input 
range of frequencies. Actually, because of the nature of the detector 
circuit 40, and the use of a second local oscillator 70, a pair of detect 
pulses will be generated for each signal found within the input range of 
frequencies. The spacing between these pulses may be used, if desired, to 
identify whether the input signal is located in the X- or K-band. This 
feature of the detector circuit 40 is more fully described in U.S. Pat. 
No. 4,313,216. 
Some other types of police radar detecting receivers themselves generate 
radio frequency signals within the range of frequencies assigned to police 
radar, and detection of these receiver generated radio frequency signals 
could result in a false alarm, i.e., an indication that a radar output 
signal has been detected when in fact the only thing present in the range 
of frequencies are low power, varying frequency signals from a nearby 
receiver. Thus, means have been provided in this invention for 
distinguishing the pulsed or continuous signals emitted from a fixed 
frequency source, such as a police radar transmitter, from other signals 
generated by variable frequency sources, such as radar receivers. 
Accordingly, means are provided to respond to fixed frequency input 
signals, but to reject variable frequency input signals. 
The time relation or position of the detect pulse on line 110 from the 
comparator 100 with respect to the beginning of each sweep is 
representative of the frequency of the detected signal since the variable 
frequency of local oscillator 30 is swept through the same range of 
frequencies during each cycle of the sweep generator 50. 
This is illustrated in FIGS. 2 and 3 where the horizontal axis represents 
frequency and the vertical axis represents the amplitude of the signals 
detected within the range of frequencies scanned by the receiver. The 
range of frequencies may be, for example, from 10.475 to 10.575 GHz. The 
single signal shown in FIG. 2 is at approximately 10.525 GHz. Thus, this 
signal will be detected approximately midway during the sweep of the local 
oscillator. In FIG. 3, several input signals are represented, some with 
relatively large amplitudes, in the range of from 10.500 to 10.530 GHz. 
Any signal of constant frequency will remain in the same relative position 
during each sweep. If any signal varies in frequency, then that signal 
will, of course, be detected at different times, or at different 
positions, from sweep to sweep of the local oscillator. 
Which one, if any, of the signals in FIG. 3 is from a radar transmitter? 
Amplitude alone cannot determine this, since the radar transmitter signal 
can be much weaker than a nearby variable frequency signal from a radar 
receiver. To determine the radar transmitter signal, reference will be 
made to the simplified waveforms of FIGS. 4-6 which illustrate three 
typical situations: where police radar alone is detected; where only 
signals from a radar receiver are detected; and, where a police radar 
signal is detected along with signals from a radar receiver. 
In each of FIGS. 4, 5 and 6, waveform A represents the signals detected 
during the previous sweep of the local oscillator 30; waveform B 
represents the signals detected during the current sweep of the local 
oscillator; waveform C represents the detect pulse outputs on line 110 
from the detector circuit 40 for the previous sweep; waveform D represents 
the detect pulse outputs on line 110 during the current sweep; waveform E 
indicates correlated detect pulses of the previous and the current sweep; 
and waveform F indicates the uncorrelated pulses between the previous and 
the current sweep of waveforms C and D. 
In FIG. 4, a single, fixed frequency radar signal is present at the 
antenna, and this signal remains at the same frequency during the scan 
represented by waveform A and the scan represented by waveform B. As 
previously noted, the output pulse 110 from the detector circuit 40 will 
be a double pulse (because the second local oscillator 70 is very close to 
the first IF frequency), and this double pulse will appear in both 
waveforms C and D at the same position, or at the same time with respect 
to the beginning of the scan, since frequency is constant. Means are 
provided to sense the correlation between the time-position of the pulses 
of waveforms C and D, and this is represented as the correlated output 
pulse in waveform E. 
In FIG. 5, only frequency variable radio frequency signals are detected at 
the antenna, and because they are variable in frequency, their position 
with respect to the beginning of the scan varies from scan-to-scan, as 
shown in waveforms A and B. Accordingly, the detect pulses on line 110 
from the previous and the current scan will appear at different times, as 
shown by waveforms C and D. Since the pulses are not correlated, there is 
no output shown in waveform E, but the pulses do appear in waveform F. 
In FIG. 6, three radio frequency signals are detected in the previous scan 
of waveform A, only one of which is from a fixed frequency source. In the 
current scan of waveform B, the variable frequency signals have changed 
position, but the fixed frequency signal remains at the same position. 
Accordingly, when the detect output pulses of waveforms C and D are 
compared, one set of detect pulses will appear in waveform E as 
representing the fixed frequency source, whereas the other detect pulses 
will appear in waveform F as uncorrelated detect pulses. 
In this invention, means are provided to discriminate against detect pulses 
on line 110 representing variable radio frequency signals, by comparing 
the frequency of these signals during each sweep of the local oscillator 
30. Only when the radio frequency signals occur at the same frequency 
during multiple sweeps of the local oscillator is an alarm output 
provided. 
FIG. 7 illustrates a simplified version of a device for accomplishing this 
and includes input buffer 120 for receiving the detect pulses on line 110 
from the detector circuit 40, and for converting these detect pulses into 
a pulse synchronized with clock pulses occurring on line 135 from a clock 
130 (FIG. 9). The clock pulses occur at regular intervals and divide each 
sweep of the local oscillator into a fixed number of increments, for 
example, 128 pulses for each sweep cycle. The output of the flip-flop 120 
is applied to a 128 bit shift register 150. For each clock pulse, any 
information appearing on line 145 is moved into and through the shift 
register. The output of the shift register on line 155 is applied to one 
input of AND gate 160, the other input of which is line 145. Thus, the AND 
gate 160 compares the detect pulses from the current sweep found on line 
145 to the detect pulses occurring on the previous sweep of the local 
oscillator as found on line 155. An alarm output on line 170 will occur 
only when there is a coincidence of these signals. 
Because the variable frequency source signals may be numerous, as shown in 
FIG. 3, there exists the possibility that there will be an apparent 
coincidence of signals in adjacent scans of the local oscillator even 
though a fixed frequency signal is not present. For this reason, the 
preferred embodiment of the invention includes means to provide an alarm 
output when the number of correlated signals exceeds the number of 
uncorrelated signals by a predetermined number. An example of a circuit 
for performing this function is shown in the block diagram of FIG. 8. 
This embodiment includes those same elements shown in FIG. 7, and in 
addition, a counter 180 which is initially preset to a predetermined 
number, such as 7, upon receipt of the retrace signal 56 from the sweep 
generator 50. Thus, the counter starts at a preset number at the beginning 
of each sweep of the local oscillator, and whenever a correlated output is 
generated on line 170, the counter will decrement by one. Uncorrelated 
detect pulses, on the other hand, cause the counter 180 to increment. This 
is accomplished by applying each detect pulse on line 145 to an exclusive 
OR device 190 along with the correlated signal from line 170. Thus, if a 
detect pulse on line 145 corresponds to a correlated signal on line 170, 
the counter 180 is not incremented; but if a detect pulse on line 145 is 
not correlated with a signal from a previous sweep, then the counter 180 
will be incremented. 
At the end of the sweep cycle, if the number in the counter 180 exceeds the 
preset number, then the number of uncorrelated detect signals exceeds the 
number of correlated signals and an alarm output will be inhibited. On the 
other hand, if the number of correlated signals exceeds or equals the 
number of uncorrelated signals, then an alarm output will be enabled. 
In order to determine whether the conditions mentioned above are met at the 
end of a sweep cycle, a second shift register 200 is provided to store 
temporarily the correlated output signals during each sweep. As shown in 
FIG. 8, the shift register 200 is a 32 bit register, and a divide by four 
counter 212 reduces the number of clock pulses from 128 per sweep to 32 
per sweep. The output of the counter 180 on line 195, and the output of 
the shift register 200 on line 210 are applied to an AND gate 220. This 
gate will provide an alarm output on line 230 only if there are correlated 
pulses in shift register 200 and the number of correlated pulses sensed by 
the counter 180 equals or exceeds the number of uncorrelated pulses 
occurring during the current sweep. 
Also shown in FIG. 8 is a simplified version of a fail-safe circuit 250 
which is designed to provide an alarm output after a predetermined number 
of consecutive sweep cycles each include at least one correlated signal 
even though there are more uncorrelated signals than correlated signals 
present during each sweep. This is designed to prevent a failure of the 
detector from providing an alarm output even though a fixed frequency 
radar signal is masked by variable frequency input signals existing over a 
long period of time. The fail-safe circuit includes a counter 260 
receiving the correlated signals on line 170 through a "N-1" counter 
circuit 270. The counter circuit 270 is merely for reducing by one the 
number of pulses applied to the counter 260. As may be noted in waveforms 
E of FIGS. 4 and 6, each correlated detect pulse is actually a pair of 
pulses, and the circuit 270 is provided to eliminate one of these pulses. 
Referring now to the schematic drawing of FIG. 9, which shows a preferred 
embodiment of the invention using CMOS technology, the detect pulses on 
line 110 are applied to the input buffer 120. This buffer is a D-type 
flip-flop for converting the detect pulses on line 110 which come in in an 
asynchronous fashion, into synchronous pulses under control of the clock 
pulses on line 135 so that pulses within the remainder of the circuit are 
synchronized, thus standardizing the timing. 
As shown in FIG. 9, the clock pulses on line 135 are generated by a clock 
130. This clock, or divider, receives system clock pulses from the main 
receiver and divides them by a factor of four. Since the start pulses on 
line 56 occurs at a 64 Hz rate, the clock generates 128 clock pulses for 
each sweep of the local oscillator 30. In other words, the entire sweep of 
the local oscillator is divided into 128 segments, each 3.906 milliseconds 
in duration. 
As shown in FIG. 9, the clock pulses on line 135 in some cases go directly 
to the various components within the circuit, and in other cases they go 
through inverters 136 and 137. The reason for this is that various 
components require different polarity clock pulses in order to work 
properly within this circuit. 
The output of the input buffer 120 is applied on line 145 to the shift 
register 150. This device functions as a delay line of one sweep length. 
It is a 128 bit shift register, and it too is controlled by the clock 
pulses on line 135. Any signal entering the shift register on line 145 
will exit the shift register on line 155 after 128 pulses from the clock 
have occurred. Thus, the shift register 150 will hold one entire sweep's 
worth of data, and its output on line 155 represents the input detect 
pulses delayed by one sweep. These pulses are compared to the current 
input detect pulses by AND gate 160. As previously stated, the output on 
line 170 will occur only when these two pulses are correlated, or in other 
words, there is a coincidence of signals. 
Both the present detect pulses on line 145 and the previous detect pulses 
on line 155 are applied to an OR gate 191. The OR gate has an output 
connected to AND gate 192, the output of which is applied as an input to a 
counter 181. Any signal, correlated or uncorrelated, coming from either 
lines 145 or 155 will be applied as an input to the counter 181. This 
counter also has an input at pin 10 from line 170, which represents the 
correlated detect signals. Whether the counter increments or decrements 
upon the application of detect pulses to pin 15 from AND gate 192 will 
depend on whether there is a correlated signal present. The counter 181 
will count up if no correlated signal is present; it will count down if 
there is a correlated signal on line 170. 
At the beginning of each sweep, counter 181 is preset by the start signal 
on line 56 from the sweep generator 50. The start signal is applied to pin 
1 and causes whatever input is applied to the preset inputs (P0, P1, P2 
and P3) to be loaded or preset into the counter. In FIG. 9, P0, P1 and P2 
are connected to a source of voltage whereas terminal P3 is grounded. 
Therefore, the counter 181 is preset to the binary equivalent of the 
decimal number 7. The B/D input of the counter 181 determines whether the 
counter will count in a decimal or binary fashion, and since this is 
connected to a source of voltage, the counter will count in binary. 
As the local oscillator sweeps across the input range of frequencies, and 
as a correlated signal is encountered, the counter 181 will decrement down 
from 7 to 6 to 5 as the two detect pulses are received, as illustrated in 
FIG. 4. If uncorrelated signals are encountered, the counter will 
increment from 7 to 8 to 9; each uncorrelated input signal being 
represented by a pair of detect pulses. 
The counter 181 is prevented from exceeding its maximum capacity (decimal 
15) by connecting the output from pin 7, the CO output, to one of the 
inputs of AND gate 192. When the counter 181 reaches its maximum capacity, 
the enabling voltage on pin 7 will be removed, and no further input 
signals will be permitted to pass through the gate 192 into the counter. 
The output of the counter 181 is on pin 2 or the Q3 output, which 
corresponds to the value of 2.sup.3 or the decimal value of 8. 
At the end of the sweep, the output of the counter 181 is clocked into a 
storage register 280. In FIG. 9, this register is a D-type flip-flop. This 
is done by connecting the Q3 output on line 195 of the counter 181 to the 
"D" input of the flip-flop 280. 
Correlated signals on line 170 are also applied to a shift register 201. 
This is a similar to the shift register 200 of FIG. 8; however, shift 
register 201 in this embodiment of the invention is a 128 bit device. 
Therefore, it stores the correlated output signals from the shift register 
150, further delayed by one sweep of the local oscillator. As previously 
explained, it is necessary to store the correlated detect pulses until it 
is determined whether these correlated pulses equal or exceed the number 
of uncorrelated pulses. Therefore, the shift register 201 stores these 
pulses until the comparison can be made by the counter 181. 
In the presence of a large number of uncorrelated signals, there is a 
possibility that many will be apparently correlated, even though no fixed 
frequency source is in fact detected. Occasionally, the number of apparent 
correlated signals will be greater than the number of uncorrelated signals 
detected by the circuit, and under these circumstances, the output of the 
counter 181 would indicate the presence of a radar signal. However, the 
probability that this condition will occur during two consecutive sweeps 
is quite low. Therefore, the present invention includes circuit means for 
rejecting any alarm output under these circumstances, and does so by 
requiring that at least two consecutive valid outputs occur from the 
counter 181. 
If the number of uncorrelated pulses exceeds the number of correlated 
pulses, then the Q3 output on line 195 from the counter 181 will be high. 
Output 195 is applied to the D input of register 280, and when the start 
pulse on line 56 is received, the Q output of this device will go high. 
Since this output on line 295 is connected to both the D and the set 
inputs of register 300, that register will immediately be set, and its Q 
output on line 290 will be low, thus removing the enabling signal on line 
290 to the gate 220. Under these circumstances, those correlated signals 
stored in register 201 will not be permitted to pass through the gate 220 
to the output circuitry. 
On the other hand, if the output of counter 181 on line 195 is low, 
indicating that there are more correlated signals than uncorrelated 
signals detected, then register 280 will be reset upon receipt of the 
start pulse, the output on 295 will be low, and register 300 will reset 
upon the next clock pulse on line 56, provided the output on line 195 
remains low. Thus, two consecutive sweeps are required wherein the output 
on line 195 must be low in order to cause an alarm output to be enabled 
through gate 220. 
Turning now to the fail-safe circuit 250, in the event that there is a 
legitimate signal emanating from a radar transmitter is detected, but this 
is being masked by a nearby receiver generating a large number of variable 
frequency signals, and therefore the number of uncorrelated pulses exceeds 
the number of correlated pulses during each sweep, means are provided to 
sound an alarm, provided there is a predetermined number of consecutive 
sweeps of the local oscillator during which a correlated signal is found 
in each. In other words, after the receipt of a given number of known 
valid or correlated input detect signals, an alarm will be sounded 
notwithstanding the presence of uncorrelated detect signals. 
The fail-safe circuit senses the correlated input signals by means of the 
circuit 270, and divides the number of pulses detected by two. As noted 
above, each valid or correlated input signal is represented by a pair of 
detect pulses, and for convenience, one of these pulses is eliminated from 
consideration before further processing. 
First of all, AND gate 271 correlates three separate input signals: one 
from line 145, representing the current sweep input; the second from line 
155, representing the previous sweep; and the third from line 210, 
representing the output of the second shift register 201. Thus, for AND 
gate 271 to provide an output, it must find correlation among three detect 
pulses during three consecutive sweeps of the local oscillator. 
The output of AND gate 271 is applied to the D-type flip-flop 272. This 
device, along with gate 273, acts as a divide-by-two circuit, and 
functions in the following manner. When correlation is found among the 
three detect pulses mentioned above, an output from gate 271 will be 
applied to toggle the flip-flop. The flip-flop is set at the beginning of 
each sweep with a start signal on line 56, and since it is set, one input 
to gate 273 is disabled. Therefore, the first detect pulse that passes 
through gate 271 is prevented from propagating through gate 273 to the 
remainder of the fail-safe circuit. This first pulse, however, will toggle 
the flip-flop, and therefore the second of the pair of pulses will pass 
through gate 273 to the circuit shown generally at 260. This second pulse 
will also toggle flip-flop 272 and return the circuit to the condition 
mentioned above. 
The output of gate 273 will be applied to set flip-flop 261, and its output 
is applied to increment counter 262. At the same time, the Q output will 
inhibit any start signal on line 56 from passing through gate 263 to reset 
the counter 262. 
Thus, as long as two detect pulses are sensed by the circuit 270 during 
each sweep, flip-flop 261 will be set, and as long as flip-flop 261 is set 
during each sweep cycle, the start pulse is prevented from being applied 
to reset the counter 262. If 64 consecutive sweeps each contain a 
correlated output signal, then the counter 262 will generate an output at 
Q7, and a reset signal will be applied to the R input of register 280, 
forcing the Q output on line 295 of that device low. Upon the next start 
pulse on line 56, the register 300 will reset and an enabling voltage will 
be present on line 290 and the correlated signal stored in register 201 
will pass through AND gate 230 to provide an alarm. 
On the other hand, if a pair of detect pulses does not successfully 
propagate through gate 271 then flip-flop 261 will not be set, and a start 
pulse on line 56 may therefore pass through gate 263 to reset counter 262. 
Sixty-four consecutive sweeps, each containing a correlated signal, was 
selected because having a number less than that made the receiver tend to 
provide alarms at an unacceptable rate in a high radio frequency pollution 
environment, whereas making the number significantly larger than 64 would 
increase the delay in sounding an alarm even though a valid fixed 
frequency signal had been detected. Sixty-four sweeps of the local 
oscillator represents approximately 1 second of time. 
In the embodiment of the invention illustrated in FIG. 9, the following 
components are used, although it is to be understood that this circuit 
could take other forms and be implemented in other ways without departing 
from the scope of this invention. 
______________________________________ 
Reference Component 
Number Number Item 
______________________________________ 
120,280,300 
U1 Type 4013 flip-flop 
150 U2 Type 4562 shift register 
201 U3 Type 4562 shift register 
160,220, U4 Type 4081 AND gate 
263,285 
192,271 U5 Type 4023 NAND gate 
181 U7 Type 4029 up-down counter 
191,273 U8 Type 4001 gates 
136,137, U9 Type 4049 inverters 
264 
270 U10 Type 4013 flip-flop 
262 U11 Type 4040 counter 
130 U12 Type 4040 counter 
______________________________________ 
While the form of apparatus herein described constitutes a preferred 
embodiment of this invention, it is to be understood that the invention is 
not limited to this precise form of apparatus, and that changes may be 
made therein without departing from the scope of the invention which is 
defined in the appended claims.