Note: Descriptions are shown in the official language in which they were submitted.
01 This invention relates to an apparatus and
02 method for continuously measuring the frequency of a
03 variable frequency signal having a period which varies
04 with slower frequency than the frequency itself, with
05 a high degree of precision.
06 It is sometimes required to measure the
07 fre~uency of a ~requency varying signal with high
08 resolution on a continuous basisO For example,
09 variations in frequenc~ of radiation from the earth as
detected by a magnetometer or the like during an
11 airplane scan of the terrain can indicate the present
12 of magnetic ore bodies. The speed by which the
13 scanning is conducted is usually dependent on the
14 ability of the frequency determining apparatus to
de-termine frequency variation of the detected input
16 signal as anomolies are overflown. By increasing the
17 resolution of the frequency determining apparatus and
1~ by increasing the signal sampling rate, a much faster
19 scan of the terrain could be achieved. This would
reduce the cost of conducting the scan, or would
21 facilitate conducting the scan over a much greater
22 area during a given scanning interval.
23 Present frequency determining methods
24 require the use of relatively slow propeller driven
survey airplanes to conduct the scanO This invention
26 increases the sampling rate of the frequency
27 determining apparatus to such a great extent that it
28 is believed that fast jet airplanes can be used to
29 conduct the scan, at much greater speeds than before.
An understanding of this invention will be
31 obtained by a consideration of the description below,
32 of the general concept and of a preferred embodiment
33 of the invention, in conjunction with the following
34 drawings in which:
Figures 1-4 are waveform diagrams used to
36 explain prior art forms of the invention,
37 Figure 5 is a waveform diag~am used to
38 - 1 -
01 explain the present invention,
02 Figure 6 is a block diagram of the basic
03 form of the present invention, and
04 Figure 7 is a block diagram illustrating
05 the preferred form of the present invention.
06 Turning to Figure 1, a received signal
07 whose frequency is to determined is shown as waveform
08 1, which has been corrected into squarewave form.
09 Within a certain period P, there are n pulses which
can vary in number according to variations in
11 frequency. According to one prior art technique Eor
12 determining the frequency of an input signal, the
13 number of pulses within a period P (e.g. 1 second) is
14 counted, thereby representing the ~requency. In this
1~ technique it ta~es a full second (assuming that is the
16 time interval of the count) to determine the frequency
17 of the input signal.
18 In order to increase the resolution and
19 thus the accuracy of the described technique, the
number of input pulses is sometimes multiplied by a
21 predetermined factor, and then the number of pulses is
22 measured over a defined period of time (e.g. 1
23 second), as before. However this also requires a long
24 time period to perform the frequency determination.
According to a technique for decreasing
26 the required period of time for frequency
27 determination, the time period of a single input pulse
28 2 is measured (Figure 2). However this measurement is
29 very limited in re~olution.
To increase the resolution of the period
31 measurement technique, a period measurement of a large
32 number n cf periods is measured as shown in Figure 3.
33 The total time period required to determine the
34 frequency is nxT, where T is the period of each pulse
(assuming equal pulse periods). ~owever this
36 increases the required time period while i~ increases
37 the resolution.
38 - 2 -
01 The technique of Figure 3 is also used in
02 the technique shown in Figure 4, in which successive
03 pulse group ~ntervals of the input signal are
04 measured, to determined changes in frequency. For
05 example, six pulses during the period nxTl are
06 measured, followed by the measurement of a similar
07 number of pulses during the interval nxT2. Thus it
08 may be seen that two successive time periods, each six
09 pulses long are required to detect any change in
frequency. Clearly this is a relatively slow and
11 ineffective technique for determining the frequency
12 change of a terrestrial signal caused by a stationary
13 ore body, from a fast moving airplane, particularly
1~ when the ore body is small.
According to prior art techniques,
16 increases in resolution result in decrease in speed of
17 frequency determination, and vice versa.
18 The present invention achieves high
19 resolution obtained by the multiple pulse period
measurement technique, but simultaneously achieves a
21 fast sampling rate similar to that of the single pulse
22 measurement technique. A consideration of Figure 5
23 will illustrate the technique.
24 The -time period for n pulses (shown as six
in number ~or example), each having its own time
26 period, for example, To-Ts, is measured, as in the
27 technique described with reference to Figure 3. The t
28 total time for the six periods is nPl. Then the
29 ~irst-in pulse time period To is dropped, and the time
period of the next pulse T6 is added to time periods
31 Tl-T5, to provide a total time period of nP2.
32 Following this the next first-in pulse time period P2
33 is dropped and the time period T7 is added to
34 establish the total time period nP3. It may be seen
that should the frequency change~ the period of each
36 new incoming pulse also changes~ and thus the total
37 changes. Accordingl~ we have a moving total
38 - 3 -
01 representing the frequency of a given number of
02 pulses.
03 It should be noted that while the total
04 over a predetermined number of pulses provides a
05 relatively high re~solution result, a sample is taken
06 each pulse period. Consequently the sampling rate is
07 as high as the pulse rate. Thus we have the result of
03 high resolution at the same time as a high sampling
~9 rate.
While the actual pulse rate of the input
11 signal can be used~ it is preferred, Eor ease of
12 design and stablity of the circuitry to divide the
13 relatively high frequency input signal by a factor
14 whereby the pulse rate which is measured is of the
order of 100 hertz. This also provides time for
16 relatively inexpensive processors to generate the
17 average calculation.
1~ It may be seen that the method of the
19 present invention provides a substantially improved
freguency measurement technique, which provides the
21 desirable result of high resolution at the same time
22 as high sampling rate, which was not possible in the
23 aforenoted prior art techniques. Accordingly a high
24 speed scanning survey airplane can be used to conduct
the survey.
26 The apparatus of the invention, in
27 general, is a continuous frequency measurement
28 apparatus comprising apparatus for providing a first
29 signal having frequency related to the frequency of
the signal to be measured, and apparatus for totalling
31 the pulse period of the first signal over a
32 predetermined number of the pulses successively as a
33 moving total.
34 More particularly, the invention is a
continuous variable frequency measurement apparatus
36 comprising apparatus for providing a first signal
37 having a frequency related to the frequency of a
38 _ ~ _
01 signal to be measured, a source of a calibration
02 signal having a prede~ermined constant frequency which
03 is greater than the frequency oE the ~irst signal,
04 apparatus for coun~ing the number of cycles of
05 calibration signal occurring during each successive
06 pulse of the Eirst signal, to provi~e successive count
07 signals, apparatus ~or s-toring a predetermined number
08 of the successive count signals, apparatus for
09 totalling the values of the stored count signals to
provide an output signal representative of the average
11 frequency to be measured over a period of the
12 predetermined number of successive count signals,
13 apparatus for deleting a first-in count signal and
14 adding a new count signal with each successive pulse
of the first signal, and apparatus for successively
16 totalling the values of the stored count signals each
17 time a new count signal has been stored, whereby the
1~ output signal is continuously representative oE the
19 average Erequency of the signal to be .neasured updated
each pulse of the first signal.
21 It should be noted that the average time
22 for a predetermined number of pulses can be used,
23 rather than the to~al (i.e. the time divided by the
24 number of pulses). Alternatively, a running average
of a prede~ermined number of totals could be used, to
26 smooth a curve defined by the totals.
27 Thus the me~hod and apparatus provides an
2~ advantage over prior art techniques for both short
29 lasting but constant frequency samples and for
continuous and drifting frequencies. The accuracy of
31 the scanned frequency in proton continuous types of
32 magnetometers, such as helium or cesium and the
33 reliability of the result Erom the freguency
34 distribution, is considerably improved. The technique
and apparatus is of course not limi~ed to the
36 geophysics field, and can be used in any field in
37 which a varying frequency is to be measured with high
3g - 5 -
6~
01 resolution.
02 A block diagram of the basic form of the
03 preferred embodiment o~ the invention is shown in
0~ Figure 6. An input signal is received at input 3 at a
05 signal condltioner 4. The signal conditioner reduces
06 the frequency of the input signal. For example, the
07 input signal from a proton magnetometer typically
08 would be between l kHz and 5 kHz, the output of a
09 helium ma~netometer would be typically between 200 kHz
and l megahertz, while the input signal from a cesium
11 magnetometer might be twice as high as from the latter
12 magnetometer. The signal conditioner, which could be
13 a phase locked loop or counter, provides an output
14 frequency of the order of lO0 hertz. This signal is
applied to a moving totalizer 5. An output signal of
16 the moving totalizer is obtained at port 6.
17 The moving totalizer stores the time
18 periods of a first predetermined number of pulses
l9 output from the signal conditioner 4. A total ti~e
period is then determined, which is representative of
21 the average frequency of the first predetermined
22 number of pulses ~e.g. 6 in number). Then the time
23 period of the first-in pulse is dropped from the total
24 time and the time period of the next pulse output from
signal conditioner 4 is measured and is added with the
26 remaining total of the time periods. The time period
27 for the next first-in pulse is then dropped and the
28 time period of the next-in pulse received from signal
29 conditioner 4 is stored and added with the time
periods of the remaining pulses. Each total is
31 determined and a signal designating the total is
32 output at port 6. Each total is of course
33 representative of the frequency of the input signal.
34 A total i9 struck once each pulse.
As the frequency of the input signal
36 changes, the time periods of the pulses of the output
37 signal from signal conditioner 4 change, and
38 - 6 -
01 consequently the time period signal output from moving
02 totalizer 5 changes, which provides an output signal
03 indicative of the frequency and of the changeO
04 Accordingly an output signal representative of the
05 frequency of the input signal is produced having high
06 resolution which corresponds to a large number of
07 pulses, but with a sampling rate corresponding to the
08 frequency of single pulses output from the signal
09 conditioner.
Figure 7 shows a more detailed block
11 diagram of the preferred form of the invention. An
12 input signal is received at input 3 to a signal
13 conditioner 4. The reduced frequency output signal of
14 signal conditioner 4 is applied to a two output di~ide
by two counter 7, which can be in the form of a J-K
16 flip-flop.
17 A calibration signal fcAL having a constant
18 frequency much higher than the output signal of the
19 signal conditioner is applied to a lead 8 which is
connected to the inputs of a pair of switches 9 and
21 10. The outputs o~ switches 9 and 10 are connected
22 respectively to counters 11 and 12. The outputs of
23 divide by two counters 7 and 8 (each of which is high
24 during successive alternate pulses of the input signal
from the signal conditioner1 are connected to the
26 enable inputs of corresponding switches 9 and 10.
27 Consequently when one output of counter 7 is high,
28 switch 9 is caused to close and when the other input
29 of counter 7 is alternately high, switch 9 is caused
to open and switch 10 is caused to close.
31 During one output pulse from signal
32 conditioner 4, the signal fcAL is applied to the
33 input of counter 11, while during the next output
34 pulse of conditioner 4, the signal fcAL is applied
to the input o~ counter 12. The number of fcAL
36 signal pulses stored in counters 11 and 12 are thus
37 representative of the corresponding time periods of
38 - 7 -
01 two succeeding output pulses of signal conditioner ~.
02 The output ports of the counters 11 and 12
03 are connected via a processor 14 to a memory 15, which
04 stores a predetermined number of pulse time data
05 signals from the counters, from which an total can be
06 taken by the processor.
07 When one output of counter 7 goes high, an
08 enable circuit 13 is caused to input an interrupt
09 signal to processor 14, having inputs connected to the
outputs of counter 11 This interrupt causes
11 processor 14 to receive the count signal stored in
12 counter 11 and to reset lt. Processor 14 causes
13 memory 15 to erase the data stored at a first memory
14 location in memory 15 if necessary, to shift the data
stored in each of a predetermined number of locations
16 te.g. the following five locations for six pulses) to
17 lower numbered locations including the erased
18 location, and to store the count data signal from
19 counter 11 in the last or freed memory location of
memory 15.
21 Since at the beginnin~ of operation all of
22 the memory locations of memory 15 will be empty or
23 have non-sensible values stored therein, until the
24 count outputs from the counters are stored in all
memory locations, the data stored in memory 15 will
26 not have validity as a whole.
27 With the next pulse leading edge from
28 counter 7 input to enable circuit 13, processor 14 is
29 interrupted causing it to receive output data ~rom
counter 1:2 and to reset counter 12. The data stored
31 in the first-in location of memory 15 is erased or
32 otherwise deleted if necessary, and the data stored at
33 the second to the sixth memory locations are shifted
3~ down one memory location to the first to the fifth
memory loc:ations. The data output from counter 12 is
3~ then stored at the sixth memory location of memory 15.
37 In similar manner, each successive pulse
38 - 8 -
01 output from signal conditioner 1~ causes the count
02 data representing the time of one pulse period
03 alternately from counter 11 and counter 12 to be
04 stored at the sixth memory location in memory 15,
05 after the data in the second to sixth memory locations
06 are shifted to the first to the fifth memory locations.
07 Each time a data signal is stored in the
08 sixth memory location of memory 15, processor 14 is
09 caused to read the data in the six memory locations
and to total them. Alternatively it can merely
11 average the time value data signals. This provides an
12 indication of the time period of six pulse periods
13 output from signal conditioner 4. The total or
14 average signal is output from output port 6 to an
output line, and can be plotted on a graph or
16 displayed on a meter or digital display, as a
17 representation of the absolute and varying frequency
13 of the input signal.
19 As noted earlier, the average of a
predetermined number of totals can also be used, to
21 smooth the output signal.
22 Since the total is determined each time
23 the sixth memory location is written, the output
24 signal representative of the input frequency is
updated at the frequency of each pulse output ~rom
26 signal conditioner 4. It is clear that the sampling
27 rate is very high. Since the number of pulses
28 measured can be predetermined, the resolution can also
29 be established, and can be very high. The larger the
number of pulses and the higher their frequency,
31 however, the faster the processor required.
32 Appendix A illustrates the algorithm
33 performed by processor ~, which is believed to be
34 self-explanatory. The variable m represents the
number of pulses during a totalling period, while n is
36 the count number stored in a counter. The variable C
37 is a constant multiplied by the total count from the
38 _ 9 _
01 counters over the totalizlng period, divided by the
02 reference frequency. A consideration of Appendix A
03 with respect to the description noted above will make
04 clear the manner of operation of the processor. It is
05 of course assumed that a person skilled in the art
06 understanding this invention will also understand the
07 techniques of preparing a program for operation of the
08 processor according to the algorithm described above.
09 It will be understood that since signals
representing the time periods of each of the pulses
11 within a totalizing period are stored in the memory,
12 other operations can also be performed based on these
13 values~ such as determining the statistical
14 distribution of the samples, obtaining parameters such
as signal to noise ratio, establishing the reliability
16 of the readings, etc.
17 Further, it should be noted that the
18 system can be used to provide a first indication of
19 the presence of ore bodies during a fast exploration
scan of a territory. Should an interesting anomoly be
21 located, a slow pass can be initiated to pinpoint its
22 location more accurately or, alternatively, the
23 resolution of the system can be changed by increasing
24 the number o~ samples totalized.
A person understanding this invention may
26 now conceive of other embodiments or variations and
27 additions, using the principles described herein. All
28 are consi~ered to be within the sphere and scope of
29 the inven~ion as defined in the claims appended
hereto.
31 - 10 -
~ APP~.~DIX A
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