Note: Descriptions are shown in the official language in which they were submitted.
CM--76799
1067589
This invention relates to a tracking oscillator and,
more particularly, an improved tracking oscillator and use
of the tracking oscillator in a frequency-to-voltage converter.
BACKGROUND OF THE INVENTION
In many RF communications area, it is often necessary
to track change over in the frequency of signals relative to
a reference frequency in incoming signals~ According to the
prior art, phase locked loops are often used for this
purpose. The prior art phase locked loop circuitry are,
however, usually complex and require a large number of
elements which are costly. Prior art frequency-to-voltage
converters such as FM discriminators usually use a wide
variety of such phase locked loop circuitry or a network of
analog filters which are generally complex and costly.
:
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved
tracking oscillator.
It is still another object of the present invention to
provide less costly and less complex tracking os,cillator.
It is yet another object of the present invention to
provide an improved frequency-to-voltage converter using a
simplified and improved tracking oscillator.
The foregoing and other objects are achieved in accordance
with the present invention by providing a tracking oscillator
that includes an integrator and a hysteresis switch connected
in series wherein the output of the switch is fed back to
the integrator for marking the change of direction of inte-
gration, i.e., decreasing or increasing direction as the
. ,:
., .
- 2 ~
. .
CM-76799
~067589
input train of waves traverses back and forth between a
lower and a higher voltage and introduce phase delay in the
output train of waves.
In accordance with another aspect of the present inven-
tion, the aforementioned tracking oscillator is advantageously
used in a frequency-to-voltage converter by interposing the
tracking oscillator between a limiter and a phase detector.
The tracking oscillator provides a phase delay and the phase
detector measures the phase delay by comparing the output of
the tracking oscillator with the output of the limiter. The
output of the phase detector is in the form of a train of
waves with pulse width proportional to the phase delay
introduced by the tracking oscillator. The output of the
phase detector may be advantageously converted into a DC
voltage proportional to the phase delay, that is, propor-
tional to the pulse width of the phase detector, by using a
suitable circuitry, such as a low pass filter and an averaging
circuit.
:~
The foregoing and other objects and features of the
present invention will be more clearly understood from the
- following detailed description of an illustrative embodiment
of the present invention in conjunction with the accompanying
drawings,
,1
BRIEF DESCRIPTION OF THE DRAWINGS
_ _
Figure 1 shows an embodiment of a tracking oscillator
and use of the tracking oscillator in a frequency to voltage
converter in accordance with the present invention.
, Figure 2 shows typical waveforms of the frequency to
voltage converter operating in a steady state.
;
,
CM-76799
1067589
Figure 3 shows steady state waveforms of the tracking
oscillator in a free running mode using an inverting integrator.
Figure 4 shows steady state waveforms of the tracking
oscillator in the tracking mode using an inverting integrator
wherein the output voltage of the hysteresis switch has
greater weight than the input voltage to the integrator.
Figure 5 shows waveforms of the tracking oscillator
shown in Figure 4 after they have been adjusted for symmetri-
cal voltage swings.
DETAILED DESCRIPTION
Figure 1 shows a frequency-to-voltage converter in a
functional block diagram form that embodies the present
invention. Figure 2 shows timing waveforms of the frequency-
to-voltage converter that are helpful in understanding the
operation of the frequency to voltage converter Referring
to Figures 1 and 2, an input signal may be either a sinusoid
S or a square wave A, as illustrated in Figure 2. A suitable
conventional limiter 10 can be used, if required, to square
; the input signal by amplifying and clipping the input to
provide a square wave B of a desired amplitude and polarity
as shown in Figure 2.
The output of the limiter is applied to a tracking
oscillator 13 comprising an integrator 21 and an hysteresis
switch 23 coupled in series as shown in Figure 1. An output
of the hysteresis loop is fed back to the integrator. The
I integrator is also adapted to receive two inputs; one from
t! the limiter and the other from the output of the hysteresis
switch. The output of the hysteresis switch is used to
, provide a reference signal C, with a phase inversion, as
,1 .
-- 4 --
.. ., ~ ~ -. .. . . .;,.. , ~ ,. ,, .. . .. : .: .
CM-76799
10~7S89
desired, with respect to the square wave input B, as shown.
If necessary, the hysteresis switch can be adapted to provide
reference signals without a phase inversion. The tracking
oscillator 13 produces a square wave output D of the same
frequency as its input signal B, but shifted in phase with
respect to it. As shown this phase shift is directly propor-
tional tG the difference in frequency between the nominal,
that is, free running frequency of the tracking oscillator
13 and the frequency of the input singal S or A. A phase
detector 15 is coupled to receive the phase shifted output D
from the tracking oscillator to produce, as its output
signal, a pulse train E whose pulse width is directly propor-
tional to the phase shift.
A suitable circuitry~ such as a low pass filter/averaging
circuit 17 is coupled to the phase detector to provide an
output signai F which is directly proportional to the
frequency shift defined above and which may range within a
maximum VmaX~ and a minimum, Vmin.
Referring to Figure 1 again, an aspect of the present
invention is the tracking oscillator 13 used in the frequency-
; to-voltage converter, as illustrated in Figure 1. The
tracking oscillator includes the integrator 21 and the
hysteresis switch 23 connected in series. The oscillator is
designed so that in a steady state operation, it exhibits
the characteristics as illustrated in the waveforms shown in
Figures 2 and 3. In the absence of an input signal from the
limiter 10~ the tracking oscillator operates as a free
running oscillator. The integrator 21, as illustrated, is
an inverting integrator, although the integrator need not be
an inverting integrator. The operation of the tracking
~. j
- 5 -
i
:;
~ ~ CM-76799
~0~7589
oscillator will be described with reference to the inverting
integrator shown.
1. Refer to Figures 1, 2 and 3. At an arbitrary
time, T=Q, assume the tracking oscillator is at a steady
state condition.
2. With the instantaneous voltage of-the output of
the hysteresis switch 23 ~Fig. 2,V and Fig. 3, Vc) just
switched to Va and with the output voltage, VOUt (~ig. 3)
applied to the hysteresis switch to a voltage Vc, the output
voltage VOUt~ of the integrator will then decrease linearly
with time from a voltage Vc to a voltage Vd. If a non-
inverting integrator were used, Vc and Vd would be inter-
changed and the polarity of the voltage transitions of the
. . .
voltage output, VOUt, of the integrator is reversed. -
3, When the voltage output, VOUt~ reaches the lower
hysteresis voltage Vd, at time To~ the hysteresis switch 23
will change its output state, so that its output voltage,
namely, Fig. 2; C: Fig. 3; Vc applied to t~e input of the
integrator and its output ~Fig. 2;B) applied to the phase
detector 15 are switched.
4, This transition of the output of the hysteresis
switch from Va to Vb reverses the slope of the output signal,
VOUt~ of the integrator 21 so that the output, VOUt, will
rise linearly from voltage Vd to voltage Vc~ during the
interval from To to 2To-
~ 5. When the output, VOUt of the integrator reaches the
i upper hysteresis voltage limit Vc at time 2To, the hysteresis
~l switch 23 will change back to its previous state, so that
.1. .
I its output voltage- (Fig. 2;D) switch back.
.
6. The steps 2 through S hereinabove are repeated
cyclically, The resulting free running oscillator has a
frequency fO = 2To
6 -
CM- 7 6 7 9 9
106~589
Now suppose an input is applied to the limiter and the
input is such that the limiter produces a square wave at the
output thereof as illustrated in Figure 2A and suppose
further that it swings symmetrically above and below the
integrator reference voltage, V f = 2- - This will
modify the integrator output waveform as illustrated in
Figure 4. As described hereinbelow in detail, with the
inverting integrator, the output of the hysteresis switch 23
(Fig. 2;D) has greater influence on phase detector 15 than
the voltage Fig. 2;A applied to the integrator from the
limiter.
1. Assume that the tracking oscillator has been
; operating under the foregoing conditions for a sufficient
time to achieve a steady state operation.
2. During the interval from time a to ~T, the voltage
output of the limiter, Fig. 2, B~ is equal to Valt the
feedback voltage from the hysteresis switch is equal to Vb'.
Therefore the integrator output voltage, VOUt' will increase
linearly with its larger positive slope.
3. At time ~T, the voltage output of the integrator
VOUt' will reach Vcl, causing the hysteresis switch 23 to
change its state and switching its output from Vb' to Val.
4 During the interval from ~T to T the oscillator
input voltage is equal to Ve and the output voltage of the
hysteresis switch is equal to Va', Therefore, the output of
the integrator, VOUt, will decrease linearly with smaller
negative slope.
5. At time T ~ the oscillator input voltage B changes
from Ve to Vh tFig. 4). Thus, during the interval from T to
'~ 30 T + ~T the drive voltage Vh is equal to the switch voltage
~' -7-
':: . ' .: :
CM-76799
lOG75t39
output Va. Therefore, the output of the integrator VOUt
will decrease linearly with its larger negative slope.
6. At time T + ~T, the integrator output will reach
Vd', Vd' causing the hysteresis switch to change its output
from Va to Vb -
7. During the interval from T ~ QT to 2T, the oscilla-
tor input is at Vh and the switch voltage reference signal
is Vb'. Therefore, the output of the integrator VOUt will
increase linearly with its smaller positive slope.
8. At time 2T, the oscillator input voltage changes
from Vh to Ve and the steps 2 through 8 described hereinabove
are repeated.
The result of the operation described above is to
generate a signal at the output (Fig. 2; D) of the hysteresis
switch 23, which is of the same frequency as the driving
signal applied to the tracking oscillator at the limiter.
In addition, the output of the switch is inverted and shifted
to the right by ~T. ~T iS dependent on the frequency of the
oscillator input (Fig. 2; B) and the ratio of the two resistors
Rl and R2 in the integrator 21. In fact, it is found that
the phase shift ~f, that is, the difference between the free
running frequency and the frequency of the osci:Llator input
signal, B, i9 directly proportional to ~T and to the ratio
of the two resistors Rl and R2.
The tracking oscillator will operate according to the
; same principle whether the integrator is inverting or non-
inverting, whether the influence of the switch output is
greater than, equal to, or less than the influence of the
input~ In practice, it was found that the inverting integra-
tor was found easier and less costly to implement. Also, it
.. : - 8 -
CM-76799
1C~67589
was found that the transfer ratio of the frequency to voltage
converter is proportional to the ratio of the influence of
the output voltage, C, of the hysteresis switch to that of
the input drive voltage B so that influence of the switch
output would usually be chosen equal to or greater than that
of the input voltage B.
The phase detector 15 operates as follows. The detector
produces a pulse train whose pulse width is proportional to
~T. Referring to Figure 2, note that the two input signals,
one from the output of the tracking oscillator (Figure 2;D)
and the other from the output of the limiter (Fig. 2;D) are
applied to the phase detector. The oscillator output is
phase shifted by virtue of the behavior of the tracking
oscillator as described hereinabove. The resulting output
signals of the phase detector shown in Figure 2;E. The
detector output is a pulse train wherein a pulse appears
whenever the two input signals are at different voltage
levels. This can be achieved by an Exclusive-OR gate. An
Exclusive-OR gate is inexpensive and small. However, other
suitable means may be used in place of the Exclusive-OR
gate.
The present tracking oscillator can be advantageously
used in a frequency-to-voltage converter. This i~ readily
accomplished by applying the output of the phase detector 15
to a suitable circuit, such as a conventional low pass
filter/averaging circuit 17 and adapted to provide DC
voltage proportional to the pulse width ~T. If the frequency
of the input signal to the frequency to voltage converter is
varying at the slower rate than the frequency of the tracking
oscillator itself~ then the low pass filter provides an
"
" '
. g _
- - ; : : ~ . ,- , .: : . , , -
CM-76799
1~67589
output voltage which varies in direct proportion to the
input signal frequency. In such a case, the converter
functions as an FM discriminator. If the input voltage
shifts between the two frequencies, then the converter
functions as frequency shift key detector.
Tracking oscillator 13 shown in Figure 1 and the overall
circuit configuration of the frequency-to-voltage converter
shown in Figure 1 may be advantageously implemented in a
; large scale integrated circuit form. The limiter 10 may
include a three stage inverter of CMOS gates connected in
series in a conventional manner (not shown). The output of
the second gate may be applied to the phase detector and may
be used to provide a phase inversion with respect to the
; output of the third gate applied to the tracking oscillator
21. The integrator 21 comprises a CMOS gate G6, a capacitor
C7, resistors Rl, R2, and R3 as schematically shown and
connected. The hysteresis switch 23 includes CMOS gates G7
through GlQ and resistors R8, R9, R10, connected, as illus-
trated to form the tracking oscillator 23. The transfer
ratio of the frequency to voltage converter is proportional
to the ratio Rl + R2 . The maximum and minimum hysteresis
voltages, Vc' and Vd' are determined by the ratio of the
~ resistances of the resistors, R3 and R10. The phase shifted
; output of the hysteresis switch 23 of the tracking oscillator
is taken from the output of CMOS gate G9 to provide the
desired polarity of the frequency-to-voltage transfer ratio.
The phase detector 15 may include an Exclusive-OR gate
Gll. The low pass filter/averaging circuit 9 may contain an
inverter and two active second order bypass low filters in
series ~not shown) of a conventional design.
- lQ
:
,
.
CM-76799 1~67S89
The reference Yoltage Rref may be maintained throughout
the circuit at the threshold voltage of the CMOS gates and
the voltage swings are symmetrical above and below the
threshold voltage, as illustrated in Fig. 5~ This is accom-
plished using negative eedback around various gates. This
factor accounts for the use of various circuit components
such as resistors R3, R8, R9, and gate Gl0 in the tracking
oscillator and similar elements that may be used in the low
pass filter and averaging circuit 17. In accordance with an
aspect of the present invention, the tracking oscillator and
the remaining circuit elements can be readily implemented by
using conventional CMOS integrated circuits, capacitors and
resistors and can be readily made in a hybrid circuit.
In summary, then, the present invention provides a
tracking oscillator of a greatly simplified circuitry and
novel utilization of such a tracking circuitry in combination
with a limiter and a phase detector for providing a frequency-
,.
to-voltage converter, The converter may be advantageously
used as an FM discriminator, or frequency shift keying
circuit as desired~ These features of the invention are
shown implemented in the form of low cost CMOS integrated
circuits. The overall frequency-to-voltage converter circuitry
can be readily implemented essentially in two CMOS integrated
circuits: one to include the limiter and tracking oscillator
and the other to include the phase detector and the low pass
filter and averaging circuit~ Each of these CMOS integrated
circuits can be implemented with a single low cost digital
; integrated circuit.
Since C~OS integrated circuits are low cost elements,
cost savings for tne overall circuitry is very significant.
. CM-76799 1~67589
Moreover, the use of the limited number of small circuit
components renders the circuit more reliable. Also the
present circuit is more suitable for implementation as a
hybrid circuit than the prior art frequency-to-voltage
converter circuits.
.
;
.~
.
.`.i :
l 12 -
