Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Phase shift by less than period of input
Reexamination Certificate
1995-09-22
2002-06-11
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Phase shift by less than period of input
C327S231000, C327S235000, C327S237000, C327S248000
Reexamination Certificate
active
06404255
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to phase shift apparatus and particularly to variable phase shifters of a type in which plural phase shifted components of an input signal are formed and combined in proportions determined by a phase shift control signal to provide a phase shifted output signal.
BACKGROUND OF THE INVENTION
Variable phase shift apparatus are useful, for example, as phase control elements in voltage controlled oscillators, tuning circuits, filters and in similar applications where controlled phase shift is needed. For example, in U.S. Pat. No. 4,020,500 entitled CONTROLLED OSCILLATOR which issued Apr. 26, 1977 to L. A. Harwood, there is described a voltage controlled crystal oscillator in which phase shift for controlling the oscillator frequency is provided by vector summation. Specifically, an oscillator signal is phase shifted by +90 degrees by means of a two-pole L-C low pass filter and combined with the original signal to produce a resultant vector lying between 0 degrees and a leading angle (e.g., +45 degrees) with the angle being controllable by varying the amplitude of the +90 degree (quadrature) signal. For providing lagging phase shifts (e.g., from 0 degrees to about −45 degrees) the polarity of the quadrature vector is inverted to be −90 degrees rather than +90 degrees and the inverted quadrature vector is added to the reference or input signal. Here the resultant vector lies between 0 degrees and a lagging angle between 0 degrees and about −45 degrees with the angle being controllable by varying the amplitude of the −90 degree vector.
A phase shift circuit, in an oscillator application, which avoids the need for quadrature phase shifting elements (e.g., 90 degree two-pole filtering) is described by Paul D. Filliman in his U.S. Pat. No. 4,797,634 entitled CONTROLLED OSCILLATOR which issued Jan. 10, 1989. Unlike the Harwood arrangement, the Filliman phase shifter combines three vectors at a time rather than two. The vector angles used are −45 degrees, 0 degrees and +135 degrees with respect to the input signal and the mixture of these three vectors provides all phase shifts, both positive (leading) and negative (lagging). Also, since the +135 degree vector is produced from the −45 degree vector by inversion, and since the −45 degree shift may be provided by a one-pole RC filter, the entire phase shift circuit may be readily constructed in integrated circuit form in applications such as providing phase control of color oscillators in television related products.
FIG. 1
herein presents a simplified block diagram of the Filliman oscillator. As shown, a phase shifter
10
is provided with two feedback paths comprising (i) a DC path (
12
) via a resistor (
14
) which regulates the DC operating point of the oscillator by means of negative feedback and (ii) an AC feedback path (
16
) through a resonator (
18
, e.g., a crystal) which provides positive feedback with a gain of unity for causing oscillations to occur.
Phase shifter
10
, as illustrated in
FIG. 1
, is non-inverting whereas oscillator applications require inversion to provide the proper negative DC feedback for bias stabilization and positive AC feedback signal at the nominal resonator frequency, Fr, for causing oscillations to occur. The inversion of the feedback signal for oscillator applications is provided in
FIG. 1
by an inverter
35
at the phase shifter output
22
. Optionally, inversion may be applied at various places. For example, the inverter
35
may just as well be placed at the input
20
of the phase shifter
10
or within the phase shifter. Also, inversion may be provided by simply using the inverting output of a differential output amplifier in the phase shift network
10
or by using an inverting form of summing amplifier
24
at the phase shifter output. What is of importance to the present invention is not whether the shifter inverts or not, but how the phase shifting is accomplished as will be described.
Phase shifter
10
includes an input terminal
20
for receiving the oscillator input signal S
1
to be phase shifted and an output terminal
22
for providing a phase shifted output signal S
2
. The input signal S
1
separated into three “intermediate” signals or “vector” components “A”, “B” and “C” which are summed via a summing circuit
24
to provide a phase controlled output signal S
2
at the network
10
output terminal
22
. Hereinafter, the terms “intermediate” signals or components may be used interchangeably with the term “vector” signals or components.
The processing of the input signal S
1
to form the intermediate signals or vector components “A”, “B” and “C” is provided by a constant gain amplifier
28
, two variable gain amplifiers
30
and
32
(the latter of which is inverting) and a 45 degree phase lag network
26
. Vector “A” in
FIG. 1
has a phase angle of zero degrees with respect to the input signal S
1
and is produced by applying the input signal to the summing circuit
24
via a non-inverting fixed gain amplifier
28
.
Vector “B” has a phase angle of −45 degrees relative to the input signal S
1
and is produced by phase shifting the input signal by 45 degrees (lagging) at the nominal frequency of the resonator
18
in the phase shift network
26
(e.g., a low pass filter). The phase shifted signal “B” is then applied to summing circuit
24
via a variable gain non-inverting amplifier
30
.
Vector “C” has a phase angle of +135 degrees relative to the input signal S
1
and is produced by inverting the output of the 45 degree phase shift network
26
with a controllable gain inverting amplifier
32
for application to summing circuit
24
. A phase shift control signal
53
is applied directly to the gain control input of amplifier
30
and is inverted by means of an inverter
38
for application to the gain control input of inverting amplifier
32
.
In operation, due to the action of the inverter
38
, an increase in control signal S
3
at terminal
22
will cause an increase in the amplitude of the vector “B” and concurrently will cause a decrease in the amplitude of the vector “C”. This will produce a lagging phase for the output signal S
2
relative to the input signal S
1
. Conversely, a decrease in the control signal S
3
will cause a decrease in the amplitude of vector “B” and an increase in the amplitude of vector “C”. This will produce a leading phase for the output signal S
2
relative to the input signal S
1
. When changing phase angles, the amplitude of vector “A” is held constant. It is the variation of the amplitudes of vectors “B” and “C” which controls the net phase shift of the network and thus the frequency of the oscillator.
SUMMARY OF THE INVENTION
It has been found that under certain special circumstances (e.g., improper amplifier gains or gain variations) the Filliman type of oscillator, employing a three-vector phase shifter of the type described, may exhibit one, or more, of the following three operating difficulties: (i) stopped oscillations; (ii) overtone oscillations; and (iii) bias instability.
The problems noted above have been found to be traceable to the phase shift portion of the oscillator. It has also been found that the problems with the oscillator may be reduced by the expedient of restricting the phase range of the phase shifter. This, however, is not a completely satisfactory solution since it limits the oscillator frequency range and thus limits the range of useful applications for the oscillator.
Phase shift apparatus, embodying the invention, includes a source providing an input signal to be phase shifted and a combining circuit for combining first, second and third intermediate signals that are derived from the input signal, and have differing phase shifts, to form a phase shifted output signal. A first amplitude control circuit, responsive to a phase control signal supplied thereto, varies the amplitudes of the second and third intermediate signals in opposite directions for controlling the phase of the phase shift
Filliman Paul D.
Rumreich Mark Francis
Lam Tuan T.
Shedd Robert D.
Thomson Licensing S.A.
Tripoli Joseph S.
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