Oscillators – Automatic frequency stabilization using a phase or frequency... – Search sweep of oscillator
Reexamination Certificate
1998-12-28
2001-03-13
Kinkead, Arnold (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Search sweep of oscillator
C331S017000, C329S307000, C375S327000
Reexamination Certificate
active
06201447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase synchronization circuit which is applicable to, for example, a carrier wave reproduction time shortening circuit of a synchronization detector installed in a demodulator of a digital wireless apparatus.
2. Description of Related Art
A synchronization detector installed in, for example, a demodulator of a digital wireless apparatus, detects a signal by demodulating a modulated signal based on a carrier wave reproduced on the receiving side. A phase synchronization circuit used to reproduce the carrier wave of this synchronization detector controls the phase and frequency of an output signal comparing the phase of the input signal with that of the output signal.
FIG. 1
is a block diagram showing the fundamental configuration blocks of this phase synchronization circuit. The fundamental operation of this circuit will be described below. First, the phase comparator (PC)
10
detects the phase difference between the phase of an input signal to be used as a reference and the phase of the output signal of the voltage control oscillator (VCO)
30
that is oscillating near a frequency to be output. The loop filter
20
then generates a control voltage based on the detected phase difference and supplies the control voltage to the VCO
30
to change the oscillation frequency of the VCO
30
. In this case, the oscillation frequency of the output signal is stabilized by feeding back the changed oscillation frequency of the VCO
30
to the PC
10
so that phase difference between the input signal and output signal of the VCO
30
will be eliminated.
FIG. 2
shows the relation between the synchronization holding range (lock-in range), frequency pull-in range, and free-running frequency of the VCO (oscillation frequency when the input to VCO is zero). Here, the synchronization holding range is a range of frequency in which the state of synchronization (the state in which the phase is locked in) can be held when the frequency of the input signal is gradually removed from the free-running frequency of the VCO. The frequency pull-in range is a range of frequency in which the output signal in the state of non-synchronization can be synchronized with the input signal when the frequency of the input signal is approached gradually to the free-running frequency of the VCO. In general, the SN ratio (signal-to-noise ratio) deteriorates as the synchronization holding range and frequency pull-in range become wider.
FIG. 3
is a block diagram showing the configuration blocks of the phase synchronization circuit having the above-described fundamental structure and operation. This phase synchronization circuit is used in a synchronization detector installed in a demodulator of a digital wireless apparatus. In
FIG. 3
, the same reference numerals are assigned to those components that are shown in FIG.
1
.
In
FIG. 3
, the phase comparator
10
has a distributor
11
, two-phase demodulators
12
and
13
, analog LPFs (low pass filters)
14
and
15
, an amplifier
16
, A/D converters
17
and
18
, and a waveform reshaping demodulating circuit
19
. The loop filter
20
has a level shift
21
. The voltage control oscillator
30
has a reference carrier wave VCO
31
, a distributor
32
, ½ dividers
33
and
34
, and a &pgr;/2 shifter
35
. In addition, this phase comparator
10
has a clock reproduction circuit
40
, an alarm circuit
50
, a sweep control signal generating unit
58
, and a sweep circuit
60
. The sweep control signal generating unit
58
may be installed inside the wave-form reshaping demodulating circuit
19
.
In
FIG. 3
, the distributor
11
distributes a received modulated signal to the two-phase demodulators
12
and
13
. The carrier signal reproduced by the VCO
30
is also supplied to the two-phase demodulators
12
and
13
. Each of the two-phase demodulators
12
and
13
generates a phase difference signal in accordance with the phase difference between these two signals. These two-phase demodulators
12
and
13
have a multiplication function like, for example, a ring demodulator. Output signals of the two-phase demodulators
12
and
13
contain unwanted higher harmonics. The analog LPFs
14
and
15
remove these unwanted higher harmonics.
The amplifier
16
amplifies the phase difference signals supplied from the analog LPFs
14
and
15
that have eliminated the unwanted higher harmonics and then send the resultant phase difference signals to the A/D converters
17
and
18
, respectively. The A/D converters
17
and
18
then convert the phase difference signals into digital signals, and send the converted digital signals to the waveform reshaping demodulating circuit
19
.
The waveform reshaping demodulating circuit
19
reshapes the given phase difference signal while removing noise and distortion from the given phase difference signal to generate a reshaped phase difference signal and demodulates the original data signal based on the reshaped phase difference signal. In addition, the waveform reshaping demodulating circuit
19
generates a control signal based on the phase difference signals from the A/D converters
17
and
18
and supplies the control signal to the level shift
21
. The level shift
21
then generates a control voltage and supplies the control voltage to the reference carrier wave VCO
31
.
The reference carrier wave VCO
31
reproduces a reference carrier signal having a frequency that corresponds to the given control voltage, and sends the reference carrier signal to the two-phase demodulators
12
and
13
via the distributor
32
, ½ dividers
33
and
34
, and &pgr;/2 shifter
35
.
The clock generating circuit
40
reproduces a clock signal based on the phase difference signal supplied from the amplifier
16
. The alarm circuit
50
generates an alarm signal when the modulated signal cannot be received.
The phase synchronization circuit shown in
FIG. 3
further has a sweep control signal generating unit
58
and a sweep circuit
60
in addition to the basic circuit structure of the phase synchronization circuit shown in FIG.
1
. The control signal output from the waveform reshaping demodulating circuit
19
is supplied to the sweep control signal generating circuit
58
. The sweep control signal generating circuit
58
determines whether the modulated signal is in synchronization with the output signal of the voltage control oscillator
30
based on this control signal. If these signals are not synchronized, the sweep control signal generating circuit
58
sends a sweep start control signal to the sweep circuit
60
. The sweep control signal generating circuit
58
sends a sweep stop control signal to the sweep circuit
60
when the modulated signal becomes synchronized with the output signal of the voltage control oscillator
30
.
FIG. 4
shows an exemplary operation characteristic of the sweep circuit
60
. In this example, the sweep circuit
60
is a Wien-Bridge oscillator whose oscillation frequency is several Hz. The sweep circuit
60
oscillates or stops oscillating based on an impedance difference. When the sweep start control signal is supplied to the sweep circuit
60
, the sweep circuit
60
outputs a sweep wave, which is a sine wave as shown in FIG.
4
. This sweep wave is supplied to the reference carrier wave VCO
31
. The reference carrier wave VCO
31
changes its free-running frequency based on the level of the sweep wave. As a result, the frequency pull-in range is expanded.
FIG.
5
(
a
) shows the frequency characteristic of the phase synchronization circuit shown in
FIG. 1
that does not have the sweep circuit
60
. The centers of the frequency pull-in range and synchronization holding range of the phase synchronization circuit shown in
FIG. 1
coincide approximately with the free-running frequency of the phase synchronization circuit.
FIG.
5
(
b
) shows the range of the free-running frequency of the phase synchronization circuit having the sweep circuit
60
shown in FIG.
3
. The oscillation of the sweep circuit
Kinkead Arnold
OKI Electric Industry Co., Ltd.
Rabin & Champagne, P.C.
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