PLL control method in data receiving apparatus

Details PLL control method in data receiving apparatus PLL control method in data receiving apparatus

1998-11-16

2002-06-25

Corrielus, Jean (Department: 2631)

Pulse or digital communications

Equalizers

Automatic

C708S323000

Reexamination Certificate

active

06411650

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a PLL control method of controlling a phase of a clock of a PLL (Phase Locked Loop) for taking synchronism of a receiving signal in a data receiving apparatus such as MODEM etc and, more particularly, to a PLL control method for obtaining a PLL control signal by making use of a tap coefficient of an auto equalizer.
2. Description of the Related Art
A MODEM (modulator-demodulator) is widely utilized in a data communications network using lines. In this type of MODEM, it is necessary for stable data communications to generate a PLL clock for taking the synchronism of a receiving signal.
FIG. 9
is a diagram showing a spectrum in a Nyquist transmission.
FIG. 10
is a diagram showing a spectrum in a non-Nyquist transmission.
FIG. 11
is a diagram illustrating a construction in the prior art.
FIGS. 12A-12C
are explanatory diagrams each showing the prior art.
According to the prior art PLL control method in the MODEM, a timing phase component is extracted from a receiving carrier in steady-state communications after an end of training, and a PLL control signal is obtained. This PLL control method is, in the case of a Nyquist transmission system, because of an existence of a Nyquist frequency, capable of easily extracting the timing phase from the receiving carrier.
Namely, as shown in the spectrum diagram of
FIG. 9
illustrating a frequency distribution in the Nyquist transmission, a Nyquist transmission method is a method of transmitting at a Nyquist interval signal points disposed on at the Nyquist interval (a Nyquist frequency) on the time-axis. According to the Nyquist transmission method, the signal points can be transmitted without any inter-symbol interference with other signal points.
Incidentally, some of the MODEMs recently proposed have a transmission speed as high as, e.g., 28.8 kbps. These MODEMs adopt a non-Nyquist transmission method without using the Nyquist transmission method.
As shown in the spectrum diagram of
FIG. 10
illustrating a frequency distribution in the non-Nyquist transmission, the non-Nyquist transmission method is a method of transmitting at the Nyquist interval the signal points disposed between the two Nyquist intervals on the time-axis. According to the non-Nyquist transmission method, the signal points are transmitted having the inter-symbol interference with other signal points.
In the thus structured non-Nyquist transmission method, there exists no Nyquist frequency, and hence it is difficult to extract the timing component.
Further, a phase delay distortion might occur in the signal because of being transmitted via a carrier link depending on the line through which the signal is transmitted. it is therefore difficult to extract a timing phase.
Thus, in the case of using the non-Nyquist transmission method and in the steady state, the extraction of the timing component from the receiving carrier becomes difficult depending on the state of the line. Consequently, the PLL becomes unstable, and there might be induced a deterioration of performance such as an increase in error rate of the MODEM.
There is proposed a method of obtaining a stable PLL control signal without using the above timing extracting method in the steady state (Japanese Patent Application Laid-Open Publication No.8-335962). This method is explained with reference to
FIGS. 11 and 12A
through
12
C.
FIG. 11
is a diagram showing a construction of a receiving unit of a MODEM. As shown in
FIG. 11
, an analog signal (a receiving signal) received via the line is converted by an analog/digital converter
90
into a digital signal. The digital signal outputted from the A/D converter
90
is demodulated by a demodulation unit
91
.
An output from the demodulation unit
91
is waveform-shaped by a roll-off filter (ROF)
92
. An output of the roll-off filter
92
is subjected to auto gain control in an auto gain control unit (AGC)
93
and is thereafter waveform-shaped by an auto equalization unit (EQL)
94
. Then, an output of the auto equalization is inputted to an unillustrated judging unit, whereby receiving data is regenerated.
On the other hand, on the occasion of transmitting the data, before transmitting the data, training data having a predetermined pattern is to be transmitted. A receiving apparatus demodulates this item of training data, and executes an initializing process of each unit. Herein, a timing extraction/phase rotational quantity calculating unit
95
extracts the timing component in the training signal transmitted anterior to the data signal, and calculates a phase rotational quantity of the timing component.
A selection unit
97
selects this phase rotational quantity as a PLL control signal during the training period. With this selection, during the training period, the timing PLL unit (TIM PLL)
98
is phase-controlled by the timing phase of the training signal.
Namely, the timing PLL unit
98
controls a phase of an internal clock so that the phase rotational quantity is zeroed. This phase-controlled sampling signal (the clock) is outputted to the A/D converter
90
. The phase of the sampling signal is thereby synchronized with the phase of the training signal.
During data communications (a steady state) subsequent to the training period, as described above, it is difficult to extract a timing phase component from a carrier of the receiving signal. Therefore, the PLL control signal is obtained from a tap coefficient of the auto equalization unit
94
. The auto equalization unit
94
is constructed of a transversal filter to eliminate a distortion of waveform of the receiving signal due to a line distortion etc.
The auto equalization unit
94
dynamically controls the tap coefficient so as to obtain an optimal equalization characteristic suited to a line state. During the training period, the tap coefficient is initialized based on the training data. Corresponding to the line state, the tap coefficient of a center tap located at the center of a group of taps of the auto equalization unit
94
, grows most, and the tap coefficients of the taps adjacent on both sides to the center tap grow to some extent corresponding to the line state. The optimal equalization characteristic at that point of time is thereby attained.
Herein, if the timing phase is synchronized therewith, the tap coefficient holds the same value as a value immediately after the training. Whereas if the timing phase is shifted, the tap coefficient set at the training time moves correspondingly in the right or left direction on the basis of the center tap.
FIG. 12A
shows a distribution of the tap coefficients of the respective taps when the timing phase is synchronized. In this case, the center tap value is largest, and the right and left tap coefficients are set to values corresponding to the line state.
FIG. 12B
shows a distribution of the tap coefficients of the taps when the timing phase lags. In this case, for compensating the phase lag, the tap coefficients are controlled so that the tap coefficient of the tap disposed on the right side of the center tap takes a large value, while the tap coefficient of the tap disposed on the left side of the center tap takes a small value.
FIG. 12C
shows a distribution of the tap coefficients of the taps when the timing phase advances. For compensating the phase advancement, the tap coefficients are controlled so that the tap coefficient of the tap disposed on the left side of the center tap takes a large value, while the tap coefficient of the tap disposed on the right side of the center tap takes a small value.
Accordingly, the phase advancement/lag of the receiving signal can be detected corresponding to the values of the tap coefficients. Since the tap coefficients are properly set at the training time, the PLL control is conducted so that the distribution of the tap coefficients is held during the steady state as it is at the training time. Therefore, according to the prior art, there is provided a tap right-and-left difference extraction unit
96

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