Generator for the production of clock signals

Details Generator for the production of clock signals Generator for the production of clock signals

2001-10-30

2004-03-09

Nguyen, Minh (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Synchronizing

C327S160000, C375S376000, C331SDIG002

Reexamination Certificate

active

06703880

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to phase-locked loop type clock-signal generators that produce a high-frequency clock signal from a low-frequency clock signal. Among these generators, the invention relates more specifically to those using an oscillator comprising series connected inverters.
BACKGROUND OF THE INVENTION
A prior art generator
10
of this kind as shown in
FIG. 1
includes a frequency divider
12
, a phase comparator
14
and an oscillator
16
that are series connected. An output OUT of the oscillator
16
is connected to an input of the frequency divider
12
. The loop
10
gives a high-frequency clock signal CKHF (f=FHF) as a function of a reference low-frequency signal CKBF (f=FBF).
The frequency divider
12
receives the clock signal CKHF and gives a signal CKHF_N that is an image of the signal CKHF, and has a frequency equal to f=FHF/N. N is an integer whose value is chosen as a function of the desired frequency FHF
0
for the clock signal CKHF, and of the frequency FBF of the reference signal CKBF used: N=FHF
0
/FBF.
The phase comparator
14
has a positive input and a negative input. The signals CKHF_N and CKBF are respectively applied to these inputs. When the signals CKHF_N and CKBF are equal to 1, the phase comparator
14
determines the phase difference between the signal CKHF_N and CKBF by comparing the relative position of the trailing edges of the clock signal CKHF_N and CKBF. The comparator then produces two logic control signals UP, DOWN as a function of the result of the comparison.
The signals UP, DOWN have the following characteristics. If a trailing edge of CKBF is detected first (instants T
1
and T
3
in
FIGS. 2
a
to
2
d
), and the signals CKHF_N and CKBF are previously at a 1, CKBF has a phase lead over CKHF_N. The comparator
14
then gives an active signal UP which, for example, takes the logic value 1. UP is then deactivated on the next trailing edge of the CKHF_N (instants T
2
and T
4
in
FIGS. 2
a
to
2
d
).
If a trailing edge of CKHF_N is detected first (instants T
5
and T
7
in
FIGS. 2
a
to
2
d
), and the signals CKHF_N and CKBF are previously at a 1, CKBF has a phase delay with respect to CKHF_N. The comparator
14
then gives an active signal DOWN which, for example, takes the logic value 1. DOWN is then deactivated on the next trailing edge of CKBF (instants T
6
and T
8
in
FIGS. 2
a
to
2
d
). Otherwise, the signals UP, DOWN remained constant, active or inactive as the case may be.
The oscillator
16
receives the command signals UP, DOWN and, at its output OUT, it gives the clock signal CKHF. Two types of known oscillators used to form a clock signal generator of the kind are shown in
FIG. 1. A
first type of oscillator, known as an analog oscillator, comprises a voltage generator and a chain of inverters. The voltage generator produces a controlled voltage VC which is a rising voltage if OUT is active, a falling voltage if DOWN is active, and if not, a constant voltage.
The variation &Dgr;VC of the controlled voltage VC is proportional to the duration of the signals UP, DOWN. In the case of an analog oscillator, the chain of inverters comprises a fixed odd number of series connected identical inverters. A data output of the first inverter is connected to a data input of the last inverter. The chain of inverters produces the clock signal CKHF whose frequency FHF is proportional to the number of inverters in the chain, and to the switching time in the inverters. The switching time of the inverters is itself proportional to the variation &Dgr;VC of the control voltage given by the voltage generator.
The frequency FHF of the clock signal CKHF obtained thus follows the variations of the controlled voltage and, therefore, those of the control signals UP, DOWN: FHF increases if the signal UP is active, FHF decreases if the signal DOWN is active, FHF is constant if the signals UP, DOWN are inactive.
A second type of oscillator, known as the digital oscillator, comprises an up/down counter and a chain of inverters. The up/down counter produces a control number NR whose value varies as a function of the signals UP, DOWN: NR decreases if the signal UP is active, NR increases if the signal DOWN is active, and NR is constant if the signals UP, DOWN are inactive. The variations &Dgr;NR of the control number NR are proportional to the duration of the pulses UP, DOWN.
In the case of the digital oscillator, the inverters of the chain of inverters are all identical and, in particular, they have identical propagation times &Dgr;td. However, the total number ND of inverters in the chain is variable as a function of the control number NR given by the counter. The variations &Dgr;AND and ND are proportional to the variations of the control number NR.
Since the frequency FHF of the signal CKHF obtained is universally proportional to the number of inverters present in the chain, it varies as a function of the number given by the up/down counter, and therefore, as a function of the signals UP, DOWN as follows: FHF increases if the signal UP is active, FHF decreases if the signal DOWN is active, and FHF is constant if the signals UP, DOWN are inactive.
Thus, regardless of the oscillator chosen, whether analog or digital, the variation &Dgr;FHF of the frequency FHF generated by a pulse UP, DOWN is proportional to the duration &Dgr;UP, &Dgr;DOWN of the signal UP, DOWN applied: &Dgr;FHF=K*&Dgr;UP or &Dgr;AF=K*&Dgr;DOWN.
The general functioning of the clock signal generator
10
is as follows. If a trailing edge of CKBF is detected first (instants Ti and T
3
in
FIGS. 2
a
to
2
d
), the signals CKHF_N, with CKBF being previously at a 1, CKBF has a phase lead over CKHF_N. It is estimated in this case that the frequency of the CKHF_N is lower than that of CKBF. That is, the frequency of CKHF is lower than the desired value FHF
0
=N*FBF. The comparator
14
then gives an active signal UP and the frequency FHF of the clock signal CKHF rises. UP is then deactivated on the next trailing edge of CKHF_N (instants Ti and T
3
in
FIGS. 2
a
to
2
d
). The duration of the signal UP applied is thus proportional to the phase difference between CKHF_N and CKBF.
Conversely, if a trailing edge of CKHF_N is detected first (instants T
5
and T
7
in
FIGS. 2
a
to
2
d
), with the signals CKHF_N and CKBF being previously at a 1, CKBF has a phase delay with respect to CKHF_N. In this case, it is estimated that the frequency of the CKHF_N is higher than that of CKBF. That is, the frequency of CKHF is higher than the desired value FHF
0
=N*FBF. The comparator
14
then gives an active signal DOWN and the frequency of the clock signal CKHF decreases. DOWN is then deactivated on the next trailing edge of CKBF (instants T
6
and T
8
in
FIGS. 2
a
to
2
d
). The duration of the signal DOWN applied is thus proportional to the phase difference between the signals CKHF_N and CKBF.
When the generator
10
is powered on, the frequency FHF of the signal CKHF is very low. For example, it is equal to the frequency FBF of the reference signal CKBF. The frequency FHF will then vary as a function of the pulses UP, DOWN produced by the phase comparator. It will increase on an average because the pulses UP are more numerous and their duration is greater than that of the pulses DOWN. The frequency FHF will finally converge towards its borderline value FHF
0
. The variations &Dgr;FHF of the frequency FHF are a function of the duration &Dgr;UP, &Dgr;DOWN, of the pulses UP and DOWN, which is itself proportional to the phase difference between the signals CKHF_N and CKBF. It may be recalled that the frequency of CKFH_N is equal to FHF/N.
The duration of the pulses UP, DOWN is random with respect to the difference in frequency &dgr; between the real frequency FHF of the signal CKHF and the desired frequency FHF
0
(&dgr;F=FHF=−FHF
0
). This sometimes leads to an excessively large increase or an excessively large decrease in the frequency FHF. That is, major oscillations of the frequency FHF,

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