Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
1999-02-10
2001-05-08
Kim, Jung Ho (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S113000, C327S157000
Reexamination Certificate
active
06229361
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a system and method for speeding up frequency switching time and particularly frequency switching time of a voltage controlled oscillator in a wireless communication system.
BACKGROUND AND BRIEF DESCRIPTION OF THE PRIOR ART
Charge pumps are generally a part of a phase lock loop system and are used in transmitter/receivers of wireless communication systems in conjunction with a voltage controlled oscillator to speed up the change from an existing frequency of operation to a different frequency of operation.
A typical phase lock loop system of the type used in wireless communication is shown in FIG.
1
and includes a reference frequency oscillator
1
which is a very accurate frequency source. The frequency emitted by the oscillator
1
is divided down by a predetermined factor N in a divider circuit
3
and fed to one input of a comparator
5
. A voltage controlled oscillator (VCO)
7
having a known tuning or frequency output range, the output frequency of which is controlled by a control voltage applied thereto, provides an output frequency which is divided down by a predetermined factor M in a divider
9
and fed to the other input of comparator
5
. The comparator
5
provides an output error signal indicative of the difference and direction of the difference in frequency output between the reference oscillator
1
and the VCO
7
. The output frequency of the VCO
7
is the frequency at which the associated transmitter/receiver operates as is well known. The error signal is applied to a charge pump
11
which provides a charge onto a low pass or loop filter
13
composed of a capacitor
27
to ground with a series connected resistor
29
and capacitor
25
in parallel therewith. The filter
13
is coupled to the VCO
7
to control the output frequency of the VCO and drive the output of the VCO to the desired and correct frequency.
In operation, it is often necessary to change the frequency of operation of the VCO
7
. The amount of frequency adjustment within the frequency output range of the VCO can vary from very small up to the entire limit of the output range. The greater the amount of frequency adjustment required, the greater is the amount of current (translated as voltage) required to make the frequency adjustment. The speed of this change of frequency is generally controlled by the use of a charge pump which is generally composed of a pair of current sources
15
,
17
. The error signal will be small from the comparator
5
to the charge pump
11
when the frequency of operation remains unchanged or the charge pump will supply small changes in current for small frequency adjustments via the current sources
15
,
17
. However, for a rapid change in operating frequency, a large amount of current must be sinked or sourced into the loop which results from the large error signal supplied by the comparator
5
. In the prior art, this has been accomplished by applying an even larger current from a separate current source
19
in addition to the current from the current sources
15
and
17
with a switch (not shown) being enabled to create a path for a predetermined limited time under control of a timer
23
to the capacitor
25
of the filter
13
in the loop to provide additional large current for a short time period to provide the rapid change in operating frequency of the VCO
7
. This speed up circuit effectively opens the bandwidth of the low pass or loop filter
13
to improve speed-up time when the synthesizer switches from one frequency to another. The limitation is that when the switch (not shown) is enabled in the integral leg of the loop filter, the charge pump blindly delivers a charge to the capacitor with little control over the amount of current being delivered. This can and usually does result in significant undershoot or overshoot of the new target VCO frequency. Also, the charge pump delivers current only when there is a phase difference in the phase frequency detector. Although the lock times are better than without speed-up mode, the switching requirements in the Global System of Mobile Communications (GSM), the most used digital wireless standard in the world, and other wireless systems are such as to preclude prior art approaches to speed-up mode circuits for conventional integer N synthesizers or fractional N synthesizers.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described problems inherent in the prior art are minimized if not overcome by providing an improved charge pump whereby the amount of current delivered by the charge pump can be more readily controlled.
Briefly, the charge pump in accordance with the present invention takes advantage of the fact that, during the speed-up mode, there are normally several different current ratios (speed-up current(current required during a frequency transition of the VCO)
ormal mode current (normal operating current)) that can be chosen, this depending upon the degree of frequency change of the VCO within its operating frequency range. During speed-up mode, rather than closing a switch where a single charge pump circuit is enabled, the charge pump circuit is mirrored to a secondary charge pump circuit which has a voltage setting resistor that provides a voltage (determined by the selected speed-up current level) on the proportional leg of the loop filter. During speed-up, the mirrored charge is enabled to source current continuously by a programmable timing counter. Since this current being delivered to the capacitor of the filter (effectively a voltage) is continuous and not dependent upon the phase difference coming out of the phase frequency detector, a voltage level at the capacitor can be set very quickly. Thus, this system provides a procedure for moving the tune voltage very close to the final voltage value very rapidly for the programmed timed period with the regular charge pump circuits then bringing the tune voltage to the final correct level.
The above is accomplished by providing an auxiliary charge pump circuit with a voltage setting resistor, a timing counter and a high speed, low capacitance switch to connect the auxiliary charge pump to the proportional capacitor for a predetermined time period to provide the initial rough change in frequency of the VCO.
The benefits of the above described improvement provide a charge pump circuit that charges the integral capacitor to a voltage very close to the final voltage in a very short time period. After this initial charging, the regular charge pump and phase detectors then bring the loop to the final required voltage. This is especially valuable when the VCO must be tuned from the low part of the its frequency band range to the upper part of that range or vice-versa. This lowers the overall lock time for a traditional integer-N or fractional-N synthesizer. Thus, the faster lock times are obtained.
REFERENCES:
patent: 5646562 (1997-07-01), Abe
patent: 5864249 (1999-01-01), Shoji
patent: 5942949 (1999-08-01), Wilson et al.
Brady III Wade James
Kim Jung Ho
Neerings Ronald O.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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