Oscillators – Ring oscillators
Reexamination Certificate
2000-02-17
2002-07-02
Kinkead, Arnold (Department: 2817)
Oscillators
Ring oscillators
C327S261000, C327S266000, C327S272000, C327S274000
Reexamination Certificate
active
06414557
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to ring oscillator circuits and, in particular, to a ring oscillator or VCO circuit that provides an improved level of noise rejection for noise originating from both the voltage supply and ground.
2. Description of the Related Art
High-speed digital circuits such as microprocessors and memories often employ a phase-locked loop (PLL) circuit to suppress timing skews between the on-chip clock and the system clock. PLL circuits typically include a voltage-controlled oscillator (VCO) circuit to generate a periodic digital signal.
FIG. 1
shows a conventional VCO circuit
100
. The VCO
100
, which is a type of ring oscillator, and consists of N stages of differential inverters
110
with the output of the Nth stage connected back to the input of the first stage. The VCO circuit
100
generates an output signal VCO_out, which is a periodic digital signal having a frequency that is adjustable by varying the voltage of the frequency control signal Vctrl.
The PLL circuit is typically fabricated on the same integrated circuit as the digital circuit and as a result, the switching noise generated by the digital circuit is coupled to the PLL, including the VCO
100
. The switching noise is coupled to the PLL circuit through various sources, including the voltage supply VDD, the ground GND and the substrate. The coupling of noise to the VCO
100
causes the output signal VCO_out to suffer from jitter (i.e., rapid variations in phase), thereby reducing the accuracy of the PLL.
A conventional approach to reducing the effect of switching noise is to construct the VCO
100
with differential, rather than single-ended, inverters
110
so that the switching noise is rejected by the inverters as common-mode noise. However, under the large-signal conditions under which most VCOs operate, the differential inverters of the VCO may function as a mixer so as to combine the common-mode noise with the differential signal. Consequently, despite its differential operation, the VCO
100
at least partially couples the switching noise to its output.
Another conventional approach is to isolate the inverters from the noise source by placing a high-impedance current source between the noise source and the inverter, as shown in
FIGS. 2 and 3
.
FIG. 2
shows a differential inverter
210
including n-type MOSFET input transistors
211
and
212
and p-type MOSFET load transistors
213
and
214
connected in a conventional differential inverter configuration. The load transistors
213
and
214
are diode-connected with the gate of the transistor connected to the drain. The differential inverter
210
also includes a current source
216
placed between ground and the input transistors
211
and
212
. The current source
216
reduces the coupling of noise from ground because of its high impedance relative to the impedance of the ground. Similarly,
FIG. 3
shows a differential inverter
310
including PMOS input transistors
311
and
312
and n-type MOSFET load transistors
313
and
314
. The differential inverter
310
also includes a current source
315
placed between the voltage supply VDD and the input transistors
311
and
312
. The current source
315
reduces the coupling of noise from the voltage supply VDD because of its high impedance relative to the impedance of VDD.
A disadvantage of the differential inverters
210
and
310
is that they are capable of rejecting noise from only one of the potential switching noise sources VDD and ground, but not both. Consequently, switching noises originating from the other supply (i.e., VDD for the differential inverter
210
and ground for the differential inverter
310
) can still affect the output of the differential inverter, thereby causing jitter in the output signal VCO_out.
In view of the shortcomings of these approaches, it is an object of the present invention to provide a VCO circuit with an improved level of noise rejection for noise originating from both the voltage supply and ground, i.e., an improved power supply rejection ratio (PSRR).
SUMMARY OF THE INVENTION
The present invention comprises a ring oscillator circuit, such as a VCO, with a relatively high level of noise rejection for noise originating from both the voltage supply and ground. The ring oscillator circuit is composed of a plurality of differential delay circuits, each differential delay circuit generating a differential output signal that is a delayed (and preferably inverted) version of a differential input signal. Each differential delay circuit includes first and second input transistors for receiving the differential input signal. Each differential delay circuit also includes first and second load transistors coupled in parallel with the respective first and second input transistors. Each differential delay circuit further includes a first current source coupled between the first input transistor and a first power supply terminal, a second current source coupled between the second input transistor and the first power supply terminal and a third current source coupled between the first and second input transistors and a second power supply terminal. In a preferred embodiment of the invention, the first power supply terminal consists of a voltage supply terminal and the second power supply terminal consists of a ground terminal. In other embodiments, however, the first and second power supply terminals may be reversed.
The ring oscillator circuit of the present invention provides an improved level of noise rejection over prior art VCO circuits because the inventive differential delay circuit rejects noise originating from both the voltage supply and ground. Specifically, the first and second current sources reduce the coupling of noise from the first power supply terminal to the output, while the third current source reduces the coupling of noise from the second power supply terminal to the output.
These and other features and advantages of the invention will be better appreciated from the following detailed description of the invention together with the appended drawings.
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Broadcom Corporation
Christie Parker & Hale LLP
Kinkead Arnold
LandOfFree
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