Oscillators – Electromechanical resonator – Crystal
Reexamination Certificate
2002-01-02
2003-12-02
Shingleton, Michael B (Department: 2817)
Oscillators
Electromechanical resonator
Crystal
C331S1160FE, C331S175000, C331S182000, C331S183000
Reexamination Certificate
active
06657507
ABSTRACT:
FIELD
This invention relates generally to electronic oscillators, and in particular, to an oscillator having an active feedback resistance circuit to set the desired operating condition of the oscillator's amplifier.
BACKGROUND
Current microprocessor systems use a crystal oscillator as part of a Real Time Clock (RTC) to keep track of the date and time of day. They typically use a relatively accurate and high frequency crystal oscillator, for example 32.768 kHz, which is divided to generate seconds, minutes and hours for the system. Because the Real Time Clock (RTC) needs to be running even when the microprocessor system is off, it is directly connected to a battery. The dependency on battery power raises power consumption issues among other accuracy, stability and manufacturing issues, as will be discussed with regard to the following example.
FIG. 1
illustrates a block diagram of a prior art processor system
100
. The processor system
100
consists of a microprocessor
108
coupled to a memory controller
106
, which is sometimes referred to in the relevant art as the “north-bridge.” The memory controller
106
interfaces with the system memory
110
. The processor system
100
further consists of an input/output (I/O) bus
102
coupled to an I/O controller
104
, which is sometimes referred to as the “south-bridge.” The “south bridge” is, in turn, coupled to the “north bridge.” Typically included in the “south-bridge” circuit board is the Real Time Clock (RTC) for the processor system
100
, which keeps track of the time and date for the system.
For discussion purposes,
FIG. 1
only shows the crystal oscillator
120
portion of the Real Time Clock (RTC) for the processor system
100
. The crystal oscillator
120
consists of an amplifier
122
including a crystal resonator
124
, an external resistor
126
, and a pair of capacitors CL
11
and CL
22
. The crystal resonator
124
and external resistor
126
are connected between the input and output of the amplifier
122
, i.e. in feedback with the amplifier. The capacitor CL
11
is coupled between the input of the amplifier
122
and ground potential. Similarly, the capacitor CL
12
is coupled between the output of the amplifier
122
and ground potential. The crystal resonator
124
resonates precisely at a particular frequency, which causes the oscillator
120
to generate a periodic signal cycling at such frequency. The external resistor
126
biases the amplifier
122
which affects its gain. The capacitors CL
11
and CL
12
serve to optimize the startup and loading conditions of the oscillator
120
.
There are several drawbacks with regard to the external resistor
126
of the prior art oscillator
120
. One set of drawbacks arises from the fact that the external resistor
126
sets the gain of the oscillator
120
. In order to satisfy the condition for oscillation, the gain of the amplifier
122
should be at least one (1). However, a gain significantly over one (1) could lead to additional noise in the output signal of the oscillator
120
, could also lead to instability of the oscillator
120
, and could unduly increase the power consumption of the oscillator
120
. Thus, the external resistor
126
should be precisely selected such that the gain of the amplifier
122
is slightly above unity gain. Because there are process variations with regard to the integrated circuit in which the amplifier
122
is formed, there can be substantial trial and error in selecting an external resistor
126
that sets the gain of the amplifier
122
slightly above unity. Such trial and error increases the costs, time and complexity of manufacturing the oscillator
120
in addition to reducing the reliability of the oscillator
120
. In addition, once the external resistor
126
is selected, it becomes impractical to change the resistor later on to account for changes in the oscillator's performance due to aging or other changes in the environment and/or application.
Another set of drawbacks stems from the fact that the external resistor
126
lies external to the integrated circuit in which the amplifier
122
is formed. Since the external resistor
126
lies external to the amplifier integrated circuit, it is typically mounted on the “south bridge” circuit board along with the integrated circuit. This increases the board routing complexity as well as the manufacturing of the “south bridge” board, which leads to increased manufacturing time and costs. Also, because the external resistor
126
is situated external to the shielded integrated circuit, it is exposed to environment noise, thereby introducing additional noise into the oscillator signal.
REFERENCES:
patent: 4704587 (1987-11-01), Ouyang
patent: 6191662 (2001-02-01), Volk
patent: 6405164 (2002-06-01), Pinai
patent: 196857 (2001-07-01), None
Millman “Microelectronics” McGraw Hill 1979 pp 237.*
Todd “FETs as voltage-variable resistors” Application Notes vol 13, No 19, Sep. 13, 1965 pp 66-69.*
Gibilisco “Handbook of Radio & Wireless Technology” McGraw Hill 1999 pp 90-93.
Fulton Robert R.
Lee Tea
Senthilkumar Chinnugounder
Blakely , Sokoloff, Taylor & Zafman LLP
Shingleton Michael B
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