Oscillators – Electromechanical resonator – Crystal
Reexamination Certificate
2002-04-09
2004-11-16
Pascal, Robert (Department: 2817)
Oscillators
Electromechanical resonator
Crystal
C331S176000
Reexamination Certificate
active
06819194
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to tunable electronic devices and components and, more particularly, to crystal oscillators incorporating tunable ferroelectric components.
BACKGROUND OF THE INVENTION
Radio frequency bandwidth is a scarce resource that is highly valued and is becoming increasingly congested. Ever-increasing numbers of users are attempting to co-exist and to pass ever-increasing amounts of information through the finite amount of bandwidth that is available. The radio spectrum is divided into frequency bands that are allocated for specific uses. In the United States, for example, all FM radio stations transmit in the 88-108 MHz band and all AM radio stations transmit in the 535 kHz-1.7 MHz band. The frequency band around 900 MHz is reserved for wireless phone transmissions. A frequency band centered around 2.45 GHz has been set aside for the new Bluetooth technology. Hundreds of other wireless technologies have their own band of the radio spectrum set aside, from baby monitors to deep space communications.
Communications within a given frequency band occur on even more narrowly and precisely defined channels within that band. Hence, in virtually any wireless communication system or device, frequency agility is required and accurate frequency generation is of critical importance. Voltage controlled oscillators (VCOs) can generate a large range of frequencies, but are problematic in that they are unstable and tend to drift. Crystal oscillators, conversely, provide excellent frequency stability but can be used only over the narrow frequency range of the particular quartz crystal contained in the oscillator. A phase-locked loop (PLL) is a commonly used frequency synthesis technique that takes advantage of both the flexibility of a voltage controlled oscillator and the stability of a crystal oscillator.
FIG. 1
depicts a typical frequency synthesis circuit including a PLL
100
and reference crystal oscillator
200
. PLL
100
operates in a well-known fashion. Briefly, VCO
108
generates a range of output frequencies via variation of a DC control voltage input to the VCO. The output of VCO
108
is fed back to phase comparator
102
, which also receives an input from crystal oscillator
200
. The VCO and crystal oscillator inputs to comparator
102
are combined, and any difference in phase/frequency results in a DC voltage output. The output of comparator
102
is coupled back to the input of VCO
108
via charge pump
104
and loop filter
106
. The greater the frequency/phase difference that exists between the two signals, the larger the output voltage from comparator
102
, and the larger the adjustment that is made to the VCO output frequency. Hence, the output of VCO
108
is driven to, and eventually locks onto, the frequency of crystal oscillator
200
. Divider
110
is positioned in the feedback path between VCO
108
and phase comparator
102
. It is typically a programmable frequency divider that divides the VCO frequency down to the reference frequency produced by crystal oscillator
200
. Hence, a large range of output frequencies can be generated and frequency lock still maintained by manipulation of the divide-by number.
Crystal oscillator
200
uses a quartz crystal to provide a fixed and stable reference frequency. Since temperature affects the rate at which a crystal vibrates, crystal oscillators often include a temperature compensation network that senses the ambient temperature and “pulls” the crystal frequency to prevent frequency drift over a temperature range. A temperature compensated crystal oscillator is referred to as a TCXO. Crystal oscillators, like voltage controlled oscillators, can also be made tunable over the frequency range (albeit much smaller) of the crystal by application of a control voltage. A voltage controlled crystal oscillator is referred to as a VCXO. A crystal oscillator that combines the attributes of voltage control and temperature compensation is referred to as a VC-TCXO.
In recent years, VC-TCXO designs have been required to comply with significantly more demanding specifications. VC-TCXOs are typically required to remain frequency stable to within ±2.5 parts per million (ppm) over a temperature range of −30° C. to +85° C. Moreover, their initial accuracy or tolerance at ambient temperature must be within ±1.5 ppm.
Modern communication devices also impose very high spectral purity and phase noise demands on the VC-TCXO. Phase noise affects the receiver's ability to reject unwanted signals on nearby channels. It is the ratio of the output power divided by the noise power at a specified offset and is expressed in dBc/Hz. The reference frequency produced by a VC-TCXO may be used to phase lock an output frequency that is one-hundred times or more greater. Since phase noise performance is degraded by a factor of 20 log N, where N is the frequency multiplier or divide-by number, close in phase noise performance (i.e. at offsets of less than 100 Hz) exceeding −90 dBc is required.
Close in phase noise performance is especially important in FM systems and in position location systems. In FM systems, close in phase noise directly degrades the audio quality and manifests itself as an audio hiss. In position location systems, such as those used to receive GPS signals, close in phase noise may manifest itself as positional inaccuracy or receiver sensitivity impairment. Receiver sensitivity impairment is due to the short term frequency or phase jitter of the reference oscillator caused by the phase noise, and the subsequent inability of the receiver to accurately correlate for extended periods of time.
To meet these stringent accuracy and close in phase noise requirements, the VC-TCXO topology and components must be chosen with caution. The use of a dedicated integrated circuit (IC), a frequency control element and factory calibration are required. Each of these requirements adds considerable time and cost to production. Currently, only a handful of manufacturers world wide can economically produce VC-TCXOs meeting these requirements that are suitable for use in high volume consumer communication devices.
The Q of the oscillator quartz crystal alone can be as high as 50,000, meaning that additional losses of any significance at all in the resonator network will dramatically reduce the overall Q and seriously degrade close in phase noise performance. One element that is particularly difficult to implement and control without degrading close in phase noise performance is the voltage controlled, frequency-tuning element associated with the crystal. In a VC-TCXO, this element usually takes the form of a voltage dependent capacitor, commonly referred to as a variable capacitance diode, varicap diode or varactor.
The operation of a varactor is well understood. When a reverse voltage is applied to a varactor, the insulation layer between the p-doped and n-doped semiconductor regions thickens. A depletion region that is essentially devoid of carriers forms and behaves as the dielectric of the capacitor. The depletion region increases as the reverse voltage across it increases, and since capacitance varies inversely as dielectric thickness, the junction capacitance decreases as the reverse voltage increases. Capacitance variations effected by control voltage variations effect corresponding variations in the resonant frequency of the oscillator.
Unfortunately, a typical varactor has a relatively low Q due to its intrinsic series resistance, which may be as much as several ohms. The Q of a varactor can be expressed as X
c
/R
s
, where X
c
is the reactance of the varactor (1/[2·&pgr;·f·c]) and R
s
is the effective series resistance of the varactor. A capacitance of 5 pF at a frequency of 1.5 GHz results in a reactance X
c
of 21.22 &OHgr;. If the effective series resistance R
s
of the varactor is 0.5 &OHgr;, the resultant Q is 42.44. When compared to the extremely high Q of the crystal itself, it can be seen how much this will degrade the overall Q of the oscilla
Forrester Tim
Toncich Stanley S.
Chang Joseph
Kyocera Wireless Corp.
Pascal Robert
LandOfFree
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