Oscillators – L-c type oscillators
Reexamination Certificate
1999-12-15
2002-02-19
Grimm, Siegfried H. (Department: 2817)
Oscillators
L-c type oscillators
C331S03600C, C331S17700V
Reexamination Certificate
active
06348841
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to electronic circuits. More particularly, the present invention relates to a novel and improved wideband Voltage Controlled Oscillator (VCO) having good noise immunity characteristics.
II. Description of the Related Art
Wireless communication systems rely on the predictable performance of over the air Radio Frequency (RF) links. Wireless phone systems are required to simultaneously monitor and control numerous RF links.
A mobile unit or wireless phone integrates numerous complex circuits. An RF transceiver is used to provide the wireless communication link with base stations. The RF transceiver is comprised of a receiver and a transmitter. The receiver receives the RF transmission from the base station via an antenna interfaced to the mobile unit. The receiver amplifies, filters, and downconverts the received signal to baseband signal. The baseband signal is then routed to a baseband processing circuit. The baseband processing circuit demodulates the signal and conditions it for broadcast through a speaker to the user.
Frequency synthesizers are used to generate the local oscillator signals required to perform the downconversion in the receiver and the upconversion in the transmitter. Frequency synthesis is used to generate the local oscillator signal because of the synthesizer's frequency stability, the spectral purity of the resultant signal, and the ability for digital control.
Frequency synthesizers are classified as direct or indirect. In Direct Digital Synthesis logic circuits generate a digital representation of the desired signal and a D/A converter is used to convert the digital representation into an analog waveform. One common way of implementing DDS is to store a table of waveform phases in memory. Then the rate at which the phases are clocked out of memory is directly proportional to the frequency of the output signal. While DDS can generate an extremely accurate representation of a sine wave, the output frequency is limited by the clocking rate.
Indirect synthesis utilizes a phase lock loop locked to the output of an oscillator. Indirect frequency synthesis is more popular for high frequency designs because the output of a high frequency oscillator can be divided down to a frequency within the operating range of the phase lock loop.
FIG. 1
shows a block diagram of an indirect frequency synthesizer utilizing a phase lock loop. A VCO
110
capable of tuning over the desired frequency range is used to provide the LO output
112
. The output of the VCO
110
is also sent to the input of a frequency divider circuit
120
, denoted ÷N where N represents the divider ratio. The divided output is provided as a first input to a phase detector
130
. A second input to the phase detector
130
is the output of a reference oscillator
140
. The phase lock loop operates to tune the output of the VCO
110
such that the output of the frequency divider
120
is identical to the output of the reference oscillator
140
. The phase detector
130
provides an output signal corresponding to a phase error between the two input signals. The phase detector
130
output is conditioned through a Low Pass Filter (LPF) before it is provided to the frequency control input of the VCO
110
. Thus, the VCO
110
is controlled to maintain phase lock with the reference oscillator
140
. It can be readily deduced from the block diagram that incrementing or decrementing the value of the divider ratio N results in a frequency change in the LO output
112
equal to the reference oscillator
140
frequency. The frequency of the reference oscillator
140
determines the frequency step size of the LO.
Frequency variations in the VCO
110
output can only be corrected by the phase lock loop if the rate of the frequency variations is less than the loop bandwidth. The phase lock loop is unable to correct for VCO frequency variations that occur at a rate higher than the loop bandwidth. The settling time of the phase lock loop will depend on the initial frequency offset and the loop bandwidth. A wider loop bandwidth results in a faster settling time. A VCO with good noise immunity will reduce frequency variations thereby reducing the settling time of the phase lock loop. Therefore, it is important to design a VCO with good noise immunity while maintaining the frequency tuning characteristics.
A VCO is merely a tunable oscillator. A typical oscillator circuit is comprised of an amplifier and a resonant circuit, commonly referred as a resonant tank. The resulting oscillator has a frequency output where the gain is greater than unity and the phase is equal to zero. The resonant circuit sets this frequency of oscillation. The relationship is most easily seen on a Bode diagram.
FIG. 2A
illustrates a Bode diagram for a typical oscillator. Curve
210
is representative of the gain in decibels of the oscillator as referenced to the left vertical axis and Curve
220
is representative of the phase in degrees as referenced to the right vertical axis. As indicated by Point
230
, the oscillation occurs when the oscillator gain is approximately 14 dB and the phase is zero producing an oscillation at approximately 124 MHz.
To create a VCO the resonant circuit is comprised of at least one variable component wherein the reactance of the variable component is a function of a control signal, typically a voltage level, so that the frequency of zero phase, and consequently the frequency of oscillation, is also variable. When the VCO is required to tune over a large frequency range the variable component must be capable of tuning the resonant circuit over the large frequency range. Possible circuit implementations for a variable resonant circuit capable of covering a large frequency range include a resonant circuit incorporating a highly sensitive variable component or a resonant circuit requiring an extended control voltage range. The first alternative presents some problems because the VCO gain, measured in terms of MHz/Volt, becomes very high. This results in large frequency changes for relatively small control voltage changes and makes the VCO more susceptible to noise induced on the tuning line. The second alternative also has disadvantages since the required control voltage range is very large. Large control voltages can present a problem in mobile battery powered electronics having limited available supply voltage ranges.
It is not sufficient that the resonant circuit tune the oscillator to the desired operating frequency. The Q of the resonant circuit is important in maintaining a specific output frequency at a given control voltage level.
FIG. 2B
depicts the phase response of two resonant circuits having different Q values. A lower circuit Q generates a more gentle phase response, whereas a higher circuit Q generates a sharper phase response. A higher circuit Q is desired to minimize the effects of small phase variations on output frequency. The phase response of a circuit having a relatively low circuit Q is shown in curve
240
. Curve
250
illustrates a circuit having a higher circuit Q. It can be seen for a given phase variation the change in frequency is more pronounced in the circuit having the lower circuit Q. The magnitude of f
2
, the frequency change in a low Q circuit for a given phase variation, is greater than the magnitude of f
1
, the frequency change in a high Q circuit for the same phase variation.
Frequency synthesizer IC's are available that integrate nearly all of the required synthesizer circuits onto one chip. Typically, the user of one of these IC's only needs to provide a resonant circuit, loop filter, and reference oscillator in addition to the IC in order to produce a synthesized LO. The remaining elements of the synthesizer, the amplifier portion of the VCO, the frequency divider, and the phase detector, are integrated onto one IC. The user provides the resonant circuit required generating the desired output frequency. The user also provides the low pass filter design generating the
Brown Charles D.
Grimm Siegfried H.
Hom Raymond
Qualcomm Incorporated
Wadsworth Philip R.
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