Oscillators – Automatic frequency stabilization using a phase or frequency... – Plural a.f.s. for a single oscillator
Reexamination Certificate
1998-11-23
2001-10-23
Pascal, Robert (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Plural a.f.s. for a single oscillator
C331S016000, C331S017000, C331S17700V
Reexamination Certificate
active
06307439
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of Invention
In general, this invention relates to a communication device utilizing voltage controlled oscillators and more specifically to a voltage controlled oscillator circuit incorporating an adaptive closed loop coarse tuning mechanism.
Description of the Related Art
Voltage controlled oscillators (VCO) are well known in the field of communication devices. In general, the operating frequency of a VCO is controlled by the application of a voltage to a tuning varactor incorporated into the VCO design. Some communication devices implement a single varicap diode to apply corresponding reference voltages over an entire band of desired frequencies. The problem encountered when using a single varicap diode is that the VCO becomes sensitive to noise and experiences reference frequency leakage in the case of high VCO gain. In some devices, this problem is addressed by varying the closed loop bandwidth of a single loop filter using an adapt switch to change the loop filter components. Such single loop adaptive filtering requires multiple switches and filters to cover a desired frequency range.
As an alternative solution, the problem of noise and frequency leakage is overcome by dividing the frequency band into sub-bands using either diode switches or dual tuning diodes from two varicap diodes, one for coarse tuning and the other for fine tuning. When using dual tuning diodes, the coarse tune varaetor is normally controlled open loop using a D/A converter. A problem with this approach, however, is that the voltage produced by the D/A converter for each individual communication device must be calibrated over the entire band, substantially increasing the overall system cost per unit. As an alternative, the VCO is sometimes installed with microstrip line etching containing precalibrated frequencies for the entire band. The microstrips are driven by an adapt switch connected directly to the VCO. However, initial calibration of the microstrip etching requires laser trimming which equates to a large up front cost, thereby also substantially increasing the cost per unit. Because single varicap adaptive filtering does not effectively reduce noise without multiple components and because a D/A converter and microstrip line etching increase overall system costs, a need exists for a less expensive, single unit coarse tuning mechanism.
SUMMARY OF THE INVENTION
Based on the above noted deficiencies in the related art, it is an object of the present invention to eliminate the need for individual calibration per unit of the coarse tune voltage across a band of desired frequencies. It is the further object to reduce VCO gain while eliminating the need for a D/A converter and without sacrificing resolution. Still another object is to allow for adaptive VCO sensitivity without the need and implementation of switching diodes. Finally, it is the object to offer a noise reduction advantage over single loop adaptive filtering used in VCOs.
These and other objects of the present invention are achieved by the incorporation of an adaptive closed loop coarse tuning mechanism into a voltage controlled oscillator (VCO) circuit. Particularly, a reference oscillator is set with the desired frequency for the VCO. The resulting wave form is sent to a synthesizer, which is connected in series with the reference oscillator. The synthesizer then compares the reference frequency to the frequency at which the VCO is currently set and a charge pump within the synthesizer converts any phase difference between the two frequencies into a corresponding current that is proportional to the phase difference. If there is no phase difference, the charge pump will not emit a current, as the VCO is presently at the desired frequency. Therefore, the greater the phase difference, the higher the current the charge pump will produce.
Connected in series with the synthesizer is a loop filter which receives the incoming current from the synthesizer and produces a corresponding voltage. This voltage is applied directly to the fine tune port of the VCO. In addition, the adaptive closed loop coarse tuning mechanism is connected in parallel with the fine tune port and develops the same voltage, which is the reference voltage for the coarse tune.
The adaptive closed loop coarse tuning mechanism consists of an configured in a noninverting feedback loop connected to a parallel resistor/adapt switch. The switch is followed in series by a shunt capacitor filter which is connected directly to the coarse tune input of the VCO. When the adapt switch is closed, the voltage produced by the coarse amp flows through the filter and to the coarse tune port of the VCO. The voltage resulting from the adaptive closed loop coarse tune overrides any voltage being received in the fine tune port making the coarse tune loop controlling in frequency acquisition. This process is known as the wide band (fast lock) mode. After a preset time has expired, the adapt switch is opened thereby effectively eliminating the coarse tune circuit from the loop because of a large time constant associated with the parallel adapt switch/resistor circuit. At that point, the fine tune port voltage will make any minor adjustments in driving the VCO to the desired frequency. This process is known as the narrow band (low noise) mode.
These as well as other novel advantages, details, embodiments, features, and objects of the present invention will be apparent skilled in the art from the following detailed description of the invention, the attached claims and accompanying drawings, listed herein below, which are useful in explaining the invention.
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James A Crawford, Frequency Synthesizer Design Handbook, 1994, p. 249.
Garmin Corporation
Glenn Kimberly E
Pascal Robert
Rolf Devon A.
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