Abstract:
A bridge-stabilized oscillator with feedback control includes an RF amplifier connected to a first bridge path and a second bridge path. Each first and second bridge path has a variable gain amplifier to receive and modify the respective signals to maintain the resistance of a resistor in the first bridge path, so the resonator in the second bridge path oscillates. A power detector provides a control signal to each of the variable gain amplifiers to maintain the phase of the output with respect to the input and constrain the gain in each of the first and second bridge paths.

Description:
BACKGROUND 
     This invention relates to oscillator circuits and, more particularly, to a circuit that incorporates a bridge-stabilized oscillator with feedback control for generating an oscillator signal having a relatively low rate of energy loss. 
     High precision oscillators are required in many electronic devices as a master clock or frequency source, from which all other time intervals and operational frequencies are derived. Quartz crystal resonators are often used in such oscillators because the resonant frequency of the crystal is very stable. However, quartz crystal resonators are expensive and physically large with a large footprint. 
     SUMMARY OF THE INVENTION 
     A bridge-stabilized oscillator with feedback control is disclosed. The oscillator includes an RF amplifier to convert a single input signal to a first bridge input signal and a second bridge input signal and provide those signals across a first bridge path and a second bridge path. Each bridge path has a variable gain amplifier to receive and modify the first bridge input signal and the second bridge input signal. A resistor is connected in series with the variable gain amplifier in the first bridge path and a resonator is connected in series with the variable gain amplifier in the second bridge path. A power detector provides a control signal to each of the variable gain amplifiers to maintain the phase of the output with respect to the input and constrain the gain in each of the paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present invention will be apparent by reference to the following detailed description of the illustrated embodiments when taken in conjunction with the following list of drawings, where like reference numerals refer to like elements: 
         FIG. 1  is a schematic of a bridge-stabilized oscillator with feedback control. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a bridge-stabilized oscillator with feedback control circuit  100 . Circuit  100  includes a bridge-stabilized oscillator  102 . Bridge-stabilized oscillator  102  includes an RF amplifier  104  with a relatively high input and output impedance that converts an unbalanced single-ended single input signal to a balanced differential output signal. The output signal is provided across two parallel paths  106 ,  108  that correspond with a first bridge input signal to first bridge path  106  and a second bridge input signal to second bridge path  108 . First bridge path  106  includes a resistor (resistive load)  110  and second bridge path  108  includes a resonator  112 , which provides an oscillating output signal. 
     Any type of resonator  112  can be used. In fact, circuit  100  can compensate for lower quality-factor components in resonator  112 . Resonator  112 , for example, can be a metal-insulator-metal (or other type) capacitor  107  in parallel or series with a wire bond (or other type) inductor  109 . The shape of wire bond inductor  109  can be tuned to correspond with the capacitance of capacitor  107  to inexpensively obtain the desired resonance frequency of resonator  112 . 
     Bridged-stabilized oscillator  102  maintains constant phase at output node  114  with respect to the input at input node  113  to RF amplifier  104  by summing the output of first bridge path  106  and second bridge path  108 . The phase at output node  114  is held constant by operating first bridge path  106  180 degrees out of phase with respect to second bridge path  108  and summing the output of first bridge path  106  and the output of second bridge path  108 . Bridged-stabilized oscillator  102  also maintains a slightly unbalanced gain difference between first bridge path  106  and second bridge path  108 , so that bridged-stabilized oscillator  102  can oscillate at the desired resonance frequency. 
     A variable gain amplifier (VGA)  116   a ,  116   b  in each first bridge path  106  and second bridge path  108 , respectively, provides feedback control for each first bridge path  106  and second bridge path  108 . By controlling the gain in each first bridge path  106  and second bridge path  108 , circuit  100  compensates for the changing resistance in resistor  110 . A power detector  118  detects the amplitude of the output signal at output node  114  and provides a first control signal to VGA  116   a  in first bridge path  106  and a second control signal to VGA  116   b  in second bridge path  108 . The amplitude of an RF signal is the difference between its maximum and its minimum value during one cycle, and is measured in volts. The amplitude is directly related to the strength, or power, of the RF signal. In that regard, by determining the amplitude of the RF signal at output node  114 , power detector  118  quantifies the signal strength and can determine the relative gain between input node  113  and output node  114 . 
     Power detector  118  in cooperation with VGAs  116   a  and  116   b  maintains the gain with respect to paths  106  and  108  almost identical, predictably in the 0.01 to 0.2 dB range or any range therebetween, although some circuits may tolerate a higher dB gain. This slightly unbalanced gain difference between first bridge path  106  and second bridge path  108  at output node  114  permits precise control of the resonance frequency by maintaining the phase of the output signal with respect to the input signal. 
     VGAs  116   a  and  116   b  maintain a constant phase relationship across all gain stages. A slight variation in phase of the signal in first bridge path  106  with respect to second bridge path  108  will produce an output signal at output node  114  that is out of phase with respect to the input signal to RF amplifier  104 . So for every change in gain provided by VGAs  116   a  and  116   b , it is desirable that the phase remain substantially unchanged, otherwise the desired resonance frequency cannot be maintained. According to one embodiment, VGAs  116  may be a high-dynamic range amplitude controller, such as the one disclosed in U.S. non-provisional patent application entitled, “HIGH-DYNAMIC RANGE PRECISION VARIABLE AMPLITUDE CONTROLLER,” Ser. No. 13/737,777, filed on Jan. 9, 2013 and commonly owned with the present application, the entirety of which is hereby incorporated by reference. 
     A digital signal can be provided to an on-chip integrated divider  138  that allows the user to decrease the output frequency. For example, where an operator may desire a 10 MHz oscillator, which would take up substantial die space, circuit  100  could be designed to provide a 100 MHz oscillation and divider  138  can divide the oscillation by 10 to produce the 10 MHz signal. 
     A variable phase shifter (VPS)  144  combined in a feedback loop between power detector  118  and RF amplifier  104  allows highly linear phase control of circuit  100 . In that regard, the resonance frequency of circuit  100  can be precisely tuned with a tuning signal from an external source without impacting the gain of circuit  100 . According to one embodiment, VPS  144  may be an ultra-precision phase shifter, such as the one disclosed in U.S. non-provisional patent application entitled, “ULTRA-PRECISION LINEAR PHASE SHIFTER WITH GAIN CONTROL,” Ser. No. 13/714,209, filed on Dec. 13, 2012, and commonly owned with the present application, the entirety of which is hereby incorporated by reference. 
     The present disclosure is implemented in the context of a BJT-based integrated circuit on a single piece of semiconductor containing the entire oscillator  100  on a single integrated circuit. Any combination of FET-based and BJT-based components, however, may be selectively arranged to produce the desired design requirements. These components can be manufactured using various fabrication technologies, including on a semiconductor substrate such as silicon (SI) substrate, silicon-germanium (Si—Ge) substrate, gallium-arsenide (GaAs) substrate, or gallium-nitride (GaN) on silicon substrate, and various transistor types, including bipolar terminal transistor (BJT), metallic oxide semiconductor (MOS), complementary metallic oxide semiconductor (CMOS), a bipolar CMOS (Bi-CMOS), heterojunction bipolar transistor (HBT), metal semiconductor field effect transistor (MES-FET) and high electron mobility transistor (HEMT) design technologies. 
     Bridge-stabilized oscillator with feedback control circuit  100  is intended for operation in the 100 MHz to 10 GHz range, or any range therebetween, but is a useful circuit at any in any frequency range, including in the terahertz range. Bridge-stabilized oscillator with feedback control circuit  100  is less expensive to construct and has a significantly smaller footprint than other high performance oscillators that operate in such ranges. Oscillator  100  can use a standard resistor  110  for the resistive load  110  in path  106 , and any type of resonator  112  in path  108 . Rather than trying to control imprecision or variability in resistor  110  by directly controlling the resistance of resistor  110 , such imprecision or variability is compensated for by controlling the gain of each VGA  116   a ,  116   b  in first bridge path  106  and second bridge path  108 , respectively, to maintain first bridge path  106  out of phase with respect to second bridge path  108 , as well as maintain a slightly unbalanced gain difference between first bridge path  106  and second bridge path  108 . 
     In the illustrated embodiment, resonator  112  is a metal-insulator-metal-capacitor  107  in parallel with a wire bond inductor  109 , although any LC circuit can be used, which is significantly cheaper and smaller than a typical quartz crystal resonator found in prior art oscillators. Even more significant, oscillator  100  with metal-insulator-metal-capacitor  107  in parallel with a wire bond inductor  109  can have performance consistent with a resonator of a very high quality-factor, typically only seen in a quartz crystal resonator. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it should be understood by those of ordinary skill in the art that various changes, substitutions and alterations can be made herein without departing from the scope of the invention as defined by the appended claims and their equivalents.