Patent Publication Number: US-2010127786-A1

Title: Low noise oscillators

Description:
TECHNICAL FIELD 
     The present invention relates to RF oscillators and more particularly to RF oscillators having low levels of phase noise. 
     BACKGROUND 
     As is known in the art, low noise oscillators have a wide range of applications such as in navigation, radars and communication systems. As is also known in the art, with transistor oscillators, flicker noise from the transistors may significantly degrade oscillator phase noise. One technique used to produce low noise oscillators is to screen oscillator transistors for devices having low phase noise. This is time consuming, costly and can sometimes lead to unpredictable yields. Obtaining RF transistors with flicker noise much less than 1 kHz is desired, but is generally considered impractical. More particularly, RF oscillator phase noise is a dominant factor limiting the performance of many systems. A time based related attribute is the short-term stability or Allan variance. The basic mechanisms of phase noise generation in oscillators are well understood and described in the literature. An example is the model described D. B. Leeson in an article by D. B. Leeson, entitled “A simple model of feedback oscillator noise spectrum,” Proc. IEEE, vol. 54, pp. 329-330, February 1966. This oscillator model is commonly referred to as “Leeson&#39;s model”. Many techniques are employed to reduce the phase noise of oscillators, but generally these techniques relate to using transistors with lower 1/f phase noise or higher Q resonators in the feedback circuit. 
     Phase noise is often described by its spectral properties. For example, phase noise can have a 1/f n  characteristic, with n being an integer. For oscillator circuits, n generally varies from 0 to 3. As described in by D. B. Leeson, in the paper entitled “A simple model of feedback oscillator noise spectrum,” Proc. IEEE, vol. 54, pp. 329-330, February 1966, electronic noise within the resonator bandwidth is increased such that flicker noise is converted into 1/f 3  phase noise when the device is embedded into a high Q oscillator circuit. The implication of this conversion is that noise within the resonator bandwidth is greatly increased. Obtaining lower phase noise then requires either lower 1/f phase noise transistors or higher Q resonators. In particular, the 1/f phase noise of a RF transistor relates to the phase noise at small offset frequencies from the center resonance frequency of the oscillation signal. For example, when referring to 1/f phase noise in a 1 GHz oscillator, the 1/f term applies to noise having a 1/f spectral shape when offset from the 1 GHz output. Although transistor 1/f phase noise is generally attributed to material and surface defects, the precise mechanisms are not well understood. 
     The origin of 1/f phase noise can be associated with the actual flicker noise of the transistor, but the specific mechanisms of conversion are also not well understood. Obtaining RF transistors having very low 1/f phase noise is quite difficult due to compromises between RF performance and flicker noise. 
     An analysis was presented by Eva S. Ferre-Pikal, Fred L. Walls, in a paper entitled Guidelines for Designing BJT Amplifiers with Low 1/f AM and PM noise, IEEE Transactions on Ultrasonics, Ferroelectronics and Frequency Control, Vol. 44, No. 2, March 1997 which relates amplifier 1/f phase noise with low frequency voltage fluctuations. Modulation of the collector-base capacitance was proposed as a means of converting flicker noise to residual 1/f phase noise. 
     In a paper entitled Reduction of Phase Noise in Linear HBT Amplifiers Using Low-Frequency Active Feedback by Eva S. Ferre-Pikal, IEEE Transactions on Circuits and Systems, Vol. 51, No. 8, August 2004 the author attempted to stabilize a conventionally biased RF transistor by use of an instrumentation amplifier. The instrumentation amplifier was configured in a conventional topology. There was evidence that additional stability of the transistor bias point could suppress 1/f phase noise. However this topology also introduced several additional resistive components as potential sources of noise and had limited noise suppression. These devices were not embedded into or related to low phase noise oscillators. 
     The desire is to provide an RF oscillator with very low phase noise. In addition, it is desired to minimize RF power variations with temperature and process variations. 
     SUMMARY 
     In accordance with the present invention, an oscillator is provided having: a transistor; a resonant circuit coupled between an output electrode of the transistor and a control electrode of the transistor; and a dc bias circuit for the transistor. The de bias circuit comprises: a voltage producing circuit and a differential amplifier. The differential amplifier includes: a first input coupled to a fixed reference voltage; a second input coupled to the voltage producing circuit, such voltage producing circuit producing a voltage at the second input of the difference amplifier related to current passing through the output electrode of the transistor; and an output coupled to the control electrode of the transistor. 
     In one embodiment, the oscillator includes a voltage source having: one potential coupled to one terminal of the voltage producing circuit; and a second potential coupled to a second terminal of the voltage producing circuit; and wherein a third terminal of the voltage producing circuit is coupled to the second input of the differential amplifier. 
     In one embodiment, the voltage producing circuit includes a first resistor coupled between the first potential and the second input of the differential amplifier and a second resistor between an additional electrode of the transistor and the second potential. 
     In one embodiment, the oscillator includes an inductor coupled between the second input of the differential amplifier and the output electrode of the transistor. 
     In one embodiment, the oscillator includes a capacitor coupled between the first input of the differential amplifier and the output of the differential amplifier. 
     In one embodiment, the oscillator includes a third resistor and a fourth resistor connected to the third resistor at a node, such node being coupled to the second potential through a capacitor, the third resistor being coupled between the output of the differential amplifier and the node and the fourth resistor being coupled between the node and the control electrode of the transistor. 
     In one embodiment, the fixed voltage is a voltage produced by a resistor divider coupled between the first and second potentials. 
     Thus, with such an arrangement, flicker noise of the oscillator is reduced by actively controlling biasing and low frequency modulation. This invention uses a novel topology to reduce flicker noise and improve phase noise. The technique is applicable to a broad class of oscillators. 
     The details of one or more embodiments of the invention are set forth in the accompanying SINGLE FIGURE of an oscillator according to the invention and description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWING 
       The SINGLE FIGURE is a schematic diagram of an RF oscillator according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the SINGLE FIGURE, an oscillator  10  is shown. The oscillator includes a transistor Q 1 ; a resonant circuit  12  coupled between an output electrode, here collector electrode, of the transistor Q 1  and a control electrode, here base electrode, of the transistor Q 1 ; and a dc bias circuit  14  for the transistor Q 1 . The dc bias circuit  14  includes: a voltage producing circuit  16 ; and a differential amplifier  18 . The differential amplifier  18  has: a first input (inverting (−) input) coupled to a fixed reference voltage; a second input (non-inverting (+)) coupled to the voltage producing circuit  16 , such voltage producing circuit producing a voltage at the second input (non-inverting (+)) of the difference amplifier  18  related to current Ic passing through the output electrode (collector) of the transistor Q 1 ; and an output  20  coupled to the control electrode (base) of the transistor Q 1 . A voltage source V 1  has: one potential (+) coupled to one terminal of  22  the voltage producing circuit  14 ; and a second potential (−) coupled to a second terminal  24  of the voltage producing circuit  14 . A third terminal  26  of the voltage producing circuit  14  is coupled to the second input (non-inverting (+)) of the differential amplifier  18 . The voltage producing circuit  14  includes a first resistor R 4  coupled between the first potential and the second input of the differential amplifier (non-inverting (+)) and a second resistor R 5  between an additional electrode (emitter) of the transistor Q 1  and the second potential (i.e., terminal  24 ). An inductor L 1  is coupled between the second input (non-inverting (+)) of the differential amplifier  18  and the output electrode (collector) of the transistor Q 1 . A capacitor C 3  is coupled between the first input (inverting (−)) of the differential amplifier  18  and the output  20  of the differential amplifier  18 . A third resistor R 3  and a fourth resistor R 6  are connected together at a node  30 , such node  30  being coupled to the second potential (i.e., terminal  24 ) through a capacitor C 4 , the third resistor R 3  being coupled between the output  20  of the differential amplifier  18  and the node  30  and the fourth resistor R 6  being coupled between the node  30  and the control electrode (base electrode) of the transistor Q 1 . The fixed voltage is a voltage produced at node  32  by a resistor divider  34  made up of resisters R 1  and R 2  coupled between the first and second potentials of the supply V 1 . 
     More particularly, the transistor Q 1  is the oscillator transistor. The differential amplifier  18  is chosen to have low flicker noise properties. A resistor R 7  is the RF load resistor with typical value of 50 ohms. Inductor L 1  is used for RF isolation and may also take the form of a distributed transmission line. Capacitor C 1  is a bypass capacitor having very low reactance at the oscillation frequency. The two port device is the resonant feedback circuit  12  and could be a lumped element LC, an acoustic resonator such as SAW, or a distributed resonator such as a transmission line or a dielectric resonator. The two-port could include a means of tuning the oscillator frequency such as a varactor diode. 
     Here the differential amplifier  18  is used to bias and stabilize the oscillator transistor Q 1 . Transistor Q 1  is shown as a bipolar device, but may also be a FET; in which case the control electrode is the gate electrode. The semiconductor material may be silicon, GaAs, GaN or other semiconductor materials. 
     Biasing is provided by using as the differential amplifier  18  a differential amplifier having low flicker noise. For example, commercially available differential amplifiers are available with a typical flicker noise intercept of less than 10 Hz. A reference voltage formed by the voltage divider of R 1  and R 2  and also having low flicker noise is used as the inverting input, and a voltage proportional to collector current of the RF transistor is used as the non-inverting input. The feedback path from the voltage at the R 4 -L 1  node is applied to the positive differential amp input due to the 180 phase shift of transistor Q 1  at low frequencies. Effectively the amplifier  19  positive input (non-inverting input (+)) becomes a negative feedback path, and the reference voltage at node  32  is applied to what is commonly used as the negative input to the op-amp. The output  20  from the differential amplifier  18  is used to provide a voltage for biasing the input (here emitter) to the RF transistor Q 1 . Resistor R 3 , R 6  and capacitor C 4  serve to isolate the RF signal from the biasing function. An additional capacitor C 3  serves as a phase shift component to establish adequate phase margin and ensure that noise processes are not regenerated by the very high differential voltage gain. The biasing configuration ensures that the voltage of the non-inverting input (+) of the differential amplifier  18  will be essentially equal to the voltage of the inverting input (−). Since the noise at the inverting input (−) is derived from a reference voltage at node  32  with very low noise, the noise at the non-inverting input (+) will also be similarly quiet. Any noise in the collector current Ic of the RF transistor Q 1  will now be sensed by the biasing circuit  14  and the voltage present at the base of the RF transistor Q 1  will be adjusted to compensate for that noise. Noise which is normally present at the collector of the RF transistor Q 1  will essentially be translated back to the base of said transistor Q 1 . However, since the transistor Q 1  has a voltage gain from the collector to base electrode, voltage noise will similarly be reduced by this voltage gain. Noise processes associated with modulation of the collector to base capacitance, and within the bandwidth of the biasing circuitry, will similarly be reduced. Resistor R 5  provides additional negative feedback to stabilize the oscillation circuit. 
     Since the biasing circuitry extends down to DC, the oscillator frequency is also stabilized with respect to variation in temperature and parametric variations of the RF transistor. The circuit can be implemented from discrete devices or as an integrated circuit. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the invention applies to crystal, SAW, LC and microwave resonant oscillators, and can be implemented with discrete components or as integrated circuit devices. Additionally, inductors and capacitors could be replaced with equivalent function distributed elements, such as microstrip transmission lines, for use at microwave frequencies. Accordingly, other embodiments are within the scope of the following claims.