Patent Publication Number: US-2009219102-A1

Title: Stabilized electrical oscillators with negative resistance

Description:
BACKGROUND 
     Electrical oscillators are used in digital systems, communications systems and electronic test equipment, to name only a few applications. One type of electrical oscillator is known as a voltage controlled oscillator (VCO). A VCO is a component that can be used to translate DC voltage into a time dependent voltage or signal. In general, VCOs are tunable oscillators designed to produce an oscillating signal of a particular frequency ‘f’ corresponding to a given tuning voltage. The frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage applied to the oscillator. The frequency ‘f’ may be varied from f min  to f max  and these limits are referred as the tuning range or bandwidth of the VCO. For many applications, particularly for test instrumentation and communication systems, a comparably wide tuning range is beneficial. 
     Many VCOs incorporate varactors, which are reverse biased diodes that function as voltage controlled capacitors, as the tuning mechanism. Varactors are comparably small, low cost, use negligible bias power and are available as integration elements in some semiconductor processes. Varactors are used in conjunction with fixed inductors to realize tunable LC resonators. The quality (Q) factor (or simply, Q) of varactors is usually high at low frequencies and degrades as with increasing frequency. While a tuning bandwidth of more than an octave is common in varactor-based VCOs at low frequencies, at microwave frequencies and above (i.e., frequencies greater than about 10 GHz) it is difficult to achieve a tuning bandwidth of more than one octave. Thus, the tuning range can be undesirably limiting. 
     As is known, varactor-tuned VCOs have modest phase noise at microwave frequencies. When lower phase noise is required, tunable high-Q Yttrium-Iron-Garnet (YIG) resonators are often used. Alternatively, when low phase noise is not a requirement, multivibrator VCOs can be used. Multivibrators do not include tunable resonators, but rely on varying the current charging a fixed capacitor to tune the oscillation frequency. There are two main advantages in using a multibrator. First, multivibrators do not require varactors, which simplifies the circuit and makes multivibrators suitable for integration in semiconductor processes that do not have varactors. Second, they have very wide tuning range, typically multi-octave. 
     While multivibrator-based VCOs have a greater tuning range than varactor-based VCOs, their tuning range can nonetheless be limited at high frequencies due to non-ideal behavior of the active devices. For example, transistors become less unilateral and their gain decreases with frequency (due to parasitic transistor elements), preventing multivibrators from achieving a wide tuning range. 
     Another disadvantage of many known VCOs is that the oscillation amplitude is typically established by the limiting action of the non-linear active device characteristics, which for some bipolar transistors can cause the transistor to operate in an unreliable saturation mode. 
     There is a need, therefore, for electrical oscillators, including VCOs that overcome at least the shortcoming of known oscillators described above. 
     SUMMARY 
     In accordance with a representative embodiment, an electrical oscillator includes a first oscillating transistor and a second oscillating transistor. The electrical oscillator also includes a first non-linear load connected to a terminal of the first oscillating transistor and a second non-linear load connected to a terminal of the second oscillating transistor. The electrical oscillator also includes a negative resistance generated between the terminal of the first oscillating transistor and the terminal of the second oscillating transistor, wherein the electrical oscillator does not include a tunable resonator. 
     In accordance with another representative embodiment, a voltage controlled oscillator (VCO) includes a first oscillating transistor and a second oscillating transistor. The VCO also includes a first non-linear load connected to a terminal of the first oscillating transistor and a second non-linear load connected to a terminal of the second oscillating transistor. The VCO also includes a negative resistance generated between the terminal of the first oscillating transistor and the terminal of the second oscillating transistor, wherein the VCO does not include a tunable resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present teachings are best understood from the following detailed description when read with the accompanying drawing figures. The features are not necessarily drawn to scale. Wherever practical, like reference numerals refer to like features. 
         FIG. 1  is a simplified schematic diagram of an electrical oscillator in accordance with a representative embodiment. 
         FIG. 2  is a simplified schematic diagram of an electrical oscillator in accordance with a representative embodiment. 
         FIG. 3  is a graphical representation of a reflection coefficient showing negative resistance as a function of frequency and collector load of an electrical oscillator in accordance with a representative embodiment. 
         FIG. 4  is a graphical representation of a reflection coefficient showing negative resistance as a function of frequency and collector load of an electrical oscillator in accordance with a representative embodiment. 
         FIG. 5  is a graphical representation of output frequency versus tuning voltage of an electrical oscillator in accordance with a representative embodiment. 
     
    
    
     DEFINED TERMINOLOGY 
     It is to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. 
     As used in the specification and appended claims, the terms ‘a’, ‘an’ and ‘the’ include both singular and plural referents, unless the context clearly dictates otherwise. Thus, for example, ‘a device’ includes one device and plural devices. 
     As used in the specification and appended claims, the term tunable resonator includes a resonator tuned thermally, mechanically, electrically or magnetically. Examples of such tunable resonators include, but are not limited to: varactors, YIG resonators, cavity-tuned resonators and dielectric resonant oscillators (DROs). 
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. Descriptions of known systems, devices, materials, methods of operation and methods of manufacture may be omitted so as to avoid obscuring the description of the example embodiments. Nonetheless, systems, devices, materials and methods that are within the purview of one of ordinary skill in the art may be used in accordance with the representative embodiments. 
     As described more fully herein, the representative embodiments relate generally to electrical oscillators having oscillating transistors, with a terminal loads comprising a negative resistance. As will become clearer as the present description continues, the negative resistance may be a differential negative resistance. Frequency tuning of the oscillating transistors comprises varying an impedance of a positive feedback connection. While the embodiments are described primarily in connection with VCOs, electrical oscillators in general are contemplated. For example, rather than varying the impedance of a positive feedback connection via an applied voltage or current, the electrical oscillators of representative embodiments may be tuned by varying the impedance of the terminal (e.g., collector) loads by varying the temperature of the load or by varying the intensity of light shining on the load. 
       FIG. 1  is a simplified schematic diagram of an electrical oscillator  100  in accordance with a representative embodiment. The oscillator  100  includes a first oscillating transistor  101  (often referred to as ‘transistor 101’ for simplicity) and a second oscillating transistor  102  (often referred to as ‘transistor 102’ for simplicity). As will become clearer as the present description continues, the transistors  101 ,  102  are substantially identical in performance. An emitter capacitor  103  and emitter resistors  104  are connected to respective emitters of the transistors  101 ,  102  as shown. An input  105  for a tuning voltage is connected to the emitter resistors  104 . The circuit  100  also includes a first emitter-follower transistor  106  and a second emitter-follower transistor  108 , with emitter resistors  107  and  109 , respectively, connected thereto. Inputs  110  and  111  provide emitter voltages to the emitter follower transistors  106 ,  108 , respectively. 
     The first oscillating transistor  101  includes a first collector  112  and the second oscillating transistor  102  includes a second collector  113 . The first collector  112  is connected to a first tap  114 , which in turn is connected to an emitter of a second diode-connected transistor  116 . The second collector  113  is connected to a second tap  115 , which in turn is connected to an emitter of a first diode connected transistor  117 . The first diode-connected transistor  117  and the second diode-connected transistor  116  may alternatively be diodes, and are often referred to below as first diode  117  and second diode  116  for ease of description. As will become clearer as the present description continues, the diodes  116 ,  117  are substantially identical in performance. 
     The first diode  117  includes a first collector resistance  118  and a second collector resistance  119  connected differentially as shown. Likewise, the second diode  116  includes a third collector resistance  120  and a fourth collector resistance  121  also connected differentially. Completing the circuit is an input  122  for the collector voltage, which is at ground in the present configuration; and positive output terminal  123  and negative output terminal  124 . 
     The first and second oscillating transistors  101 ,  102  are illustratively npn InP heterojunction bipolar transistors (HBTs). However, this is merely illustrative, and it is emphasized that other three terminal devices are contemplated by the present teachings to provide oscillation. For instance, HBTs based on other materials (e.g., other III-V semiconductors) may be used. Alternatively, pseudomorphic high electron mobility transistors (pHEMTs) may be used for the first and second oscillating transistors  101 ,  102 . Alternatively, field-effect transistors (FETs) may be used for the first and second oscillating transistors  101 ,  102 . Illustrative FETs include metal-oxide-semiconductor (MOS) FETs may be used. Moreover, metal semiconductor FETs (MESFETs) may be used. Again, a wide variety of materials are available for fabricating the transistors  101 ,  102 , including but not limited to Si, Ge, SiGe, and a variety of III-V semiconductors. 
     Similarly, as noted above the first and second diodes  116 ,  117  may be diode-connected transistors or diodes. If diode-connected transistors, the first and second diodes  116 ,  117  may one of the types of transistors described above with the shorting of two terminals to effect the diode. Alternatively, one of a variety of pn-junction diodes or metal-semiconductor junction (Schottky) diodes may be used for the first and second diodes  116 ,  117 . 
     Selection of alternative devices (e.g., FETs, pnp transistors) may require modification of parameters, connection, etc. to realize a functioning oscillator. For instance, if a FET is selected, rather than a collector, the drain of the first and second oscillating transistors  101 ,  102  would be connected to a negative resistance. As such, more generally therefore, a terminal of a three-terminal device is connected to the negative resistance. As one deft in circuit design will appreciate the need for such modifications, these modifications are thus contemplated by the present teachings. 
     Finally, and as will be appreciated by one of ordinary skill in the art, the fabrication of the circuit  100  in large-scale processing is advantageous. Thus, in certain embodiments employing wafer-scale fabrication, the selection of materials is predicated on the selection of devices for the first and second oscillating transistors  101 ,  102  is related to the selection of the diodes  116 ,  117 . As such, if one were to select a GaAs-based HBT for transistors  101 ,  102 , the diodes likely would be GaAs-based diodes as well. 
     In operation, the cross-connection of the first collector  112  to the second diode  116  and the second collector  113  to the first diode  117  as shown results in a differential load at each collector, and a negative resistance. The differential loads of the circuit  100  comprise non-linear terminal (collectors in the presently described embodiments) loads comprising of resistors  118 - 121  and first and second diodes  117 ,  116 . The connection of the second diode  116  via the tap  114  provides a bias voltage from the first transistor  101  to the second diode  116 ; and the connection of the first diode  117  via the tap  115  provides a bias voltage from the second transistor  102  to the first diode  117 . 
     In the representative embodiments described in conjunction with  FIG. 1 , the negative resistance is provided by the connection of the base of the first oscillating transistor  101  to the emitter of the second emitter-follower transistor  108 ; the connection of the base of the second oscillating transistor  102  to the emitter of the first emitter-follower transistor  107 ; and the emitter capacitor  103 . As will be appreciated, the negative resistance of the representative embodiments, among other things, completes a positive feedback connection. 
     The non-linear terminal load (e.g., non-linear collector load) of the representative embodiments function as a limiting mechanism for the oscillator. To this end, at lower oscillation amplitudes the closed loop gain of the oscillator  100  is greater than unity (1), which allows the oscillation to start. The limiting action provided by the non-linear terminal loads reduces the gain to unity and the amplitude of oscillation stabilizes at the final oscillation condition. Thus, the non-linear collector loads function as stabilizing limiters since the closed-loop gain decreases as the oscillation amplitude increases. 
     Decreasing the gain reduces the tendency of the first and second oscillating transistors to oscillate and results in a substantially stable oscillation amplitude. In accordance with the presently described embodiments, the limiting action is manifest as a decrease in resistance across the first and second diode-connected transistors  117 ,  116  (or alternatively diodes) as the oscillation amplitude increases. As will be appreciated by one of ordinary skill in the art, the limiting action is a function of the voltage across the first and second diode-connected transistors  117 ,  116 ; and a function of the DC bias through the first and second diode-connected transistors  117 ,  116 . 
     In the presently described embodiments, the voltage across the first and second diodes  117 ,  116  is illustratively an RF voltage. Increasing the forward DC bias current through the first and second diodes  117 ,  116  limits oscillation to a lower RF voltage level. Moreover, lowering the load impedance increases the frequency of oscillation (as can be seen by the negative resistance curves in  FIGS. 3 and 4  discussed below.) Consequently, the oscillator frequency may be tuned by changing the impedance and thus the DC current through the diodes. Illustratively, the DC current through the first and second diodes  17 ,  116  may be changed by changing the oscillating transistors&#39; bias or by incorporating a dedicated bias line in the oscillator  100 , or both. Thus, the non-linear load (comprised of resistors and first and second diodes  117 ,  116 ) functions as both a stabilizing limiter and a frequency tuning mechanism. 
     Certain clear benefits are provided by the electrical oscillator  100 . In a typical oscillator the limiting action is provided by the oscillating transistors. By contrast, in accordance with the representative embodiments, by having a limiting action that does not rely on or otherwise comprise additional or external oscillating transistors, the first and second oscillating transistors  101 ,  102  can be biased for optimum high frequency operation, or maximum signal-to-noise ratio, or both, without regard to the desired limiting RF amplitude level. Moreover, the first and second oscillating transistors  101 ,  102  are able to operate in a substantially linear mode which can improve reliability and maintain the loaded Q of the oscillator. 
     In illustrative embodiments, varying the differential impedance of the cross-connected first and second oscillating transistors  101 ,  102  and thereby tuning the electrical oscillator  100 , involves adjusting the tuning voltage at the input  105 . Specifically, as the tuning voltage, V tune , is made more negative, the emitter current increases in the first and second oscillating transistors  101 ,  102  and the voltage across the second collector resistance  121  and the fourth collector resistance  121  also increases. This increases the forward bias across first and second diodes  117 ,  116 , respectively. As such, as V tune  is made more negative, the differential impedance between the collectors of the first and second oscillating transistors  101 ,  102  decreases due to decreased load impedance and increased capacitance. 
     As described above, the tuning of the electrical oscillator  100  is effected by varying the differential impedance presented to the collectors of the cross-connected first and second oscillating transistors  101 ,  102 . In another representative embodiment, an external bias voltage is applied to increase the tuning range of the oscillator.  FIG. 2  is a simplified schematic circuit diagram of an electrical oscillator in accordance with a representative embodiment and includes an external bias input  201 . Many of the components described in connection with the representative embodiments of  FIG. 1  and their function are substantially identical. As such, details are not duplicated, but rather differences are described. 
     The input  201  provides a bias voltage, V bias     —     adjust , which biases the first and second diodes  117 ,  116  through a first bias resistor  202  and a second bias resistor  203 , respectively. With this control there is additional flexibility in adjusting the oscillation frequency. In particular, the oscillation frequency and amplitude may be controlled by setting the bias of the first and second diodes  117 ,  116  as described previously. If greater frequencies of oscillation are desired, a greater bias on the first and second diodes  117 ,  116  will increase the oscillation frequency by increasing the current and thereby decreasing the differential impedance between the first and second collectors  112 ,  113 . Moreover, control of the amplitude by gain reduction is also realized. With this arrangement, a substantially optimum transistor bias can be established at each frequency. This prevents the transistor bias from dropping too low (with a consequent drop in gain and negative resistance) or too high (with a consequent operation at a high junction temperature). With this added bias control the tuning range can be increased over what can be achieved with the variation of the differential impedance described in connection with the embodiments of  FIG. 1 . 
     As will be appreciated, in the embodiments described in connection with  FIGS. 1 and 2 , the oscillation frequency of the oscillating transistors are controlled by controlling the load impedance of the terminal (e.g., collector) loads by controlling the voltage/current to the diodes  116 ,  117 . As alluded to previously, frequency tuning of the oscillating transistors  101 ,  102  by varying an impedance of a positive feedback connection may be effected by means other than voltage/current variation. In accordance with an illustrative embodiment, rather than diode  116 ,  117  (or diode connected transistors), a thermistor (not shown) may be connected to each terminal (e.g., each collector  112 ,  113 ) to provide the terminal load. Because the diodes  116 ,  117  are foregone, the resistors required for biasing would not be needed. By varying the temperature of the thermistors, the load impedance varies as needed to tune the oscillating transistors  101 ,  102 . Notably, the limiting function of the oscillating transistors is provided by the thermistors, which provide increasing power as the impedance drops. 
     In accordance with another representative embodiment, a photoresistor (not shown) or similar light-dependent resistor (not shown) could supplant the diodes  116 ,  117 . Variation of the intensity of light directed to the photoresistor will result in a variation of the impedance at the terminals (e.g., collectors  112 ,  113 ). Like the thermistors, the photoresistors provide the limiting function that reduces the impedance with increasing power. Still other devices and configurations for frequency tuning of the oscillating transistors  101 ,  102  by varying an impedance of a positive feedback connection within the purview of one of ordinary skill in the art are contemplated. 
       FIG. 3  is a graphical representation of a reflection coefficient showing negative resistance as a function of frequency and collector load of an electrical oscillator in accordance with a representative embodiment. The electrical oscillator may be electrical oscillator  100  or electrical oscillator  200 , having components described in conjunction with representative embodiments above. The graphical representation of  FIG. 3  may be derived from a circuit model comprising the electrical oscillator  100  or the electrical oscillator  200  as a negative resistance connected to a load. Such a technique for modeling the oscillators  100 ,  200  is known to those skilled in the art of high-frequency circuit design and, as such, details of the method of modeling the oscillators  100 ,  200  are omitted to avoid obscuring the present teachings. Curve  301  is the graph of negative resistance (1/Γ&lt;1) versus frequency (referred to as negative resistance curve  301 ) over an illustrative frequency range of oscillation of approximately 6.0 GHz to approximately 55 GHz. As will be appreciated from a review of negative resistance curve  301 , there is a broadband negative resistance generated between the collectors (or terminals depending on the type of oscillating transistor selected) due to their respective positive feedback connection. The arrow  302  indicates the variation of the negative resistance with oscillation frequency for a particular bias condition of the first and second oscillating transistors  101 ,  102 , and consequently the first and second diodes  117 ,  116 . 
     However, the negative resistance curve  301  is a function of collector voltage and will ‘move’ toward the ordinate (in an ‘upward’ direction  303 ) towards the top of the polar plot as the amplitude of oscillation increases. The model shown in  FIG. 3  represents an oscillator having a relatively high emitter current first and second oscillating transistors  101 ,  102  operating at a greater (in this example negative) V tune  voltage and corresponding the negative resistance and diode load. Under these conditions, the impedance seen by the collectors  112 ,  113  of the first and second oscillating transistors  101 ,  102 , respectively, is comparatively low. As the amplitude of oscillation increases, the negative resistance curves moves up and the collector load impedance decreases (moves in the direction  304  due to lower resistance and increased capacitance). A stable oscillation will occur when the negative resistance curve  301  moves upward and intersects the reflection coefficient of the load (Γ collector load ). At the intersection, a stable oscillation point  305  exists where the feedback in the oscillator is unity with zero phase shift. In the present illustration, the negative resistance curve and the load impedance coincide at a stable oscillation condition as shown with a frequency in this example of 26 GHz. 
       FIG. 4  is a graphical representation of a reflection coefficient showing negative resistance as a function of frequency and collector load of an electrical oscillator in accordance with a representative embodiment. The electrical oscillator may be electrical oscillator  100  or electrical oscillator  200 , having components described in conjunction with representative embodiments above. The electrical oscillator is modeled according to the method described in connection with  FIG. 3 . In the present example, a lower frequency oscillation condition is realized. Curve  401  shows the negative resistance curve over a frequency range of approximately 6 GHz to approximately 40 GHz. In the present model, the tuning voltage (V tune ) has a lesser magnitude (i.e., less negative) and a comparatively low emitter current. In this case the negative resistance and load curves are for the first and second oscillating transistors  101 ,  102 . Under these conditions the impedance seen by the collectors  112 ,  113  of the first and second oscillating transistors  101 ,  102  is relatively high. As the oscillation amplitude increases, the negative resistance curve  401  moves upwardly and a load impedance decreases (moves in a direction  402  due to lower resistance and increased capacitance) intersect at a stable oscillation condition at point  403  at a lower frequency of 8 GHz. 
     The negative resistance curves  301 ,  401  are functions of oscillation frequency and the oscillation frequency depends on the value of the collector load impedance  FIGS. 3 and 4  show that the negative resistance of the respective models decreases with increasing amplitude and the impedance of the load from diodes  116 ,  117  also drops with increasing amplitude until a stable oscillation condition is reached (with a closed loop gain of 1). Lowering the impedance of the load decreases the gain which reduces the tendency to oscillate and leads to a stable oscillation amplitude. 
       FIG. 5  is a graphical representation of output frequency versus tuning voltage of an electrical oscillator in accordance with a representative embodiment. The oscillator may be electrical oscillator  100  or electrical oscillator  200  described previously, with selected devices and component values. In a representative embodiment, the first and second oscillating transistors  101 ,  102  were HBTs with a transition frequency (F t ) of approximately 180 GHz. The measured oscillation frequency curve  601  reveals an oscillation frequency range from a low end of approximately 6 GHz to approximately 30 GHz over a tuning voltage range of approximately −2.75 V to approximately −7.0 V. 
     In view of this disclosure it is noted that variations to the electrical oscillators and VCOs described herein can be implemented in keeping with the present teachings. Further, the various topologies, devices, components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.