Abstract:
A multiple phase oscillator includes a master oscillator that injection locks a first ring oscillator. The free-running frequency of the first ring oscillator is adjustable through a control signal. A second ring oscillator has a same structure as the first ring oscillator and is connected to operate in a free-running mode. The free-running frequency of the second ring oscillator is adjustable through the control signal. A control loop senses the output of the second ring oscillator and adjusts the control signal so that the free-running frequency of the second ring oscillator matches a desired value.

Description:
TECHNICAL FIELD 
     The invention relates to oscillators, in particular to low-noise multiple phase oscillators. 
     BACKGROUND 
       FIG. 1  is a schematic diagram of a conventional LC oscillator. The oscillator comprises a pair of N-type MOS transistors M 1  and M 2  that are cross-coupled by their drains and gates. The sources of transistors M 1  and M 2  are connected to a common bias current source Ib. The drain of each transistor M 1  and M 2  is connected to a high voltage supply node through a respective inductor La, Lb of same value. A capacitor C is connected between the drains of the transistors which form the differential outputs OPUT+ and OUT− of the oscillator. These output signals are in phase opposition. The output signals exhibit a low noise and a frequency that remains relatively accurate over a wide temperature range. 
     To provide more than two phases, for instance four phases in quadrature, it is known in the art to couple together two LC oscillators of the type shown in  FIG. 1 . Such a circuit is disclosed, for instance, in U.S. Pat. Nos. 6,456,167 and 7,436,266 (both incorporated herein by reference). 
     Thus, multiplying the number of low-noise oscillators to provide more phases is costly in terms of surface area, especially due to the inductors, and in terms of power consumption. 
     SUMMARY 
     In an embodiment, a multiple phase oscillator comprises: a master oscillator; a main ring oscillator connected to be injection locked to the low-noise oscillator and having a free-running frequency adjustable through a control signal; a secondary ring oscillator of same structure than the main ring oscillator, connected in free-running mode and having a free-running frequency adjustable through said control signal; and a control loop connected to an output of the secondary ring oscillator and configured to adjust the control signal so that the free-running frequency of the secondary ring oscillator assumes a desired value. 
     The oscillator may further comprise a first current source connected to bias the main ring oscillator and adjustable through said control signal; and a second current source of same structure than the first current source, connected to bias the secondary ring oscillator and adjustable through said control signal. 
     The control loop may comprise a frequency-to-voltage converter connected to an output of the secondary ring oscillator; and a differential amplifier connected to produce said control signal by amplifying the difference between the output of the frequency-to-voltage converter and a constant reference voltage. 
     The constant reference voltage and the conversion factor of the frequency-to-voltage converter may be based on band-gap references. 
     The oscillator may further comprise circuitry connected to the secondary ring oscillator and the control loop for regularly turning on and off the secondary ring oscillator and the control loop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention provided for exemplary purposes only and represented in the appended drawings, in which: 
         FIG. 1 , previously described, is a schematic diagram of a conventional low-noise oscillator; 
         FIG. 2  is a block diagram of a conventional multiple phase oscillator based on an injection locked ring oscillator; and 
         FIG. 3  is a block diagram of an injection locked ring oscillator incorporating an embodiment of a temperature compensation circuit. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made to  FIG. 2  which illustrates a block-diagram of a multiple phase oscillator structure as disclosed in U.S. Pat. No. 6,188,291 (incorporated by reference). The oscillator comprises a master accurate oscillator MOSC, such as the LC oscillator of  FIG. 1 , connected to injection lock a ring oscillator ROSC. 
     The ring oscillator comprises multiple similar delay elements  10  connected in a ring. An odd number of the delay elements are connected to cause a phase inversion, whereby the signal is inverted at each revolution in the ring. As shown, the ring may include four differential delay elements  10 . Each differential delay element has a pair of inverting and non-inverting inputs, and a pair of inverting and non-inverting outputs. Three of the differential delay elements may be connected to cause a phase inversion, i.e. the inverting and non-inverting inputs of one delay element are respectively connected to the non-inverting and inverting outputs of the previous element. The remaining delay element, the first one in the figure, is then connected as a buffer. 
     The ring oscillator has a free running frequency equal to twice the inverse of the sum of the delays. The delay elements usually being matched, each element introduces the same delay, whereby the ring oscillator provides multiple phases that are equally distributed over 180°. 
     Four delay elements provide phases at 0°, 45°, 90° and 135°. A differential delay element further provides two opposite phases, whereby the oscillator of  FIG. 2  also provides phases at 180°, 225°, 270° and 315°. As shown, an in-phase signal +I and its opposite −I may be taken from the outputs of one of the delay elements, and a quadrature signal +Q and its opposite −Q may be taken from the outputs of the delay element two positions further. For a given number of phases, a ring oscillator consumes substantially less power and less surface area than an LC oscillator structure, 
     A ring oscillator is however known to be noisy and inaccurate (due to process variations), and to drift with temperature. 
     To improve this situation, as shown, the differential outputs of the master oscillator MOSC are connected respectively to the two inputs of one of the delay elements  10  of the ring oscillator. With this configuration, under certain operating conditions, the master oscillator imposes its frequency to the ring oscillator, this phenomenon referred to in the art as injection locking. 
     The ring oscillator then produces multiple phases with noise and accuracy characteristics commensurate with the characteristics of the master oscillator, while consuming less power and surface area than, for example, multiple LC oscillators coupled to produce the same number of phases. 
     This satisfactory operation is however subject to design constraints caused by an inevitable mismatch between the free running frequency of the ring oscillator and the frequency of the master oscillator. Higher mismatches require more power from the master oscillator to achieve injection locking. As mentioned above, the ring oscillator free running frequency is subject to temperature drift. In some applications, the temperature drift may be such that the power provided by the master oscillator, designed for nominal operating conditions, is insufficient to injection lock the ring oscillator when the operating temperature exits a relatively small range. 
       FIG. 3  is a block diagram of an injection locked ring oscillator incorporating an embodiment of a temperature compensation circuit. 
     As in  FIG. 2 , the oscillator of  FIG. 3  comprises a first ring oscillator  30  connected to be injection locked by a master oscillator MOSC. The first ring oscillator  30  may have a structure similar to that of  FIG. 2 . Each delay element of the ring oscillator may be in the form of a differential pair of N-MOS transistors biased by an individual current source. The multiple individual current sources biasing the delay elements are shown in  FIG. 3  as a single current source  32  pulling a current Ic. 
     In practice, the delay value of the delay elements, and thus the free running frequency depends on the bias current Ic. Even though the current sources can be designed to be temperature compensated, this measure is insufficient to prevent temperature drift of the free running frequency, since the temperature drift depends also on other components of the delay elements. 
     The additional elements of  FIG. 3 , described now, form an embodiment of a temperature compensation circuit  20  for the free running frequency of the ring oscillator  30 . This temperature compensation circuit comprises a second ring oscillator  34  that is designed to be matched to the first ring oscillator  30  and to operate under the same conditions (for instance placed next to the ring oscillator  30  on an integrated circuit chip). 
     Thus, the second ring oscillator  34  has the same structure as the main oscillator  30 , i.e. the same delay elements biased by the same current Ic. The current sources biasing the delay elements of the secondary oscillator are shown as a single current source  36 . 
     Contrary to the first ring oscillator  30 , the second ring oscillator  34  is configured to oscillate freely. In other terms, even though both oscillators  30  and  34  are matched and biased by a same current, they will not oscillate strictly at the same frequency. Indeed, the first oscillator  30  is normally constrained to oscillate at the frequency of the master oscillator MOSC through injection locking, whereas the second oscillator  34  oscillates at its free running frequency. 
     Hence, if the temperature varies, the frequency of the second oscillator  34  will vary, whereas the frequency of the first oscillator will normally stay locked on the frequency of the master oscillator MOSC. However, the first ring oscillator  30  will only stay locked if the power provided by the master oscillator is sufficient. A purpose of the temperature compensation circuit is to adjust the free running frequency of both the first and second ring oscillators simultaneously so that the first ring oscillator can stay locked with a relatively low power level provided by the master oscillator over a wide temperature range. 
     To this end, the current sources  32  and  36  are adjustable through a same control signal Vc that is provided by a frequency control loop. The frequency control loop is configured to regulate the free running frequency of the second oscillator  34  to a constant value, typically equal to the frequency chosen by design for the master oscillator MOSC. 
     The frequency control loop may comprise a frequency-to-voltage (F/V) converter circuit  38  connected to an output of the second ring oscillator  34 . The voltage output by the converter circuit  38 , which is representative of the free running frequency of the second oscillator  34 , is compared by a differential amplifier  40  to a reference voltage Vbg (which may, for example, comprise a fixed band-gap voltage). The output of the differential amplifier  40  provides the control voltage Vc to the current sources  32  and  36 . If the differential amplifier  40  has infinite gain, the control loop tends to regulate the control voltage Vc to be equal to the reference voltage Vbg. 
     To obtain satisfactory temperature compensation, the elements of the control loop are preferably temperature independent. Components that may be influenced by temperature in the control loop are the conversion factor of the frequency-to-voltage converter  38  and the source providing the reference voltage Vbg. The conversion factor usually depends on the value of a current source Ibg. The current Ibg and the voltage Vbg may be provided by band-gap reference sources that are temperature independent. 
     To save power, the temperature compensation circuit need not operate at all times. Indeed, it may instead be selectively turned on (for example, periodically) for short durations. In an example, a control circuit  45  is provided to control the periodic turning on of the temperature compensation circuit through an enable signal (EN 1 ), which may, for example, occur once every two seconds for a short on-time duration. While the compensation circuit is turned off by the control circuit  45 , i.e. the elements  34 ,  38  and  40  are powered down, the control voltage Vc may be stored on a capacitor  50 . 
     Alternatively, the temperature compensation circuit may be turned on as needed, for example, when the temperature variation exceeds a threshold. The temperature variation may be detected by a temperature sensor  60 , such as a diode, which outputs an enable signal (EN 2 ) configured to enable operation of the temperature compensation circuit. This enable signal EN 2  may be applied directly to the circuit  20  or applied to the control circuit  45 . 
     The foregoing description has been provided by way of exemplary and non-limiting examples of a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims.