Abstract:
Techniques to tune a frequency of a sinusoidal signal using devices having controllable signal phase delay times. One implementation may include an input terminal to receive an input signal; a first delay path to selectively receive the input signal; a second delay path to selectively receive the input signal; and a control terminal to control an extent to which the input signal flows through the first delay path and the second delay path.

Description:
FIELD 
   The subject matter disclosed herein generally relates to oscillator devices. 
   DESCRIPTION OF RELATED ART 
   A voltage controlled oscillator (VCO) may be used to generate a sinusoidal voltage signal. For example, VCOs may be used in communication systems to transmit and reproduce signals.  FIG. 1  depicts a prior art ring oscillator implementation of a VCO  100  having multiple similar ring elements  110 -A to  110 -C.  FIG. 2  depicts a prior art implementation of a ring element  110  (ring elements  110 -A to  110 -C are similar to ring element  110 ). Ring element  110  is a differential transistor pair element that uses a capacitive load  202  and resistive load  204 . To change the frequency of the sinusoidal voltage signal output by the VCO  100  (shown as Vout), the capacitance of each capacitor  202  in each ring element  110  may need to be changed. Frequency variation may be limited due to the difficulty in varying a capacitor value. In addition, the phase noise performance of VCO  100  may suffer due to thermal noise from the resistive load  204 . The phase noise performance measures the purity of frequency content output by an oscillator. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a well known ring oscillator implementation of a VCO having multiple similar ring elements 
       FIG. 2  depicts a prior art implementation of a ring element. 
       FIG. 3  depicts one possible receiver system in which some embodiments of the present invention may be used. 
       FIG. 4  depicts one example implementation of a ring element, in accordance with an embodiment of the present invention. 
       FIG. 5  depicts one suitable implementation of a delay device, in accordance with an embodiment of the present invention. 
   

   Note that use of the same reference numbers in different figures indicates the same or like elements. 
   DETAILED DESCRIPTION 
     FIG. 3  depicts one possible receiver system  300  in which some embodiments of the present invention may be used. Receiver  300  may receive signals encoded in compliance for example with optical transport network (OTN), Synchronous Optical Network (SONET), and/or Synchronous Digital Hierarchy (SDH) standards. Example optical networking standards may be described in ITU-T Recommendation G.709 Interfaces for the optical transport network (OTN) (2001); ANSI T1.105, Synchronous Optical Network (SONET) Basic Description Including Multiplex Structures, Rates, and Formats; Bellcore Generic Requirements, GR-253-CORE, Synchronous Optical Network (SONET) Transport Systems: Common Generic Criteria (A Module of TSGR, FR-440), Issue 1, Dec. 1994; ITU Recommendation G.872, Architecture of Optical Transport Networks, 1999; ITU Recommendation G.825, “Control of Jitter and Wander within Digital Networks Based on SDH” March, 1993; ITU Recommendation G.957, “Optical Interfaces for Equipment and Systems Relating to SDH”, July, 1995; ITU Recommendation G.958, Digital Line Systems based on SDH for use on Optical Fibre Cables, November, 1994; and/or ITU-T Recommendation G.707, Network Node Interface for the Synchronous Digital Hierarchy (SDH) (1996). 
   Referring to  FIG. 3 , optical-to-electrical converter (“O/E”)  355  may convert optical signals received from an optical network from optical format to electrical format. Although reference has been made to optical signals, the receiver  300  may, in addition or alternatively, receive electrical signals from an electrical signal network or wireless or wire-line signals according to any standards. Amplifier  360  may amplify the electrical signals. Clock and data recovery unit (“CDR”)  365  may generate cleaner versions of the electrical signals and corresponding clock. For example, CDR  365  may use a VCO in a phase locked loop (“PLL”) to regenerate sinusoidal electrical signals. On the regenerated signals, layer two processor  370  may perform media access control (MAC) management in compliance for example with Ethernet, described for example in versions of IEEE 802.3; optical transport network (OTN) de-framing and de-wrapping in compliance for example with ITU-T G.709; forward error correction (FEC) processing, in accordance with ITU-T G.975; and/or other layer  2  processing. Some implementations of receiver system  300  may utilize 10 Gigabit Attachment Unit Interface (XAUI), Serial Peripheral Interface (SPI), ten bit interface (TBI), and/or Gigabit Media Independent Interface (GMII) compliant interfaces (as well as interfaces based on related standards) to provide intercommunication between the layer two processor  370  and the CDR  365 . 
   Interface  375  may provide intercommunication between layer two processor  370  and other devices such as a microprocessor, memory devices (not depicted), packet processor (not depicted), and/or a switch fabric (not depicted). Interface  375  may provide intercommunication in compliance, for example, with Peripheral Component Interconnect (PCI), Universal Serial Bus (USB), Ethernet, IEEE 1394, and/or a vendor specific multi-source agreement (MSA) protocol. 
   In accordance with an embodiment of the present invention,  FIG. 4  depicts one example implementation of a ring element  400 . Ring element  400  may be used in a ring formation such as in an arrangement similar to the ring elements  110 -A to  110 -C of VCO  100  depicted in FIG.  1 . When used in a ring formation, output terminals E and F of a ring element  400  may be coupled to input terminals A and B of another ring element similar to ring element  400 . When used in a ring formation, each of the ring elements  400  may utilize the same control signal at terminal CONTROL. 
   In accordance with an embodiment of the present invention, terminal OUTPUT (nodes E and F) of ring element  400  may provide a phase delayed version of the signal provided to terminal INPUT (nodes A and B) and the delay may be tuned by the control signal provided at terminal CONTROL. In accordance with an embodiment of the present invention, a control signal provided at terminal CONTROL may tune the frequency of a signal by a ring oscillator utilizing multiple ring elements  400  by adjusting the phase delay between the input and output terminals of an individual ring element. 
   One implementation of ring element  400  may include input transistors  402 -A,  402 -B,  404 -A, and  404 -B, control transistors  406 -A and  406 -B, load  415 -A, load  415 -B, current source  410 , and delay device  450 . Input transistors  402 -A,  402 -B,  404 -A, and  404 -B as well as control transistors  406 -A and  406 -B may be implemented as MOSFET transistors although various other types of transistors may be used, such as BJT and HBT types. Input transistors  402 -A,  402 -B,  404 -A, and  404 -B may have similar gain/phase characteristics. 
   Load  415 -A and load  415 -B may be implemented as inductors. Use of inductor loads in ring element  400  may provide better phase noise performance than use of resistive loads (e.g., as used in the ring element  110  of FIG.  2 ). Moreover, the use of inductor loads may also allow operation at higher frequencies than operation provided by ring element  100 . Inductor values may be chosen to provide a maximum allowable inductance for a desired maximum frequency of operation. A high “Q value” inductor may be used to improve the phase noise of the oscillator although it is not required. 
   In one implementation, a control signal applied to gate terminals of control transistors  406 -A and  406 -B may control an amount of phase delay that a signal input to terminal INPUT is delayed before being provided at terminal OUTPUT. For example, a high value of a control signal may control current of current source  410  to flow primarily through the short-delay path (e.g., transistors  402 -A and  402 -B). For a high value of a control signal, an input signal to terminal INPUT may experience a time delay due to a signal path through transistors  402 -A and  402 -B and loads  415 -A and  415 -B prior to being provided to terminal OUTPUT. 
   Conversely, a low value of the control signal may control current of current source  410  to flow primarily through the longer-delay path which includes delay device  450  and transistors  404 -A and  404 -B. For a low value of a control signal, an input signal to terminal INPUT may experience a phase delay from a signal path through delay device  450  combined with transistors  404 -A and  404 -B as well as load  415 -A and  415 -B prior to being provided to terminal OUTPUT. 
   When the control signal is between high and low, control current of current source  410  may flow evenly through the short-delay and longer-delay paths. When the control signal is between high and low, an input signal to terminal INPUT may experience a phase delay, prior to being provided to terminal OUTPUT, from a signal path partially through (a) transistors  402 -A and  402 -B and load  415 -A and  415 -B and (b) delay device  450  combined transistors  404 -A and  404 -B as well as load  415 -A and  415 -B. The phase delay of an input signal in this scenario may be between those for low and high values of the control signal. 
     FIG. 5  depicts one suitable implementation of delay device  450 . In accordance with an embodiment of the present invention, delay device  450  may delay signals provided at input terminals (terminal INPUT) of ring element  400 . One implementation of delay device  450  may include transistors  502 -A and  502 -B, load  504 -A, load  504 -B, and current source  506 . This implementation is merely illustrative and other devices can be used to provide a delayed signal path. For example, other combinations of circuit elements can be used to provide a delayed signal path. 
   Transistors  502 -A and  502 -B may be implemented as MOSFET transistors although various other types of transistors may be used, such as BJT and HBT types. Gate terminals of transistors  502 -A and  502 -B (shown as respective terminals G and H) may be coupled to receive an input signal from respective terminals A and B of ring element  400 . Load  504 -A may couple a high supply voltage to a drain terminal of transistor  502 -A whereas load  504 -B may couple a high supply voltage to a drain terminal of transistor  502 -B. Loads  504 -A and  504 -B may be implemented as impedance elements such as resistive elements. Drain terminals of transistors  502 -A and  502 -B may provide respective terminals I and J. Source terminals of transistors  502 -A and  502 -B may be coupled to current source  506 . 
   In one implementation, terminals G and H of delay device  450  may be coupled to respective terminals A and B of ring element  400 . In one implementation, terminals I and J of delay device  450  may be coupled to respective terminals C and D of ring element  400 . 
   When ring element  500  is used in a ring formation similar to that described with respect to  FIG. 1A , the frequency of signal output by the ring (Vout) may be represented as follows:
 
Ring frequency=1/(2 T *number of ring elements  500 ), where
     T=total delay through a ring element  500 
 
In one implementation, T low &lt;T&lt;T high , where T low  may represent the lowest amount of phase delay a ring element  500  introduces (e.g., when the control signal is in a high state) and Thigh may the highest amount of phase delay a ring element  500  introduces (e.g., when the control signal is in a low state). The minimum T low  may be determined by a minimum phase delay through ring element  500 . A maximum Thigh may be determined by a maximum delay that can be accomplished without deteriorating the VCO&#39;s center frequency.
   

   Accordingly, one advantage of some embodiments of the present invention may be that when a ring element  500  is used in a VCO ring formation, the frequency of the signal provided by the VCO may be controlled using a control signal as opposed to manually adjusting capacitance values as in the prior art. A VCO tuning range, which is defined as the maximum frequency deviation the VCO can be tuned without significant degradation in its performance, can be maximized. Adjusting the frequency of oscillation in this method may increase the linearity of gain variation. 
   Modifications 
   The drawings and the forgoing description gave examples of the present invention. The scope of the present invention, however, is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of the invention is at least as broad as given by the following claims.