Abstract:
Each starved-inverter of a voltage controlled ring oscillator has an output transfer gate associated therewith. The pair of complementary switches composing a transfer gate being controlled in common with the relative current generators of the starved-inverter stage, by a frequency control voltage and by a voltage difference between a supply voltage and the control voltage, respectively. The frequency produced by the oscillator is linearly proportional to the control voltage and inversely proportional to the square root of the supply voltage, for an enhanced noise immunity and improved frequency stability.

Description:
TECHNICAL FIELD 
     The present invention relates to a voltage controlled ring oscillator (VCO) with a wide frequency range and enhanced noise immunity, particularly suited for implementing phase locked loop (PLL) control systems. 
     BACKGROUND OF THE INVENTION 
     Phase locked loop systems (PLLs) are often integrated in very large scale integration (VLSI) devices, typically in integrated circuits (ICs) for application specific ICs (ASICs), microprocessors, and the like. 
     PLLs are often used as frequency synthesizers capable of generating square wave frequencies that may vary from 10 to over 200 MHz starting from an input clock signal with a frequency commonly comprised between 1 and 4 MHz. 
     PLLs are also used for square shaping or reshaping clock signals, for recovering digital data, etc. Whichever the type of application, PLLs must have a short-term instability of the output frequency (output jitter) as low as possible. In other words, they must be highly immune to causes of short-term instabilities, in other words, to high frequency noise. 
     The presence of jitter is highly detrimental in VLSI applications because the output square wave produced by the PLL system has a degree of uncertainty of the rise and fall fronts, which, if used as system clock signal, may negatively affect systems because of a reduction of the time-scale margins for a correct handling of data and of the data retention time. 
     Very often PLL systems must ensure a maximum instability of the switching fronts of the output signal below about 0.5 nanoseconds (ns), notwithstanding the fact that they must function in a relatively noisy &#34;environment.&#34; 
     Short-term instability (jitter) in VCO, realized in an integrated circuit using VLSI CMOS technology, is primarily caused by the high frequency noise that is generated within the integrated circuit and which is injected in the VCO of a PLL through the supply rails Vdd and ground, as well as through the control voltage (Vc) line of the output frequency generated by the VCO. 
     A block diagram of a frequency synthesizer based on a PLL employing a voltage controlled oscillator (VCO) is depicted in FIG. 1. 
     Notwithstanding that the behavior of a phase locked loop is in some measure similar to that of an adaptive filter and that therefore the long-term instabilities of the input clock signal are effectively filtered, short-term instability caused by the noise coming from the supply rails and from the control voltage line remains a difficult problem to be solved, especially in VLSI devices. 
     Among the circuit blocks that comprise a PLL, the phase and frequency detector (PFD) and the low pass filter are intrinsically immune to short-term instabilities, while the frequency divider (1/N), generates a negligible jitter as compared with the high frequency noise that is normally present at the output of the VCO block. Therefore, it may be said that the control of jitter in a PLL requires a voltage controlled oscillator having a high immunity toward high frequency noise. 
     Substantially, most of the times in VLSI applications, the VCO employed for implementing a PLL consists of a voltage controlled ring oscillator. A ring oscillator offers a high gain and a great stability in a relatively simple and least burdensome way. 
     A typical architecture of a VCO is depicted in FIG. 2. The VCO is implemented by a plurality (odd number) of inverting (delay) stages connected in cascade, each delay stage commonly being a so-called starved-inverter comprising transistors M1, M2, M3 and M4. Each inverting stage is often followed by a Schmitt trigger circuit S1 for providing a partial filtering of short-term frequency instabilities. 
     In a starved-inverter, the transistors M1 and M4, controlled by the output signals produced by the voltage-current control converter, act as current sources, while the transistors M2 and M3 work essentially as digital switches by enabling the source and sink currents. Therefore, the node n1 is alternately charged and discharged, thus causing the switching of the output Schmitt trigger S1 associated with the inverting stage when its triggering thresholds (in the two switching directions) are crossed. The signals propagate through the N inverting stages of the oscillator producing a square wave output signal F-OUT. 
     Because of the hysteresis toward the input voltage signal switchings, the use of a Schmitt trigger at the output of each inverting stage of the ring oscillator tends to reduce the instability of the switching point of the inverting stage. 
     However, the noise that is injected through the control voltage line Vc as well as through the supply rails Vdd and GND, in practice, modulates the switching thresholds of the various inverting (delaying) stages that compose the ring oscillator thus causing a jitter of the switching fronts of the output signal. 
     Moreover, in known VCOs the output frequency varies linearly with the supply voltage and the power supply rejection (PSR) is intrinsically poor. Furthermore, the frequency produced by the VCO increases with the supply voltage and this makes the transfer function of the PLL (and therefore its stability) strongly dependent on the operating voltage unless effective but costly voltage regulation circuits are implemented. Notwithstanding the use of a Schmitt trigger at the output of each inverting delay stage of the ring oscillator, the amount of noise that is effectively filtered out is relatively modest. In a circuit working at 5V, a PSR of 10%/V is normal in known systems. 
     There is clearly a need for a voltage controlled oscillator (VCO) that, though being based on a ring oscillator architecture, offering a relative intrinsic stability, high gain and sturdiness with a relatively modest and least burdensome circuit complexity, has a markedly reduced sensitivity to supply voltage variations and an enhanced hysteresis of each inverting stage so as to ensure a higher rejection of high frequency noise. The circuit should remain easily integrable in CMOS technology. 
     SUMMARY OF THE INVENTION 
     A method for markedly improving the ability to reject disturbances coming from the supply rails and from the control voltage line of a voltage controlled ring oscillator has now been found and represents an object of the present invention. 
     Basically the method consists in making the output frequency produced by the oscillator directly proportional to the control voltage and inversely proportional to the square root of the supply voltage. Such an objective may be implemented by employing in place of a Schmitt trigger at the output of each inverting (delaying) stage of the ring oscillator as commonly done in the prior art VCOs, a transfer gate composed of a pair of parallel complementary switches, between the output of each inverting stage and the input of the inverting stage that follows in the chain of cascaded stages and by controlling a switch of the pair constituting the transfer gate in common with the relative current generator of the starved-inverter that constitutes the inverting stage by the control voltage and the other switch of the pair and the other current generator by the voltage difference between the supply voltage and the control voltage. 
     A further advantage deriving from such an embodiment of the invention is that an exceptionally high hysteresis is produced in each inverting stage of the ring oscillator, which may reach up to 80%-90% of the supply voltage, thus further increasing high frequency noise rejection ability. 
     Differently from the known architecture, the voltage controlled oscillator of the invention does not require a voltage-to-current converter for driving a first starved-inverter of the chain of inverters. According to the invention, the first inverting stage may be in practice a simple unit gain voltage shifting stage driven directly by the control voltage signal, while all the following inverting (delaying) stages (in an even number) are customarily starved-inverters, each followed by a transfer gate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The different aspects and advantages of the invention will become even more evident through the following description and analysis of an important embodiment and by referring to the annexed drawings, wherein: 
     FIG. 1 is a block diagram of a conventional PLL employing a VCO; 
     FIG. 2 shows the structure of a ring type VCO, according to a known technique; 
     FIG. 3 is a diagram of a voltage controlled ring oscillator made according to the present invention; 
     FIGS. 4a and 4b show the internal voltages of the VCO of the invention according to a simulated operation; and 
     FIG. 5 shows characteristic curves of the VCO of the invention simulated for different supply voltages. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the inventive VCO 2 in FIG. 3, the VCO&#39;s control circuit, responsive to a control voltage signal Vc, basically comprises of a first inverting stage in the form of a unit gain voltage shifter input stage 10, formed by a pair of complementary transistors MVN and MVP. 
     This input stage 10 outputs a voltage difference between the supply voltage Vdd and the control voltage Vc. 
     The VCO 2 further comprises a plurality of delaying inverting stages 12 of the so-called starved-inverter type, in an even number, an output driver 14 and a control logic circuitry 16 (Reset, Test By-pass Logic) functionally coupled in the feedback line of the ring oscillator. The control logic circuitry 16 may be similar to the control blocks that are normally employed in ring type VCOs for implementing a reset, stop and by-pass function for performing tests on the integrated circuit and need not be described herein. 
     The input stage 10 is preferably made by a pair of complementary transistors MVN and MVD designed so as to have an identical transconductance for ensuring a unit voltage gain. The input stage 10 shifts the control voltage Vc so that the following delaying inverting stages 12 cascaded therefrom which form the ring oscillator are controlled in their n-channel sections directly by the control voltage Vc and in their p-channel sections by a voltage difference between the supply voltage and the control voltage (Vdd-Vc). 
     An output transfer gate 18 composed of a pair of complementary transistors functionally connected in parallel is coupled to the output of each delaying inverting stage 12 that follows the first inverting stage represented by the unit gain voltage shifter input stage 10. The delaying inverting stages 12 may be in an even number so that the total number of inverters in the reaction loop of the oscillator is odd. The complementary transistors that form each output transfer gate 18 are driven in common with the respective current generators of the delaying inverting stage 12 to the output of which the transfer gate is associated, namely: by the control voltage Vc for the n-channel section and by the voltage difference Vdd-Vc for the p-channel section, respectively. 
     With reference to the diagram of FIG. 3, an analysis of the operation of the VCO 2 of the invention is hereinbelow reported. The analysis refers to a first delaying inverting stage 12, that follows the unit gain inverting stage that constitutes the control voltage shifting input stage 10, the load of which is represented by the pair of transistors MjN and MjP of a following delaying inverting stage 12. 
     As said above, the transistors MVN and MVP that form the inverting control voltage shifting input stage 10 (having a unit voltage gain) are designed so as to have the same transconductance, that is: 
     gm MVN  =gm MVP   
     This identity of transconductance may be considered verified in the range of variation of electrical parameters due to the process spread and of the operating temperature. 
     Therefore, in first approximation, MSN and MSP are both biased with a same |IVGS|. In fact, MSN is biased by the control voltage Vc, while MSP is biased by the difference Vdd-Vc, both referred to ground potential. 
     The allowed range of variation for the control voltage Vc may be set as: 
     0.25&lt;Vc/Vdd&lt;0.66 
     By assuming the node A charged to Vdd, the node B is also charged to Vdd and therefore the transistor M1P is ON while the transistor M1N is OFF. 
     In these conditions, MTN is OFF and MTP is ON. When the reaction node of the ring oscillator switches, M1N conducts and MSN starts to discharge the node A by a constant current. The voltage on the node B tracks the voltage of the node A. 
     When: 
     
         V(A)=V(B)=Vdd-Vc+|V.sub.TP |=V1 
    
     where the V TP  is the threshold voltage of the transistor MTP, the transistor MTP turns OFF too, thus the node B is practically disconnected from the node A and clamped to the voltage V1. The transistor MSN continues to discharge the node A through a linear voltage ramp. 
     When: 
     
         V(A)=Vc-V.sub.TN =V2 
    
     where V TN  is the threshold voltage of the transistor MTN, MTN rams ON, the charge distributes itself over the nodes A and B, the voltage V(B) is pulled down toward the voltage V(A) and finally both nodes A and B assume the same potential, which is much lower than the switching threshold of the following inverting stage. It should be noted that the N delaying inverting stages 12 are identical in structure to that of the first delaying inverting stage 12 and thus need not be described in detail herein. Therefore, the transistors (MjT and MjN . . . ) in the output transfer gate (not shown) of each of the successive delaying inverting stages 12 switch, thus propagating the signal through the chain of delaying inverting stages of the VCO 2. 
     The voltage swing of the signal in the first delaying inverting stage 12, before the following delaying inverting stage 12 switches, is therefore equal to Vdd-V2. 
     It may be demonstrated through similar deductions that when the node A starting from ground potential reaches the voltage Vdd, the clamp voltage V1 is given by: 
     
         V(A)=V(B(=Vc-V.sub.TN =V1 
    
     while charge distribution takes place when: 
     
         V(A)=Vdd-Vc+|V.sub.TP |=V2 
    
     the voltage swing being equal to V2. 
     In conclusion, the output signal swing of the first delaying inverting stage 12 of the VCO 2, before the following delaying inverting stage switches, in either direction, (rise and fall of the voltage on the node V(A)) is given by: 
     
         ΔV=Vdd-Vc+V.sub.T 
    
     on account of the assumption that: 
     
         V.sub.TN =|V.sub.TP |=V.sub.T 
    
     The delay of propagation through the N+1 inverting stages that form the reaction loop of the VCO 2 is given by: 
     
         T.sub.VCO =(N+1)C.sub.load ΔV/I .sub.o 
    
     wherein I o  is the charge/discharge current through MSN and MSP, that is: 
     
         I.sub.o =β(VC-V.sub.T).sup.2, where β=β.sub.MSN =β.sub.MSP 
    
     considering that MSN and MSP are designed to have the same gain, the output frequency of the VCO is given by: 
     
         f.sub.VCO =1/2T.sub.VCO =const.×C.sub.load.sup.-1 x(Vc-V.sub.T).sup.2 /(Vdd-Vc-V.sub.T) 
    
     where C load  is essentially the drain junction capacitance of M1N, M1P, MTN and MTP. The gate capacitances of transistors MjN and MjP are effectively decoupled from the node A and therefore they do not contribute to C load . 
     By assuming a step-junction profile, the following expression may be written: 
     
         C.sub.drain (V)=const. ×(1-V/φ).sup.-0.5 
    
     where φ is constant dependent from the fabrication process. 
     In first approximation, C load  corresponds to the above indicated drain capacitance, averaged over ΔV, that is: 
     
         C.sub.load =const.×((1+ΔV.sub./φ).sup.0.5 -1)/Vdd 
    
     Therefore, 
     
         f.sub.VCO =const. ×[Vdd/((1+ΔV/φ).sup.0.5 -1)]×(Vc-VT).sup.2 /ΔV 
    
     By considering the definition of the ΔV parameter, the last equation may be simplified as follows: 
     
         f.sub.VCO =const.×Vc/Vdd.sup.0.5 
    
     According to premises, the output frequency produced by the VCO 2 is a linear function of the control voltage Vc, while it varies with Vdd according to an inverse square root function of the supply voltage. 
     A number of advantages are obtained, including a high power supply rejection, a large hysteresis and an inverse relationship between frequency and supply voltage. The PSR is improved by using Vc and the voltage difference between Vdd and Vc as driving voltages for the output transfer gate 18. A variation of 10% of Vdd results in a variation of just about 3% of the frequency generated by the VCO 2. 
     A large hysteresis is achieved by virtue of the difference in rising and falling input thresholds at the nodes A and B, as discussed above. Assuming, for example, that Vdd=5 V, Vc=2 V and VT=1 V, the maximum ΔV excursion that may be reached in CMOS devices may be as large as 4 V. Such a large voltage swing tends to minimize short-term instability modulation of the output frequency produced by the oscillator (jitter modulation). 
     The inverse relationship between the frequency and the supply voltage helps in stabilizing the loop. In fact, the dependency from Vdd of the gain of the VCO 2 compensates the reduced gain of the phase and frequency detector (PFD) at low Vdd values. This is extremely advantageous by considering that the bandwidth of a PLL depends from the product of the two gains. 
     The operating parameters of the VCO 2 of FIG. 3, functioning at a nominal supply voltage of 5 V are shown as diagrams of the internal voltages in FIGS. 4a and 4b, and as a transfer characteristics of the VCO 2, for a supply voltage of 4.5 V, 5.0 and 5.5, respectively, in FIG. 5.