Abstract:
A negative resistance device for a multiphase oscillator is disclosed. The negative resistance device is coupled to taps of the multiphase oscillator so that it injects no energy into the oscillator when the oscillator is most sensitive to noise, thereby decreasing the phase noise of the oscillator. The negative resistance device also guarantees the direction of movement of a traveling wave past the taps of the multiphase oscillator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     1. Field of the Invention 
     The present invention relates generally to multiphase oscillators and more specifically to active devices that maintain the oscillation of such oscillators. 
     2. Description of the Related Art 
     Most electronic systems encounter noise from several sources, such as shot noise, thermal noise, and flicker noise, which usually arise from currents flowing in the electronic system. 
     Most electronic systems include one or more oscillators, which provide the clocks that pace or operate the system. Because oscillators are linear, time-varying (LTV) systems, electronic noise has unusual effects on oscillators. One way to characterize these effects is to determine the LTV unit impulse response h φ  for the oscillator. This function h φ  includes a key function called the impulse sensitivity function (ISF) Γ(ω 0 τ), which captures the essential information about the oscillator and allows one to compute the oscillator&#39;s response to a phase impulse. The ISF for an oscillator shows that the various noise sources mentioned above give rise to phase φ(t) modulation cos [ω 0 t+φ(t)] of the oscillator frequency ω 0 , which appears as a spectral “skirt” about the oscillator frequency ω 0 . This spectrum, actually a power spectral density            (Δω), is usually separated into three regions. The region nearest the oscillator frequency is the 1/f 3  region. At some point ω 1/f  (the 1/f noise corner), this region breaks into a 1/f 2  region, which continues until it reaches the noise floor of the system, which is a flat region at frequencies far distant from the oscillator frequency.
     Using the ISF, the power spectral density (PSD) for an oscillator in the 1/f 2  region can be shown to be 
                 ℒ   phase     ⁡     (   Δω   )       =     10   ⁢     log   ⁡     (     k   ⁢       Γ   rms   2       δω   2       ⁢         l   n   2     _       Δ   ⁢           ⁢   f         )       ⁢           ⁢   where   ⁢           ⁢         l   n   2     _       Δ   ⁢           ⁢   f               
is the power spectral density of the noise, Γ rms  is rms value of the ISF, and Δω is the offset frequency from the oscillator nominal frequency. The PSD for an oscillator in the 1/f 3  region is
 
                   ℒ   Phase     ⁡     (   Δω   )       =     10   ⁢     log   ⁡     (       kc   0   2     ⁢       ω     1   /   f         Δω   3       ⁢       i   n   2       Δ   ⁢           ⁢   f         )           ,         
where now the ISF is included in the c 0  term. These formulae for the power of the sidebands about the oscillator frequency indicate that there are both up and down frequency translations of circuit noise currents i n  into noise near the oscillator frequency. The 1/f 2  region results from down-converted white noise at integer multiples of the oscillator frequency and the 1/f 3  region results from up-converted 1/f circuit noise. The 1/f noise is particularly troublesome for some applications, but the presence of the c 0  term (the dc term) in the PSD function suggests that to minimize this noise, one should make the term as small as possible. One way of doing this is to arrange the circuitry so that disturbances during the rising edge in the oscillation are the same as those during the falling edge, thereby making the net disturbances over the cycle almost cancel out.
 
     Not only does the oscillator itself have a time-varying characteristic, but the oscillator can be subject to time-varying noise sources that vary in a periodic fashion. In fact, in a full analysis, another function called a noise modulating function (NMF) α(ω 0 t) modifies the ISF to create an effective ISF (ISF e ) that accounts for periodic time-varying noise. As with the other noise sources, one should attempt to find circuit topologies that minimize the effect of time-varying noise. One way to minimize the noise is to arrange circuitry so that time-varying noise sources have their maximum power at the minimum sensitivity point in the oscillator waveform. 
     Rotary wave oscillators are a particular type of oscillator, operating by means of a wave that travels in a closed loop that reverses the phase of the wave on every transit of the loop.  FIG. 1  illustrates this type of oscillator. The direction of the traveling wave around the closed loop can be either clockwise or counter clockwise unless some technique sets the direction of rotation. Such oscillators have the advantage that multiple phases are available by physically tapping the oscillator. However, for the taps to be useful, the one must know the direction of the wave along the taps. It is thus desirable to have control over the direction of the traveling wave. 
     In summary, it is desirable to have an oscillator with low phase noise to make it useful in a great variety of applications and it is desirable, in the case of a rotary wave oscillator, to control the direction of travel of the wave. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention is an amplifier for generating and maintaining a traveling wave on a RTWO. The amplifier is connected to the phases of the RTWO such that it turns off at the critical time when noise would otherwise be injected into the oscillator. 
     One of the advantages of the present invention is that it eliminates crowbar current. In particular, the dwell time of the amplifier eliminates crowbar current, which flows from supply to ground, thereby contributing noise power but not energy to the oscillator. 
     Another advantage is that it enforces the wave direction of oscillator of a rotary traveling wave oscillator. 
     Yet another advantage is that, with proper location of the amplifier phase taps, it reduces the sensitivity of an RTWO due to process variations. 
     The present invention thus reduces phase noise and provides a means for controlling the direction of the traveling wave in the oscillator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  shows a prior art rotary oscillator; 
         FIG. 2A  shows an amplifier for use in an embodiment of the present invention in which the amplifier is the negative resistance device; 
         FIG. 2B  shows an alternate version of the amplifier for use in an embodiment of the present invention; 
         FIG. 3  shows a portion of the transmission line on which a wave travels from left to right in the figure; 
         FIG. 4A  shows a timing diagram of the various waveforms involved in the present invention; 
         FIG. 4B , the inset in  FIG. 4A , illustrates the negative-going wave front and two times T 1  and T 2  at which the amplifier output impacts the wave front; 
         FIG. 5A  shows an equivalent circuit for the amplifier of  FIG. 2A  when a wave travels in the preferred direction; and 
         FIG. 5B  shows an equivalent circuit for the amplifier of  FIG. 2A  when a wave travels opposite to the preferred direction. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a prior art rotary oscillator  10 . The oscillator  10  includes a pair of conductors  14   a,b  connected to form a closed loop  12 , an odd number of cross-overs  18  that reverse the phase of a wave traveling on the conductors  14   a,b , and a plurality of regeneration devices  16  (of which  20   a,b  is one of many implementations) that create and maintain the traveling wave. Each of the regeneration devices  16  has negative resistance and operates to add energy to the traveling wave as it passes a device. For example, if the wave is traveling clockwise with a positive voltage on conductor  14   b  and has not reached  20   a,b , then the regeneration device  20   a,b  has a polarity opposite to the approaching wave. As the wave passes the regeneration device  20   a,b , the wave changes the state of the device  20   a,b  so that it has the same polarity as the wave, thereby adding energy to and reinforcing the wave. 
     Any negative resistance device that operates to add energy to the traveling wave will meet the requirements of a regeneration device for the rotary oscillator. 
       FIG. 2A  shows an amplifier  40  for use in an embodiment of the present invention in which the amplifier is the negative resistance device. In the version shown in  FIG. 2A , the amplifier  40  includes first  42  and second  44  p-channel transistors and first  46  and second  48  n-channel transistors. The p-channel transistors  42 ,  44  have their channels connected in series and between a first supply voltage V 1   50  and the output out  52 . The n-channel transistors  46 , 48  have their channels connected in series between a second supply voltage V 2   54  and the output out  52 . The gates of the second p-channel transistor  44  and the first n-channel transistor  46  are connected to a first input, in 1   56 . The gates of the first p-channel transistor  42  and the second n-channel transistor  48  are connected to a second input, in 2   58 . 
       FIG. 2B  shows an alternate version  60 ,  62  of the amplifier for use in an embodiment of the present invention. In the alternate version, the amplifier  62  at one position on the loop comprises two p-channel transistors  64 ,  66  whose channels connected in series between the first supply voltage V 1   50  and the output  52 . The gate of one transistor is connected to the first input in 1   56  and the gate of the other transistor is connected to the second input in 2   58 . At an adjacent position on the loop, up or downstream, the amplifier comprises two n-channel transistors  68 ,  70  whose channels are connected in series between the output  52 ′ and the second supply voltage V 2   54 . The gate of one transistor is connected to the first input in 1   56 ′ and the gate of the other transistor is connected to the second input in 2   58 ′. These two configurations thus alternate at each position on the loop, so that the traveling wave first encounters, say a p-channel amplifier  60 , then an n-channel amplifier  62 , and then a p-channel amplifier  60 . When the wave encounters a p-channel amplifier  60  it receives a boost that increases the wave&#39;s higher potential. When the wave encounters an n-channel amplifier  62 , it receives a boost that increases the waves&#39; lower potential. 
     Operation of  FIG. 2A   
       FIG. 3  shows a portion of the transmission line  14   a ,  14   b  (in  FIG. 1 ), on which a wave travels from left to right in the figure. Amplifier  40  has input  1  and input  2  connected to different positions Ph j    84 , Ph k    82  on conductor  14   b  and its output connected to a position  Ph 0     86  on conductor  14   a . The amplifier embodiment  40  in  FIG. 2A  operates as follows, adopting the convention that positive j and k values lag the wave front, while negative values lead it. If the wave front of a wave traveling in the direction from j to k is at Ph 0 , then the wave front arrives at Ph j  before it arrives at Ph k . Before the wave arrives at Ph j  or Ph k  on the RTWO, input  1  and input  2  are relative low voltages compared to their complementary voltages on the other conductor of the RTWO. This means that both p-channel transistors are on, sourcing current from the first supply voltage V 1  to the output  86 , which is at a voltage that is relatively high. 
     When a positive-going wave front V(Ph j ) arrives the Ph j  point  82  on the RTWO, input  2  of the amplifier  40  goes positive first. This causes one of the p-channel transistors ( 42  in  FIG. 2A ) in the amplifier  40  to turn OFF, and one of the n-channel transistors ( 48  in  FIG. 2A ) to turn ON. Because now there is no pair of transistors in amplifier  40  connected between a supply voltage, V 1  or V 2 , and the output  52 , the output  52  is in a high-impedance state, neither sourcing nor sinking current to the RTWO at position  86 . 
     When the positive-going wave front V(Ph k ) arrives at the Ph k  point  84  on the RTWO, now input  1  of the amplifier goes positive. This has the effect of turning ON both n-channel transistors ( 46 ,  48  in  FIG. 2A ) and turning OFF both p-channel transistors ( 42 ,  44  in  FIG. 2A ). The output of the amplifier V(  Ph 0   ) now sinks current to the second supply voltage V 2 , which reinforces the negative-going wave front at the point  86  (  Ph 0   ) on the RTWO to which the output of the amplifier  40  is connected. 
     Conversely, if we assume that the output of the amplifier is at a relative low voltage, then both n-channel transistors ( 46 ,  48  in  FIG. 2A ) are ON, sinking current from output  52  to the second supply voltage V 2 . Under this condition, both in 1  and in 2  are relatively positive. When a negative-going wave front arrives at the Ph j  point  82  on the RTWO, input  2  of the amplifier  40  goes relatively low. This causes one of the n-channel transistors ( 48  in  FIG. 2A ) to turn OFF and one of the p-channel transistors ( 42  in  FIG. 2A ) to turn ON. Because there is no pair of transistors connected between a supply voltage and the output, the output  52  is in a high-impedance state, neither sourcing nor sinking current to the RTWO at position  86 . When the negative-going wave front arrives at the Ph k  point  84  on the RTWO, input  1  goes relatively low, causing both n-channel transistors ( 46 ,  48  in  FIG. 2A ) to turn OFF and both p-channel transistors ( 42 ,  44  in  FIG. 2A ) to turn ON, thereby sourcing current from the first supply voltage V 1  to the output  52 . Again, this reinforces the positive-going wave front at the point  86  on the RTWO to which the output of the amplifier  52  is connected. 
       FIG. 4A  shows a timing diagram of the various waveforms involved. V(Phj) is the waveform of a positive-going wave at Phj, V(Phk) is a positive going wave a Phk, and V(Ph 0 ) is the output of the amplifier  40  at position  86  in  FIG. 3 . I(out p) is the output current through the p-channel transistors ( 42 ,  44  in  FIG. 2A ) and I(out n) is the output current through the n-channel transistors ( 46 ,  48  in  FIG. 2A ). I(out n+p) is the resultant output current from the amplifier  40 . ISF 2  is the impulse sensitivity function, α(ωt) is the cyclostationary noise function, and Γeff is the effective impulse sensitivity function. As is clear from the timing diagram, the output of the amplifier I(out n+p) has a dwell time τ pch    92 , which separates the time when the amplifier  40  is sourcing current from the time when the amplifier  40  is sinking current. It is the time when the amplifier  40  is in the high impedance state. It is also clear that the time when the amplifier  40  is in the high impedance state aligns with the ISF 2  for the oscillator, which is the time when the oscillator is most sensitive to phase noise. Therefore, arranging the inputs of the amplifier  40  to create a dwell time  92  reduces the phase noise of the oscillator, because it reduces the energy input to the oscillator during the time when the oscillator is most sensitive. The size of the cyclostationary noise source is also minimized, resulting in a lower effective ISF. 
     The size of the delay or dwell time τ pch  (Ph j −Ph k ) is adjustable, as shown in  FIG. 3 . The tap for Ph j  can be adjusted from Ph 4 , to Ph 3 , to Ph 2  while the tap for Ph k  can be adjusted to Ph −1  to Ph 0 , to Ph 1 . To be most effective in reducing noise, the time difference τ pch  between Ph j  and Ph k  should be on the order of the output transition time or rise and fall time of the wave front on the oscillator. This allows precise alignment of the amplifier&#39;s switching with the transition of the wave front at  Ph 0   . In fact, the goal here is to arrange the input connections so that the amplifier switches at a desirable point on the transition of the wave front at  Ph 0   .  FIG. 4B , the inset in  FIG. 4A , illustrates the negative-going wave front and two instants in time T 1  and T 2  at which the amplifier output can be designed to switch. Time T 1  is at a time above a detection threshold of the wave front and occurs when the amplifier switches after the wave front has passed Ph 0 . Time T 2  is a time below the detection threshold and occurs when the amplifier switches before or at the time the wave front passes Ph 0 . It is preferable to impact the negative-going wave front at a time T 1 , which means that it is preferable to connect the in 1  input of the amplifier to a point Ph k  after Ph 0 . This permits the oscillator to run at its natural oscillation frequency, which is that determined only by the transmission line. If the in 1  input is connected to a point before Ph 0  so that the switching of the amplifier occurs at T 2 , i.e., before Ph 0 , the oscillator is forced off its natural oscillation frequency, which consumes more power, and raises the noise floor. 
     Operation of  FIG. 2B   
     The amplifiers in  FIG. 2B  operate in a fashion similar to those of  FIG. 2A , except that the wave front encounters the p-type amplifier  60  in  FIG. 2B  and the n-type amplifier  62  in  FIG. 2B  at different times. In particular, for the p-type amplifier  60  when a positive-going wave front approaches input  2 , the amplifier  60  goes to a high impedance state. When the positive-going wave front reaches input  1 , amplifier  60  is shut off, thereby preventing any boost to the negative-going wave on conductor  14   a  when it reaches position  86  in  FIG. 3 . The wave then continues to the next amplifier, which is now an n-type amplifier  62  in  FIG. 2B . When the positive-going wave reaches input  2 , amplifier  62  is enabled and when the wave reaches input  1 , amplifier  62  is turned ON, thereby sinking current on the negative-going wave front traveling on conductor  14   a . This means that there are times during the traveling of the wave when a positive-going wave front is not reinforced, while the negative-going wave front is a short time later and vice-versa. An advantage of the configuration of  FIG. 2B  is that the capacitive load of the RTWO is reduced, thereby reducing RTWO power consumption. The configuration also simplifies the layout wiring of the RTWO. 
     Direction Control 
     To encourage a wave front to travel in a preferred direction, an additional constraint must be imposed on the amplifiers&#39; input connections. In particular, if the input connections are subjected to a constraint that input  2  is reached before input  1  and |Ph j −Ph 0 |&gt;|Ph k −Ph 0 |, then the wave front propagation from Ph j  to Ph k  in  FIG. 3  is encouraged, while a wave in the reverse direction is discouraged.  FIG. 5A  shows an equivalent circuit for the amplifier of  FIG. 2A  when a wave travels in the preferred direction. Having the positive difference between Ph j  and Ph 0  larger than that between Ph k  and Ph 0  provides sufficient time for the p-channel and n-channel transistors  42 ,  48  shown in  FIG. 5A  connected to the first V 1  and second V 2  supply voltages to reach very low resistance (resistance in the triode or linear region (R pON , R nON )) by the time the wave front reaches input  1 , after first passing input  2 . When the wave front reaches input  1 , the amplifier  40  then switches with maximum gain to maximally boost the wave. 
       FIG. 5B  shows an equivalent circuit for the amplifier of  FIG. 2A  when a wave travels opposite to the preferred direction. A wave front traveling in the reverse direction, upon reaching input  1  first, would turn ON the transistors in the amplifier  44 ,  46  connected to the output Ph 0 , as shown in  FIG. 5B . However, these transistors  44 ,  46  have high resistance Rp, Rn due to their position in the amplifier. When the wave front reaches input  2 , the amplifiers switch but with gain insufficient to maintain the wave. Therefore, waves traveling in the direction shown in  FIG. 3  are encouraged and waves traveling in the opposite direction are discouraged. 
     Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.