Abstract:
A method and an apparatus for a duty-cycled injection locked oscillator is provided for frequency shift keyed (FSK) signal transmissions. The oscillator includes a resonance LC tank and a first switching device. The first switching device is coupled to the resonance LC tank and injects an initial current pulse with a predetermined pulse magnitude into the resonance LC tank. The initial current pulse also fixes an initial phase of the duty-cycled injection locked free-running oscillator in response to the predetermined magnitude of the initial current pulse to enable fast settling of injection locking and high data rate operation of the duty-cycled injection locked oscillator. The oscillator also includes a second switching device, such as a differential pair of switching devices. The second switching device is coupled to the LC resonance tank for injecting a gated periodic reference signal having a duty cycle modified to reduce power of the reference signal by approximately seventy-five per cent.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to ultra low power, wideband oscillators and more particularly relates to oscillators for low-power frequency shift-keying (FSK) transmitters. 
     BACKGROUND OF THE DISCLOSURE 
     In biomedical applications, tethering wires impose significant restrictions on the subject under investigation and limits the free movement. Therefore, a wireless transmitter is usually preferred to send out electro-biophysiological signals. For example, in the neural signal recording of a free-moving live subject, the recorded signal could substantially differ from that of a movement-restricted subject. In order to have concurrent access to multi-channel information in neural signal recording applications, the required transmission data rate is approximately 100 Mbps with 100 channels, even when on-the-fly signal processing and time-domain multiplexing techniques are applied. 
     Power consumption is also a key constraint to wireless transmitters in biomedical applications, especially when the transmitter is implanted. When powered by a battery or by wireless telemetry, the transmitter is designed to consume low power to avoid frequent battery replacement or excessive exposure of live subjects to electromagnetic waves. However, high data rate communications is difficult to realize in a limited power budget environment. For example phase-locked loop based oscillators typically are limited to generation of low data rate signals and require high power consumption. In conventional transmitters for low power biomedical applications, power consumption may be reduced by employing open-loop frequency synthesis techniques, such as open-loop voltage controlled oscillators. Such techniques, though, result in the generated frequency being inaccurate and unstable over process, voltage, and temperature variations, making demodulation at the receiver side more difficult. 
     Low power yet accurate frequency synthesis can be obtained through known injection locking techniques. For example, a free-running oscillator will lock to the fundamental or harmonics of an injected reference signal under the condition that the targeted harmonic is within the locking range of the oscillator. The major benefits of injection lock LC oscillators include low phase noise and low power consumption. However, the use of injection lock LC oscillators is limited to low data rate communications, even though LC oscillators are preferred for better phase noise performance. One major problem with injection lock oscillators is the variable locking time, which could be as long as three microseconds. 
     In biomedical applications, frequency-shift keying (FSK) modulation schemes are preferred due to their inherent superior performance in bit-error rate (BER) and interference rejection. The problem of indefinite locking time, however, still exists. For FSK modulation, conventionally the reference signal is generated by hopping from one frequency to another. 
     Thus, what is needed is a method and apparatus for low-power transmission of signals at a high data rate. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure. 
     SUMMARY OF THE INVENTION 
     According to the Detailed Description, a method for fixing the initial phase of a free-running oscillator is provided for low power high data rate frequency shift-keying (FSK) communications. The method includes injecting an initial current pulse into the resonant LC tank of the free-running oscillator. The initial current pulse has a predetermined magnitude. The method also includes locking the free-running oscillator in response to a relationship between the predetermined magnitude of the initial current pulse and a phase of the free-running oscillator. 
     In addition, a method for signal generation as target harmonics of the free running LC oscillator for injection lock is provided. The method includes injecting a gated reference current signal into a resonance LC tank of the free running oscillator. The method further includes varying a duty cycle of the gated reference current signal to reduce power of the gated reference signal while maintaining substantially equivalent strength of injected harmonics. 
     Further, a duty-cycled injection locked oscillator is provided for frequency shift keyed (FSK) signal transmission. The oscillator includes a resonance LC tank and a first switching device. The first switching device is coupled to the resonance LC tank and injects an initial current pulse with a predetermined pulse magnitude into the resonance LC tank. The initial current pulse also fixes an initial phase of the free-running oscillator in response to the predetermined magnitude of the initial current pulse to enable fast settling of injection locking and high data rate operation of the duty-cycled injection locked oscillator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with the present invention. 
         FIG. 1  is a circuit diagram of a conventional differential complementary cross-coupled LC oscillator; 
         FIG. 2  is a signaling diagram of a duty-cycled transmitter output for frequency shift-keyed (FSK) transmissions in accordance with the present embodiment; 
         FIG. 3  is a block diagram of an injection-locked transmitter for burst mode FSK transmission in accordance with the present embodiment; 
         FIG. 4  is a circuit diagram of a duty-cycled harmonic injection-locked differential oscillator in accordance with the present embodiment; 
         FIG. 5  is a circuit diagram of a single-ended representation for the differential oscillator of  FIG. 4 ; 
         FIG. 6  is depicts timing diagrams for operation of the duty-cycled injection locked oscillator of  FIG. 4  in accordance with the present embodiment; and 
         FIG. 7  is a graph depicting the relationship between the initial phase of a free-running oscillator and the logarithm of the magnitude of the initial current pulse of the oscillator of  FIG. 4  in accordance with the present embodiment. 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and do not necessarily include any non-essential elements of the transmitter. For example, the power amplifier and antenna of the present embodiments are only depicted in the block diagram of  FIG. 3  and omitted in other figures. Those skilled in the art with the information disclosed herein will understand that the remaining circuitry can be designed in accordance with any of a number of conventional schemes. 
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
     Open-loop free-running oscillators are typically used to reduce power consumption in transmitters for biomedical applications. Referring to  FIG. 1 , a circuit diagram of a conventional complementary cross-coupled LC oscillator  100  is depicted. Such oscillators are typically used in biomedical transmitters because of its advantages of low power consumption and low phase noise. The LC oscillator includes coil L  102  and tunable capacitor banks  2 C  104 . Without locking to any reference signal, the oscillator  100  can run at a desired frequency by tuning the capacitor banks  2 C  104  in response to signal V DATA  on line  106 . In biomedical applications, a frequency shift-keying (FSK) transmission scheme is usually adopted as the modulation scheme for the oscillator  100  because of the excellent bit-error rate (BER) performance and interference rejection, and the FSK modulation is realized by tuning the capacitor banks  2 C  104 . The resonance tank of the coil L  102  and the capacitor banks  2 C  104  is accomplished by cross coupling transistor pair MN0 ( 108 ) and MN1 ( 110 ) and cross coupling transistor pair MP0 ( 112 ) and MP1 ( 114 ), while the LC oscillator is biased by current source I b    116 . 
     While low power consumption could be achieved by the oscillator  100 , it has a major disadvantage. Due to variations of process, voltage, and temperature, the oscillation frequency drifts over time. The frequency inaccuracy and instability makes demodulation difficult at the receiver side and increases the burden of the receiver. 
     The present embodiment provides a promising candidate for low power yet accurate frequency synthesis utilizing the injection locking technique. The major benefits of injection lock oscillators include low phase noise and low power consumption. And a free-running oscillator will lock to the fundamental or harmonics of an injected reference signal when the targeted harmonic is within the locking range of the oscillator. Referring to  FIG. 2 , a signaling diagram  200  of a duty-cycled transmitter output for frequency shift-keying (FSK) transmissions in accordance with the present embodiment is depicted. The signaling diagram  200  depicts burst-mode operation of an FSK transmitter where the burst-mode output signal  202 ,  204  for each data element  206 ,  208  occupies only a fraction of the bit period  210 . With the output pulse width of T p    212  and bit period of T f    210 , the duty cycle of the transmitter output can be calculated as
 
η= T   p   /T   f ×100%  [1]
 
     Referring to  FIG. 3 , a block diagram illustrates an injection-locked transmitter  300  for burst mode FSK transmission in accordance with the present embodiment. The transmitter  300  operates in burst mode to save power. An initial start-up pulse on line  302  quickly starts up an oscillator  304  and fixes the phase of the free-running oscillator  304 , including active oscillator elements  305 . With the knowledge of the phase of the free-running oscillator  304 , a reference current signal can be injected on line  306  at the optimum timing to achieve fast locking. The optimum timing is accomplished by providing an injection reference signal  307  to a duty-cycle tuning block  308 . 
     The output of the duty-cycle tuning block  308  is provided to a reference signal gating block  310  which generates the reference current signals for injection on lines  306  into the oscillator active elements  305  at optimal times. Meanwhile, a gating signal is provided to the reference signal gating block  310  on line  312 . In accordance with the present embodiment, the reference signal has a duty cycle of 33.33% instead of a conventional 50% duty cycle. The modified duty cycle of the reference signal reduces the required power of the reference signal and produces balanced output power. The gating signal is also applied on line  314  to the oscillator active elements  305  and turns the oscillator  304  on and off to realize the burst mode operation. 
     In order to achieve FSK modulation, the data on line  316  sets a frequency band of the free running frequency in band select block  318  of the oscillator  304  close to one of two harmonics of the reference signal such that one of the harmonics always falls into the locking range of the free-running oscillator  304 . The signals produced by the oscillator V out  on lines  320  are amplified and sent to power amplifiers  322  and an antenna  324  for transmission. One major contribution to the advantageous operation of the FSK transmitter  300  in accordance with the present embodiment is that an initial current pulse is applied to fix the initial phase of the free-running oscillator. This current pulse also serves the purpose of enabling quick start-up of the oscillator and, with the known initial phase of the free-running oscillator, the injection reference signal has a target to quickly lock to. Therefore, the variable settling time in prior art solutions can be avoided and high data rate can be achieved. In addition, the adjusted duty cycle of the injection reference signal obtains two-fold benefits. One benefit is the power of the injected reference signal is reduced by approximately 75% with low orders of harmonics. And the other benefit is that harmonics of equal power can be obtained such that the transmitter  300  has the same locking condition and balanced power levels at the two different harmonic frequencies. 
     A circuit diagram of a duty-cycled harmonic injection-locked oscillator  400  in accordance with the present embodiment is shown in  FIG. 4 . The oscillator  400  is a differential implementation of the injection lock oscillator for the FSK transmitter where both the oscillator  400  and the injection reference signal  402  are in differential modes to minimize the effect of power supply variations. The burst mode is realized by the signal Rst, which turns the oscillator and injection signal on and off by turning on and off switching devices  404 ,  406 , and  418 . When there is a data bit to be transmitted, the Rst signal is set to low and the tail current I b  is supplied to start up the oscillator. The data bit also selects a proper capacitor bank  408  for FSK operation as is well-known to those skilled in the art of conventional open-loop oscillators for FSK transmission. The switching device  406  is set to open to allow oscillation to build up in the oscillator  400 . When the burst mode data transmission is completed, the signal Rst is set to high. In response thereto, tail current I b  is cut off and the oscillator differential output terminals V outp  and I outm  are shorted to speed up the turning off of the oscillator  400 . 
     The free-running frequency of the oscillator  400  is set by the capacitor banks  408  to ensure the 4th and 5th harmonics of the reference signal are always within the locking range of the oscillator  400 . The strength of the initial current pulse is set by the current source I b,init    410  and operated by a switching device  412 , while the strength of the injection reference signal is determined by the current source I b,inj    402  and injected in a differential mode by the cooperative operation of a switching device  414  and a switching device  416 . The differential injection signal V injp  and V injm  to the respective switching device  414  and  416  each has a duty cycle of 33.33%, instead of 50%. The start up of the oscillator  400  is speed up by the initial current pulse supplied through the switching device  412  and the phase of the oscillator  400  during free running is determined by the magnitude of the initial current pulse. The initial current pulse extends operation of the oscillator  400  to high data rate applications. 
     Referring to  FIG. 5 , a circuit diagram of a single-ended representation  500  for the differential duty-cycled injection locked oscillator  400  is depicted with an LC tank  502  for oscillating the signal and a switching device  504  for injecting the current pulse  402 . The switching device  404  and the switching device  412  operate similarly to that described above. 
     Signal timing diagrams for the single-ended alternate of embodiment  400  for burst mode injection lock oscillation is shown in  FIG. 6 . Taking the desired operating frequency as the fourth harmonic of the reference signal as an example, in accordance with the present embodiment, the data sets the capacitor bank such that the fourth harmonic  600  of the injection reference signal  612  is within the locking range of the oscillator  400 . At time t A    602 , the signal Rst  604  is disabled such that the oscillator starts to build up oscillations by noises in the circuit, the noises being on the order of nano-volts. At time t B    606 , a current pulse  608  is injected into the oscillator with magnitude of I b,init    610 . As the injected current pulse has a magnitude many orders of magnitude higher than the noise, the state of the oscillator is over-written. When the oscillator reaches steady state, the phase of the free-running oscillation is linearly related to the logarithm of I b,init , as shown in a graph  700  in  FIG. 7 . The graph  700  depicts the relationship  702  between the initial phase of a free-running oscillator (plotted on the x-axis  704 ) and the logarithm of the magnitude of the initial current pulse of the oscillator  400 ,  500  (plotted on the y-axis  706 ) in accordance with the present embodiment. 
     Referring back to  FIG. 6  and utilizing conventional fast-locking oscillation techniques, with the phase of the free-running oscillator fixed by I b,init , one can inject a reference signal V inj    612  at an optimum timing  614  to for fast locking the oscillator  400 ,  500 . The gated periodic injection signal V inj  with a duty cycle of η′ and period T inj  can be decomposed into a one-sided exponential Fourier series as 
                     V   inj     =       ∑     n   =   0     ∞     ⁢       c   n     ⁢     ⅇ     j   ⁢           ⁢   n   ⁢       2   ⁢   π       T   inj       ⁢   t                   [   2   ]               
where c n  is the exponential Fourier series coefficients.
 
     In conventional injection lock oscillators, the duty cycle of the injection reference signal is 50% and, in order to have frequency separation of approximately 100 MHz and operating frequencies of 400 MHz to 600 MHz for FSK operation, the ninth and eleventh harmonics of a fundamental 54.24 MHz can be used, resulting in harmonic coefficients of magnitude 0.1415 and 0.1157 respectively. In accordance with the present embodiment, the duty cycle of the injected reference signal is adjusted to 33.33%, nearly doubling the harmonic coefficients. This in turn reduces the injection signal power by approximately 75% while maintaining the same harmonic strength of the injection reference signal. More importantly, as the power of the injected harmonic directly adds into the oscillator, the equal power levels of the fourth and the fifth harmonics make the power levels of the oscillator output equal at the fourth and fifth harmonics. Subsequently, the transmitted output power levels are the same at the fourth and fifth harmonics of the reference signal, relieving the receiver for demodulation. At time t C    614 , the reference signal is injected for fast locking. Subsequently, the oscillator is locked to the fourth harmonic of the reference signal. Finally at time t D    616 , the signal Rst  604  is enabled and the oscillator is turned off. 
     In accordance with the present embodiment, a duty-cycled harmonic injection locked oscillator  400 ,  500  for low-power high-data rate burst-mode FSK transmission is provided which advantageously achieves constantly fast locking instead of variable locking time. As seen in the description hereinabove, an initial current pulse is injected to fix the initial phase of the free-running oscillator. The initial phase of a free-running oscillator in steady-state is linearly related to the logarithm of the magnitude of the current pulse. With the initial phase of the free-running oscillator fixed, constantly fast locking time is always achieved, leading to high data rate transmission. 
     Another major advantage of oscillator  400 ,  500  in accordance with the present embodiment is that the power of the injection reference is reduced by approximately 75% and equal-power output of FSK transmissions is achieved by modifying the duty cycle of the injection reference signal. 
     Thus it can be seen that a burst-mode injection-locked LC oscillator has been provided for FSK transmission in neural signal recording applications. Injection locking is used to provide accurate and stable output frequencies. An initial current pulse is injected to the oscillator to quickly start up the oscillator and to fix the initial phase of the free-running oscillator. Oscillation frequency of the free-running oscillator is tuned by a capacitor array, and with the knowledge of the initial phase and free-running frequency, a reference signal is injected at the optimum timing for fast locking. The duty-cycle of the reference signal is tuned to 33.33% to reduce its power by 75% and to provide harmonics of equal power thereby enabling the FSK transmitter  400 ,  500  in accordance with the present embodiment to achieve a high data rate of 54.24 Mbps with measured bit energy efficiency of 62 pJ/bit. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, dimensions, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the steps for fabrication and elements of the apparatus described in the exemplary embodiments without departing from the scope of the invention as set forth in the appended claims.