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CIRCUITRY WITH RESISTIVE INPUT
IMPEDANCE FOR GENERATING PULSES
FROM ANALOG WAVEFORMS
CROSS REFERENCE TO RELATED 5
This application is a continuation-in-part of U.S. application Ser. No. 09/429,527 for METHOD AND APPARATUS FOR GENERATING PULSES FROM ANALOG WAVEFORMS, filed Oct. 28, 1999, which is owned by the 10 Assignee of the present invention, and is herein incorporated by reference for all purposes.
This application is related to co-pending U.S. application Ser. No. 09/429,519 for A METHOD AND APPARATUS 15 FOR COMMUNICATION USING PULSE DECODING, filed Oct. 28, 1999 and to concurrently filed and co-owned U.S. application Ser. No. 09/805,854 for "METHOD AND APPARATUS TO RECOVER DATA FROM PULSES," both of which are owned by the Assignee of the present 2Q invention and are herein incorporated by reference for all purposes.
BACKGROUND OF THE INVENTION
This invention relates to a waveform to pulse conversion 25 technique and more particularly to techniques for converting arbitrary analog waveforms to sequences of pulses.
Pulse generators are well known, for example, for DC controllers and other applications. However, the typical pulse generator is an adaptation of a conventional oscillator 30 or monostable multivibrator, which can produce undesired or spurious transients when the pulse triggering is terminated. Such transients could be confused with trailing pulses, so that the counting of pulses is an inaccurate representation of the intended pulse count. 35
A typical pulse generator is responsive to a trigger related to a threshold level; that is, pulses are generated when a level input is applied that exceeds a predetermined threshold that has established a trigger point. The duration of the input above the threshold typically corresponds to the duration of 40 the pulse train or oscillation period.
A classic van der Pol (vdP) oscillator is a simple nonlinear oscillator circuit and is a useful starting point for use as a pulse generator. However, the classic vdP oscillator is not readily controlled.
U.S. application Ser. No. 09/429,527 discloses circuitry that controls oscillations. However, the disclosed circuitry is frequency dependent. In some situations, this is an undesirable property, especially when dealing with a signal that has 50 a broadband frequency spectrum. The power transfer to the circuitry for certain frequency range is better than others. Circuitry is needed which is simple and yet which addresses needs in specialized applications. There is a need for an improved controlled relaxation oscillator. 55
SUMMARY OF THE INVENTION
According to the invention, a circuit is provided for generating output pulses or oscillations in response to input analog waveforms. The circuit has a variable operating point 60 and a transfer function characterized by an unstable operating region bounded by a first stable operating region and a second stable operating region. The circuit is further characterized by having a resistive input impedance.
The invention will be better understood by reference to 65 the following detailed description in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings:
FIGS. 1A and IB show two types of transfer functions contemplated by the invention;
FIGS. 2 and 3 show circuit arrangements for forcing the operating point between stable and unstable regions of operation;
FIG. 4 shows an embodiment of a circuit arrangement according to the present invention;
FIG. 5 is the transfer function characterizing the circuit shown in FIG. 4;
FIG. 6 shows an alternate embodiment of a circuit arrangement according to the present invention;
FIG. 7 is the transfer function characterizing the circuit shown in FIG. 6; and
FIGS. 8 and 9 are signal traces illustrating the operation of the circuits shown in FIGS. 4 and 6, respectively.
DESCRIPTION OF THE SPECIFIC
Referring to FIGS. 1A and IB, the illustrative circuits contemplated by the present invention exhibit a transfer function having either an S-shaped appearance such as shown in FIG. 1A or the N-shaped appearance shown in FIG. IB. For the purposes of the present invention, the "transfer function" of a circuit refers to the relationship between any two state variables of a circuit. For example, electronic circuits are typically characterized by their I-V curves, relating the two state variables of current and voltage. Such curves indicate how one state variable (e.g., current) changes as the other state variable (voltage) varies. As can be seen in FIGS. 1A and IB, each transfer function 102 includes a portion which lies within a region 104, referred to herein as an "unstable" region. The unstable region is bounded on either side by regions 106 and 108, each of which is herein referred to as the "stable" region.
A circuit in accordance with the invention has an associated "operating point" which is defined as its location on the transfer function 102. The nature of the output of the circuit depends on the location of its operating point. If the operating point is positioned along the portion of the transfer function that lies within region 104, the output of the circuit will exhibit an oscillatory behavior. Hence, the region 104 in which this portion of the transfer function is found is referred to as an unstable region. If the operating point is positioned along the portions of the transfer function that lie within either of regions 106 and 108, the output of the circuit will exhibit a generally time-varying but otherwise nonoscillatory behavior. For this reason, regions 106 and 108 are referred to as stable regions.
In accordance with the present invention, any arbitrary waveform can be used. Illustrative example waveforms include sinusoidal, ramp, asymmetric, sawtooth, square and channel-optimized symbol. For example, sinusoidal waveforms with frequency fl and f2 are used to represent binary 0 and 1. It is desired to receive these two sinusoidal waveforms with equal amplitude. However, the channel will attenuate one frequency, say f2, more than it will to fl. Therefore, at the transmitter end, the amplitude of sinusoidal waveform with frequency f2 can be increased to compensate for the expected higher loss in the channel such that when the waveforms are received, the amplitude of the two sinusoidal waveforms are equal. The disclosed list of waveforms is not exhaustive, of course. Rather, it is intended to illustrate the fact that various waveforms can be used.
In a more general case, a mixture of different analog waveforms can be used. Thus, the information can be represented by a sine wave for a period of time, and then by 5 other waveformns at other times. In the most general case, it is possible to vary the waveform from one cycle to the next.
It is only required that there be a corresponding circuit which has stable regions and at least one unstable region as 10 described above, whose operating point can be selectively forced between the unstable and stable regions. In practice of course, appropriate channel-optimized waveforms would be selected to represent symbols for transmission.
At the receiving end, the received transmission is fed into 15 the input of a circuit as described above. The transmitted information can then be extracted (recovered) from the resulting oscillatory and non-oscillatory behavior of the circuit. Circuitry for such information recovery is disclosed in, but not limited, commonly owned, pending U.S. appli- 20 cation Ser. No. 09/429,527, entitled "Method and Apparatus for Generating Pulses From Analog Waveforms" and in commonly owned, concurrently filed, co-pending U.S. application 09/805,854 entitled "Method and Apparatus to Recover Data From Pulses", both of which are incorporated herein by reference for all purposes.
The advantage of the present invention lies in the ability to select some arbitrary combination of arbitrary analog waveforms to represent information. Such information can 3Q then be transmitted by selecting waveforms which are appropriate for transmission by conventional methods. Hence, any conventional transmission medium, wireless or wired, can be used with the invention.
A configuration for varying the operating point of a circuit 35 is described in the related application U.S. application Ser. No. 09/429,527, and is shown here as FIG. 2. The figure shows a circuit 202 having inputs 203 and 205. A capacitive element 204 is coupled at one end to an input 205. An arbitrary analog waveform source 210 is coupled between an 40 input 203 and the other end of capacitive element 204, thus completing the circuit. For the purpose of the discussion, circuit 202 has a transfer function which appears S-shaped. The circuit arrangement shown in FIG. 2 allows the slope of the arbitrary waveform generator 210 to move the operating 45 point of circuit 202 into and out of the unstable region 104. This action controls the onset of oscillatory behavior, and cessation of such oscillatory behavior, at the output of circuit 202 as a function of the output of arbitrary waveform generator 210. 50
Another configuration for varying the operating point of a circuit is also described in the related application U.S. application Ser. No. 09/429,527, and is shown here as FIG. 3. In this case, the circuit 302 has an N-shaped transfer function. In place of the capacitive element, an inductive 55 element 304 is provided. As with FIG. 2, an arbitrary analog waveform source 210 completes the circuit. Both FIGS. 2 and 3 are using op-amp to realize N-Shape or S-Shape I-V transfer functions. By a thorough investigation into the op-amp model, the unstable region corresponds to op-amp 60 linear operation and stable region corresponds to non-linear operation. Hence, both op-am linear and non-linear operations are required to form N-shaped or S-shaped transfer functions.
However, the input impedances of the foregoing oscilla- 65 tors are reactive. Thus, the input impedance seen by the source is frequency dependent. This effect is undesirable
since the analog waveform used as the source has a broadband frequency spectrum. Thus, in order to provide a flat response across the anticipated range of frequencies, broadband matching techniques must be used. However, broadband matching itself is a difficult process, adding undesired complexity and cost to the end system. Therefore, it was desired to attain a configuration having a resistive input impedance for operation in the linear and non-linear regimes as discussed above which still permitted driving the circuit in and out of its unstable region. Because of the dual requirement that these op-amp circuits operate both in linear and non-linear regimes, it is not obvious as how to find the resistive equivalent of the controlled oscillator circuits shown in FIGS. 2 and 3.
Turning to FIGS. 4 and 5, an illustrative circuit according to an embodiment of the invention exhibits a "controlled" relaxation oscillation behavior. In the context of the present invention, the term "controlled relaxation oscillations" refers to the operation of the circuitry in such a way that the number of desired oscillations can be generated followed by a substantially instantaneous termination of the oscillations. Conversely, the circuit is able to respond, substantially without transients, from a non-oscillatory condition to an oscillatory state to yield a desired number of oscillations.
Unexpectedly, the circuit 401 shown in FIG. 4 exhibits the desired combination of a resistive input impedance circuit having controlled relaxation oscillations. The op-amp used is an LM7121 operational amplifier. A negative feedback path is provided by a voltage divider circuit comprising resistors Rt and R2. Nodes 402 and 404 are connected by a capacitor 410. One end of a resistor 420 is connected to node 402. An analog waveform source 430 is coupled across the other end of resistor 420 and node 408 to complete the circuit. The analog waveform source represents a received signal from which the pulses will be extracted.
The oscillator circuit shown in FIG. 4 has the transfer function as described above in connection with FIG. IB. The graph of FIG. IB is based on the I-V characteristic (current and voltage relations) to represent the transfer function. In this embodiment of the invention, however, the transfer function characterizing the circuit of FIG. 4 is based on the relationship between two voltages W1 and V in the circuit, shown in FIG. 5. An unstable region of operation 504 is bounded by two stable regions of operation 506, 508.
The transfer function shown in FIG. 5 is obtained by properly biasing the op-amp through pin Vcc and Wdd. The unstable region is defined as a region where dVJdV is negative and the stable region is defined as a region where dVj/dV is positive.
As an illustrative example, the transfer function shown in FIG. 5 can be obtained by a circuit configured with the following parameters: Vcc and Wdd are set to 3.5V and -1.5 V respectively. The corresponding values for Rt and R2 are 63 Q and 10 Q, respectively.
The governing equations for the circuit in FIG. 4 are the following: