Switched capacitor envelope detector
TECHNICAL FIELD 
     This invention relates generally to envelope detector circuits, and, more particularly, to a switched capacitor envelope detector. 
     BACKGROUND ART 
     Envelope detectors basically full wave rectify an input signal and filter the envelope of the resulting signal to provide an output signal which is representative of the envelope of the input signal. Envelope detectors are used in many applications including speech recognition systems which often rely upon the envelopes of several bandlimited sections of the speech input to provide a desired output. In discrete device technology, an envelope detector may be made by coupling an absolute value detector to a low pass filter. Such circuits typically are large and have diodes associated therewith which are very difficult to make in a standard MOS process. Others have implemented envelope detectors in switched capacitor technology as taught by P. E. Allen and E. Sanchez-Sinencio in a Texas A &amp; M University text entitled Switched Capacitor Circuits (2nd Preliminary Edition May 1981). However, previous switched capacitor envelope detectors typically utilize at least one capacitor having a floating plate or electrode which is generally a source of error. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an improved envelope detector. 
     Another object of the present invention is to provide an improved envelope detector utilizing switched capacitor techniques. 
     Yet another object of the present invention is to provide an envelope detector having an absolute value detector portion and a filter portion which share the same circuitry. 
     In carrying out the above and other objects of the present invention, there is provided, in one form, an envelope detector having absolute value detection means and filter means. The absolute value means provide an absolute value signal from an input signal and amplify the input signal by using input switched capacitance means and an integrating operational amplifier. The filter means filter the amplitude envelope of the absolute value signal and comprise at least two integrating operational amplifiers. The first integrating operational amplifier of the filter means and the integrating operational amplifier of the absolute value means are the same integrating operational amplifier. 
    
    
     The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates in schematic form an envelope detector constructed in accordance with a preferred embodiment; and 
     FIG. 2 illustrates in graphical form timing signals associated with the envelope detector of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in FIG. 1 is an envelope detector 10 which generally comprises an absolute value detection portion 11 and a filter means portion 12. In the preferred embodiment, all switches shown in FIG. 1 are CMOS transmission gates which are clocked in a conventional manner by signals φ 1 , φ 2 , φ 3 , and φ 4  shown in FIG. 2 and the respective complements thereof and by signals V c  and V c  to be described below. The switches are constructed to be conductive when a clock signal applied to the control input thereof is at a high logic state and nonconductive when the applied clock signal is at a low logic state. 
     In the absolute value detection portion 11, an input signal is coupled to first terminals of switches 13 and 14 and to an inverting input of a comparator 16. A noninverting input of comparator 16 is coupled to a reference voltage terminal, say analog ground, V AG , and an output of comparator 16 provides a complement of a signal V c  and is coupled to an input of an inverter 17. An output of inverter 17 provides signal V c . Signal V c  is a logic low level signal when V IN  is less than V AG  and is a logic high level signal when V IN  is greater than V AG . A second terminal of switch 13 is coupled to first terminals of switches 18 and 19. A second terminal of switch 14 is coupled to first terminals of switches 21 and 22. Switches 19 and 21 both have a second terminal coupled to reference voltage terminal V AG . A second terminal of switch 18 is coupled to both a first electrode of a capacitor 23 and to a second terminal of switch 22. A second electrode of capacitor 23 is coupled to both a first terminal of a switch 24 and to a first terminal of a switch 26. A second terminal of switch 24 is coupled to reference voltage V AG . A second terminal of switch 26 is coupled to an inverting input of an operational amplifier 27, to a first electrode of a feedback capacitor 28 and to a first terminal of a switch 29. A noninverting input of operational amplifier 27 is coupled to reference voltage terminal V AG , and a second electrode of capacitor 28 is coupled to an output of operational amplifier 27. Switches 14, 19 and 26 have the control electrodes thereof coupled to signal φ 1 . Switches 13, 21 and 24 have the control electrodes thereof coupled to signal φ 2 . Switch 18 has a control electrode coupled to V c  and switch 22 has a control electrode coupled to V c . 
     In the filter means portion 12, operational amplifier 27 and capacitor 28 of the absolute value detection portion 11 are also used as the first stage of filter means 12. A second terminal of switch 29 is coupled to a first terminal of a switch 31, to a first electrode of a feedback capacitor 32 and to a first electrode of a feedback capacitor 33. A second electrode of capacitor 32 is coupled to both a first terminal of a switch 34 and to a first terminal of a switch 36. A second terminal of switch 36 is coupled to voltage reference terminal V AG . A second terminal of switch 34 is coupled to the output of operational amplifier 27. The output of operational amplifier 27 is coupled to a first terminal of a switch 37. A second terminal of switch 37 is coupled to a first terminal of a switch 38 and to a first electrode of a capacitor 39. A second terminal of switch 38 is coupled to reference voltage terminal V AG . A second electrode of capacitor 39 is coupled to a first terminal of a switch 41, to a first terminal of a switch 42, and to a first electrode of a feedback capacitor 43. A second terminal of switch 41 is coupled to reference voltage terminal V AG . A second terminal of switch 42 is coupled to an inverting input of an integrating operational amplifier 44 and to a first electrode of a feedback capacitor 46. Operational amplifier 44 has a noninverting input coupled to reference voltage terminal V AG . A second electrode of feedback capacitor 46 is coupled to an output of operational amplifier 44 which provides an output voltage V OUT . A second electrode of feedback capacitor 33 is coupled to a second electrode of feedback capacitor 43, to a first terminal of a switch 47 and to a first terminal of a switch 48. A second terminal of switch 48 is coupled to reference voltage terminal V AG , and a second terminal of switch 47 is coupled to the output of operational amplifier 44. Switches 29, 34, 38, 41 and 48 each have a control electrode coupled to signal φ 3  of FIG. 2. Switches 31, 36, 37, 42 and 47 each have a control electrode coupled to signal φ 4  of FIG. 2. 
     In operation, comparator 16 compares input voltage V IN  with reference voltage V AG . When V IN  changes from greater than to less than V AG , comparator 16 will trip or change logic output states and provide V c  at a first logic level. When V IN  changes from less than to greater than V AG , comparator 16 will trip in an opposite manner and provide V c  at a second logic level. Signals V c  and its complement allow V IN  to be coupled to the first electrode of capacitor 23 via either switches 13 and 18 or switches 14 and 22. Switches 18 and 22 and comparator 16 perform the absolute value generation. In particular, switches 13 and 18 and switches 14 and 22 allow the input voltage to be charged onto capacitor 23 in a manner so that V IN  is always presented to the inverting input of operational amplifier 27 as a negative signal. When V IN  is at a high logic level with respect to V AG , the output of comparator 16 is low and V c  is at a high logic level. Therefore switch 18 is made conductive and V IN  is allowed to charge onto capacitor 23 so that the first electrode of capacitor 23 is at a more positive potential than the second electrode thereof. This means that V IN  is inverted before being fed into operational amplifier 27 and is a negative signal at the inverting input. Therefore after operational amplifier 27 amplifies V IN , the output of operational amplifier 27 is positive. Similarly, when V IN  is at a low logic level with respect to V AG , the output of operational amplifier 16 is high and V c  is at a low logic level. Therefore switch 22 is made conductive and V IN  is allowed to charge onto capacitor 23 so that the first electrode of capacitor 23 is at a more negative potential than the second electrode thereof. This means that V IN  is allowed to be coupled to operational amplifier 27 without any inversion and is a negative signal at the inverting input. Therefore, after operational amplifier 27 amplifies V IN , the output of operational amplifier 27 is positive. Switches 13 and 18 and switches 14 and 22 provide alternate paths to capacitor 23 depending upon whether an inversion of the input signal is required. These four switches along with switches 24 and 26 comprise a &#34;parasitic insensitive&#34;  switched capacitor structure. 
     It should be noted that operational amplifier 27 has a dual purpose. Not only is it used as an amplifier stage in absolute value detection portion 11 but it is also the first stage of filter means 12. Feedback capacitors 28, 32, 33, 39, 43 and 46 determine the poles and zeroes and frequency response of filter means 12 to create a low pass filter so that V OUT  is a filtered signal representing the amplitude envelope of V IN . Absolute value detection portion 11 may be clocked differently from filter means 12 to accurately control cutoff frequencies in filter means 12. The object is to obtain as low of a cutoff filter in frequency means 12 as possible so that V OUT  does not move very fast. An envelope is created by filter means 12 which is always positive and is low pass filtered which basically strips off and provides the envelope of V IN  which must be bandwidth limited to at least one half of the clocking frequency of φ 1  and φ 3 . Therefore, the spectral energy of an input signal V IN  at a predetermined frequency has been determined. 
     For the purpose of illustration only, assume that V IN  is a sine wave. After being rectified via the operation of switches 13, 14, 18 and 22 and capacitor 23, V IN  will have frequency components at DC and at all even harmonics. The amplitude roll-off is very gradual however, with the component which is ten times the fundamental frequency being only 40 dB below the DC component. Sampling such a signal without causing aliasing is usually a problem. The circuit of FIG. 1 overcomes the aliasing problem by sampling V IN  and then full wave rectifying V IN . The signal which is charged onto capacitor 28 is a continuous time signal that represents a rectified V IN  after integration. Capacitor 32 adds damping so that capacitor 28 does not saturate, thereby effectively moving the pole from the origin to a cutoff frequency, f o . Therefore the output signal of operational amplifier 27 is a continuous-time signal that is the rectified signal after filtering by a first order lowpass filter. Capacitor 32 does not cause aliasing because the signal of interest is the output voltage of operational amplifier 27 rather than the signal charged across capacitor 32. However, capacitor 32 does add frequency components to the output signal of operational amplifier 27 at integer multiples of its switching rate. 
     Aliasing does however occur when the output signal of operational amplifier 27 is sampled onto capacitor 39. Therefore, capacitor 32 and the sampling frequency must be chosen so that harmonics of the rectified signal are sufficiently attenuated. In a preferred embodiment, a clock frequency of 50 KHz for signals φ 1  and φ 2  is suggested and a clock frequency of 3125 Hz for signals φ 3  and φ 4  is suggested thereby providing a low pass cutoff frequency of approximately 30 Hz. However, these frequencies are given by way of example ony and any frequencies may be used. It is only important that the frequency of φ 3  and φ 4  be less than the frequency of φ 1  and φ 2  to allow the designer to choose an adequate low pass cutoff frequency and to maintain low capacitance values. It should also be noted that signal φ 1  should always be kept relative to φ 2  as shown in FIG. 2 and signal φ 3  should always be kept relative to signal φ 4  as shown in FIG. 2 regardless of the frequencies chosen for signals φ 1  and φ 2  and signals φ 3  and φ 4 . 
     Since the circuit of FIG. 1 may be implemented by using &#34;parasitic insensitive&#34; switches, parasitic capacitance errors tend to be cancelled out. By now it should be appreciated that an envelope detector implemented in switched capacitor technology and using common circuit elements for performing an absolute value detection function and a filter function has been provided. 
     While the invention has been described in the context of a preferred embodiment, it will be apparent to those skilled in the art that the present invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of this invention.