Digital waveform conditioning circuit

A digital waveform conditioning circuit includes a noninverting amplifier stage. A source of digital information is coupled to the input of the amplifier via a series resistor. The amplifier output is coupled to a digital utilization circuit. A feedback capacitor is coupled between the amplifier input and output terminals to provide integration of unwanted input noise pulses, positive feedback during threshold transitions for fast rise and fall times, and a hold-off voltage for a specified time thereafter to prevent oscillations during said transitions.

BACKGROUND OF THE INVENTION 
This invention relates generally to digital electronic systems, in 
particular waveform restoration systems used therein. Digital electronic 
systems utilize information encoded into sequential binary signals 
existing in either of two digital logic "states" to communicate or 
translate information between a number of information processing system 
components. A typical digitally encoded signal comprises a plurality of 
"squared" pulse-like signals. In order to effectively utilize such digital 
signals, a great number of encoding languages have been developed each 
being composed of combinations or sequences of binary two-state signals. 
While the languages used and systems processing them vary greatly in 
content format and encoding systems, all share the common requirement of a 
"clear" transition to perform effectively. 
However, even within the most rudimentary digital logic systems a number of 
noise or extraneous signal sources are constantly in action which often 
contaminate the digital signal. Unless the effect of such noise is 
eliminated erroneous system performance can result. One common source of 
such noise is found in the transients which accompany electrical 
switching. Most typically these transients arise as electrical contacts 
mate or part producing a "spike" that is a sharp, short duration signal. 
Transients also arise due to contact bounce which produces a series of 
closely spaced noise pulses. In either case the noise signals combine with 
the digital information and are often interpreted as desired signals by 
the system. This, of course, gives rise to erroneous system output. 
Digital logic systems are complex and perform their interpretive and 
calculative processes as well as information storage and retrieval through 
the use of great numbers of information translating devices. They 
inherently require a great variety and number of switches and switching 
systems to properly implement system operation. Even within the most 
sophisticated and up-to-date digital logic systems switches are used to 
route signal and information which must of necessity make and break a 
great number of electrical connections which give rise to transient or 
spike responses. 
In addition to switching transient problems noise may be produced outside 
the logic system. For example, one of the most promising yet challenging 
environments for digital systems is the utilization of the vast telephone 
network as a distribution system. While the potential for digital system 
use of the telephone distribution system is great, the challenge presented 
to a digital system in utilizing the telephone distribution network is 
also great. Telephone systems are in essence vast switching networks which 
during normal performance are extremely laden with switching transient 
noises making their use by digital information systems difficult. 
These and other noise problems have led practitioners in the art to develop 
numerous signal conditioning systems, the simplest of which is a passive 
integrating network such as set forth in U.S. Pat. No. 3,466,647, issued 
to John Guzak, Jr. In the system shown a number of keyboard switches are 
used to couple information into digital logic circuitry. Guzak provides a 
plurality of capacitors in parallel with the switches which integrate the 
switching transients and dampen switch action. It is also known to utilize 
active integrators such as those shown in U.S. Pat. No. 3,405,286, issued 
to R. Mudie, and U.S. Pat. No. 3,792,363, issued to Gebel et al. Such 
active integrators comprise combinations of an inverting amplifier stage 
and A.C. feedback network (such as a capacitor) which couple a portion of 
the output signal back to the input terminal to produce negative feedback. 
The basic idea is to provide a negative feedback or gain reduction for 
signals of higher frequency typical of those forming transient or noise 
spikes within the signal. While both active and passive type integrators 
can under many conditions effectively reduce the high frequency transient 
noise content of the digital binary encoded signal they unfortunately also 
introduce an often undesirable increase in the rise and fall times of the 
digital signal. 
U.S. Pat. No. 3,513,333, issued to R. T. Andersen, sets forth an inverting 
amplifier having a positive feedback system which is directed primarily to 
minimizing the effects of noise transients produced by "contact bounce" 
within the system. As mentioned, contact bounce is present when a 
mechanical circuit breaker separates or mates ineffectively and produces a 
series of connect and disconnect transients in short succession after 
switch actuation. Andersen recognizes this fact and uses an amplifier in 
combination with a positive A.C. feedback network to supply a hold-off 
voltage causing the amplifying network to ignore noise signals occurring 
during the interval following switch actuation. While the described system 
is somewhat effective against the particular family of noise produced by 
switch contact bounce, the inverting amplifier and positive feedback 
offers little in the way of protection from noise transients occurring at 
other times within the system. 
Still another apparatus for reducing the noise within digital systems is 
set forth in U.S. Pat. No. 3,824,583, issued to Quentin C. Turtle, which 
discloses a monostable multivibrator circuit (often called a "one-shot" 
multivibrator) which is characterized by the generation of an output 
signal pulse in response to an input trigger signal. The output signal has 
a duration independent of the input trigger signal duration. One of the 
primary benefits of such one-shot multivibrators arises from their 
inherent rejection of any trigger during the duration of their output 
pulse; however, they will still trigger at other times on noise pulses. A 
somewhat similar device well known in the art and often used to remove 
noise from digital encoded information signals is that of a Schmitt 
trigger. While somewhat similar to a monostable multivibrator, Schmitt 
trigger circuits tolerate more noise due to their displaced thresholds and 
enhance rise times due to their positive feedback. Schmitt triggers are 
often referred to as "squaring" circuits. Again, their use in digital 
systems is limited due to their displaced thresholds and their inability 
to reject large noise pulses. Both monostable multivibrators and Schmitt 
triggers offer some advantages in the art of noise suppression. However, 
their circuit construction is generally complex and limited in performance 
and the need arises for a simpler, more effective waveform conditioning 
circuit within the digital art. 
Accordingly, it is an object of the present invention to provide an 
improved waveform conditioning system for use in a digital electronic 
information system. It is a more particular object of the present 
invention to provide an improved waveform conditioning network which 
utilizes a minimum number of system components and produces effective 
noise elimination without substantial deleterious effect upon the rise and 
fall times of the digital signal. 
SUMMARY OF THE INVENTION 
A digital waveform conditioning circuit for use in a digital electronic 
information system in which information is arranged in a sequence of 
signals having predetermined logic states, said signals being subjected to 
extraneous noise and switching transients which tend to produce undesired 
reactions within said information system; comprises a signal buffer 
amplifier having a gain of more than one, responsive to an applied 
sequence of signals, producing an output sequence of signals having the 
same polarity as the applied signals, a resistor in series with the input 
of said amplifier, and a capacitor connected between the input and output 
terminals of said amplifier.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
The following disclosure is offered for public dissemination in return for 
the grant of a patent. Although it is detailed to ensure adequacy and aid 
understanding, this is not intended to prejudice that purpose of a patent 
which is to cover each new inventive concept therein no matter how others 
may later disguise it by variations in form or additions or further 
improvements. 
FIG. 1 shows a waveform conditioning circuit constructed in accordance with 
the present invention in which a source 10 of digitally encoded 
information is coupled via a series resistor 11 to an input terminal 16 of 
a buffer amplifier stage 13. The output terminal 17 of buffer stage 13 is 
coupled to a digital information utilization device 14. A feedback 
capacitor 12 is coupled between input terminal 16 and output terminal 17 
of buffer stage 13. A resistor 15 is also coupled between input terminal 
16 and output terminal 17. Digital information source 10 may include any 
digital system component. For example, digital source 10 may form the 
output of a computer telephone terminal which receives digitally encoded 
information over the telephone network system. Similarly, digital 
information source 10 may also comprise any of the well-known digital 
readout components such as the output of a computer or the like. 
Similarly, digital utilization device 14 may comprise virtually any 
digital system or subsystem which would normally be coupled to digital 
information source 10. For example, the device 14 may comprise the input 
terminal of a computer being fed from a telephone line source formed by 
digital information source 10. Or, digital utilization device 14 may 
comprise a second computer receiving an information transfer from a first 
computer of source 10. In other words, the present invention wave 
conditioning circuit is likely to find application between any two digital 
system components. 
Buffer amplifier stage 13 is characterized by a non-inverting relationship 
between its input and output signals. For example, if a positive-going 
pulse is applied to input terminal 16 a positive-going pulse would appear 
at output terminal 17 and be coupled to digital utilization system 14. A 
number of well-known gate and amplifier combinations may be utilized to 
perform this buffer stage function. For example, an AND gate which 
produces an output signal of the same polarity when its input terminals 
coincide will provide the noninverting output signal of buffer 13 if both 
its input terminals are commonly coupled to resistor 11 and its output 
terminal coupled to digital utilization device 14. In the same manner an 
OR gate, which produces a positive output signal when either of its input 
terminals receives a positive signal, having input terminals commonly 
coupled to resistor 11 and output terminal coupled to digital utilization 
system 14, may perform the function of buffer 13. Yet another embodiment 
for buffer stage 13 may be provided by an even number of inverters 
connected in cascade between resistor 11 and digital utilization system 
14. The even number of inversions is, of course, to assure a like polarity 
signal at input terminal 16 and output terminal 17. These examples are 
merely illustrative and not exclusionary. It will be apparent to those 
skilled in the digital art that any number of gate and amplifier 
combinations may be used to satisfy the requirements of buffer state 13 
provided they provide a gain of more than one and produce an output signal 
of the same polarity as the input signal and are in the sense 
"noninverting" stages. 
The operation of the present invention waveform conditioning circuit is 
best understood by reference to waveforms 2A, 2B and 2C shown in FIG. 2. 
More particularly, waveform 2A shows a digital pulse output signal of 
digital information source 10 having a positive going leading edge 20 and 
a negative going trailing edge 21. Also present on waveform 2A are a 
number of noise spikes or extraneous pulses 22 exemplary of the type which 
often contaminate a binary encoded signal. This signal applied via 
resistor 11 to the input of buffer stage 13 (e.g., a noninverting 
amplifier) produces a corresponding output signal having the same 
polarity. Capacitor 12 couples back a differentiated portion of the output 
signal 2C of buffer stage 13 which is combined with input waveform 2A to 
form the signal shown in FIG. 2B which appears at terminal 16 in FIG. 1. 
The integrating effect of resistor 11 and capacitor 12 forms an increasing 
ramp signal 36 after the input has changed to the positive or "high" 
state, which upon reaching threshold 35 (of the active region of the 
amplifier of the buffer stage) causes buffer stage 13 to abruptly change 
output state and produce a sharp rise 30. This large offset at the input 
prevents any immediate recrossing of the threshold 35, and hence any 
oscillations or false multiple counts at logic transitions. After the 
negative transition 21 of input waveform 2A, the integration of resistor 
11 and capacitor 12 then produces a decreasing ramp 37 until threshold 35 
is again reached causing an abrupt fall 31. Noise spikes 22 in waveform 2A 
are attenuated by the integrating effect of resistor 11 and capacitor 12 
and appear as attenuated remnants 32 in waveform 2B. The purpose of 
resistor 15 is to provide sufficient positive feedback (hysteresis) at low 
frequencies to prevent self-oscillation during extremely slow input 
transitions. It will be apparent to those skilled in the art however that 
resistor 15 may be omitted from the described embodiments without 
departing from the spirit of the present invention. Resistor 11 need not 
necessarily be a separate component but its function may be served by an 
internal resistance in signal source 10. 
The positive feedback provided by capacitor 12 causes the switching between 
logic states to be regenerative or self-enforcing. Once threshold 35 has 
been crossed the system rapidly converts to the opposite logic state. 
However, the noise signals 22 are integrated or filtered by the action of 
capacitor 12 and resistor 11 such that they are insufficient in amplitude 
to affect the logic state of buffer 13. As a result, a virtually noise 
free signal delayed in time from that which originally stimulated the wave 
conditioning circuit (shown in waveform 2C) is produced at the output 17 
of buffer stage 13. The positive feedback function of the wave 
conditioning circuit permits the maintenance of a high or fast rise time 
40 and a corresponding fast fall time 41 which is, of course, desirable in 
digital information systems. 
FIG. 3 shows the present invention waveform conditioning circuit adapted to 
utilize C-MOS logic elements. The operation and structure of the C-MOS 
embodiment of FIG. 3 is essentially the same as that set forward in 
conjunction with FIGS. 1 and 2 above. The major difference, however, is 
the imposition of a current limiting resistor 52 between input terminal 16 
of a C-MOS buffer stage 50 and the junction of resistor 11 and feedback 
capacitor 12. The function of resistor 52 is to provide current limiting 
to protect the more sensitive circuitry within C-MOS buffer stage 50. 
As an example of a specific embodiment, the non-inverting amplifier 50 is 
two stages of a 4069 C-MOS. These are well known and, for example, are 
described in Motorola's Semi Conductor Data Library CMOS, Vol. 5 - Series 
B, pg. 5-136. Resistors 11 and 52 are 10k ohms. Resistor 15 is 100k ohms 
and capacitor 12 is 0.022 mf. This example is suitable for operation at 
1000 Hertz. 
Other minor modifications may be required to adapt the principles set forth 
in this disclosure to other logic systems using means well known to those 
skilled in the art. 
From the above disclosure it is obvious that the present invention is a 
simple yet novel device which combines the best characteristics of several 
older more complex devices while avoiding their shortcomings. Like an 
integrator, it attenuates noise pulses, but it avoids the slow rise and 
fall times. Like a Schmitt trigger, it enhances rise and fall times, but 
it will not trigger even on large noise spikes and it does not have the 
displaced thresholds. Like the one shot multivibrator it has good rise and 
fall times and is immune to signals immediately after a transition, but it 
responds to both polarities and maintains its noise immunity between input 
transitions.