Pulse duration modulation multiplier syste

Carlson, A. W.

Furciniti, C. A.

English

NASA Technical Reports Server

Filed Sept. 13. 1967 2 Sheets-Sheet 1 
, 
https://ntrs.nasa.gov/search.jsp?R=19710002915 2020-03-17T02:31:20+00:00Z
S h e  ts-Shee t 2 
INVENTORS 
1 
in the presence of a large amount of ripple component. 
Similarly, to resort to  frequency multiplication would un- 
desirably change the pulse rate to a higher frequency by 
the same factor that the modulation is increased. 
We have found that we are able to  improve the linear- 
ity of a PWM system by allowing a first pulse modulator 
to operate over that portion of its range where modu- 
lation is a linear function of the modulating signal and 
then, by utilizing our invention, multiplying the modula- 
tion to any degree desired. 
U.S. en. 332-9 6 e C k i k S  
SUMMARY OF THE INVENTION 
A device for increasing, multiplying or amplifying, by In our device, the modulated pulse width signal, from 
some constant factor, the width modulation of a pulse a prior pulse width modulator, is utilized as the input of a 
width modulated signal wherein the output pulse train has modulation multiplier to produce an output pulse train 
a greater degree of width modulation than the input having a higher degree of modulation than the original 
width-modulated signal. The input width-modulated sig- input pulse signal. Included in the modulation multiplier 
nal pulse train is first integrated by a suitable R-C net- circuit is an R-C low-pass filter circuit which integrates 
work, the time constant thereof being chosen, with respect 2o the input pulse train. The factor by which the modulation 
to the pulse repetition frequency of the input signal, to of the input PWM signal is multiplied or increased is de- 
provide an essentially peaked signal. The peaked signal, pendent upon the ratio of the time constant of the low- 
when applied as an input to a trigger circuit, causes the pass integrating circuit to  the pulse period; the multipli- 
circuit to switch from one output level to another to  gen- cation is approximately 4Tc/T, where T, is the time con- 
erate the required modulated output signal. 25 stant of the low-pass integrating circuit and T is the pe- 
riod of the PWM signal pulses. When T,/T is greater 
than one, the multiplication factor is quite constant over 
The invention described herein was made in the per- the whole range of output modulation while the maxi- 
formance of work under a NASA Contract and is subject mum error in linearity is less than two percent when T,/T 
to the provisions of Sec. 305 of the National Aeronautics 30 is equal to one. The modulation muliplication factor is 
and Space Act of 1958, Public Law 85-568 (72 Stat. 435; increased and the linearity is improved as T, is increased, 
42 U.S.C. 2457).  but the peak-to-peak amplitude of the sawtooth wave- 
form VZb is decreased as Tc is increased to set a limitation 
BACKGROUND OF THE INVENTION on the magnitude of T,/T.  It is, therefore, desirable that 
35 the amplitude of the sawtooth waveform be appreciably 
greater than the level of signal needed to operate the 
switch controlled by the sawtooth. n e  sawtooth wave- 
the output voltage of the switching circuit to switch from 
input sawtooth waveform is above or below the switching 
point of the switching circuit. This generates an output 
Pulse train having the same period, but a higher degree 
It is, therefore, a principal object of the present inven- 
tion to provide a simple pulse width modulation system 
which operates upon the original pulse width modulated 
signal to generate an output pulse train having a greater 
5o degree of pulse width modulation than the input signal. 
Another principal object of the present invention is to 
provide a pulse width modulation system with improved 
linearity by allowing a first pulse width modulator to op- 
erate Over that portion of its range where modulation is a 
then multiplying 
the modulation to the desired degree+ 
This invention relates to modulation systems and, more 
particularly, to a device for increasing, multiplying or 
modulation of a pulse width modulated signal. 
Otherwise amplifying, by Some constant factor, the width form applied to the input of the switching circuit 
Earlier methods of amplification or  multiplication of 4o 
pulse width modulated (PWM) signals required the use One level to another according to whether the 
of a low-pass filtering system to the modulation 
component on the signal and the use of the recovered 
modulation as the input to another pulse width modulator. 
The modulation was capable of being increased by the re- 45 of modulation, than the input Pulse train. 
peated or cascading of operations of this sort provided 
that the modulation-demodulation process yields gain in 
signal strength. Similarly, certain types or methods of fre- 
quency multiplication of phase-modulated signals may be 
suitably modified to operate with PWM signals. 
The disadvantages of the prior art systems reside in the 
fact that they are considerably more complex than the 
subject invention. For example, if the modulating signal 
recovered by the low-Pass filter is used as the input to  an- 
other PWM, any distortion introduced by the low-pass 55 linear function of the modulating 
filter or any ripple components remaining after filtering 
become a source of distortion which \vouId then be ap- 
plied to the second modulator. Thus, the prior art has had 
to resort to large and rather elaborate filtering systems. 
The problem becomes particularly Severe in instances 
where the modulation Component to be recovered is small 
(as it normally would be when one is concerned about in- 
creasing it). The low-pass filter is now required to sepa- 
rate or recover a relatively small modulation component 
The features 'Of Our invention which we t' be 
are set forth with particularity in the appended 
claims. Our invention itself, however, both as to its orga- 
nization and method of operation, together with further 
objects and advantages thereof, may be best understood 
by reference to the following description taken in con- 
junction with the following drawings. 
3,535,657 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a partial schematic and block diagram of 
our invention; 
FIG. 2A is a plot (V vs. t )  of the waveform VZ, ap- 
pearing in FIG. 1; 
FIG. 2B is a plot (V vs. t )  of the waveform V2b ap- 
pearing in FIG. 1; 
FIG. 2C is a plot (V vs. t )  of the waveform of the 
output pulse width modulated signal V2c appearing in 
FIG. 1; 
FIG. 3 is a schematic diagram of one form of switching 
circuit which may be utilized in our invention; and 
FIG. 4 is a partial schematic and block diagram com- 
bining the disclosure of FIGS. 1 and 3. 
DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
Referring now to FIG. 1, there is shown our novel pulse 
width modulator for operating on a signal generated 
from a first pulse width modulator 10. In this embodi- 
ment, a signal derived from a generator 12 and a modula- 
tion signal 14 are applied to  the first pulse width modula- 
tor 10 which produces a pulse width modulated signal. 
The pulse width modulated signal having a waveshape 
Vza (FIG. 2A) is applied, by means of a pair of leads 
16, as an input to  our novel modulator 18. The first stage 
consists of an integrator 20 having a 
and a shunt capacitor 20.2 in the 
usual integrator configuration. The input signal Vz,, hav- 
ing been integrated, now appears as VZb at the Output 
thereof, is applied to  a pair of terminals 22.1 and 22.2, 
and represents the input to a switch means 2'4. The output 
of the switch means 24 may be derived from a pair of 
terminals 26.1 and 26.2 and appears as Vz,. 
We now refer to the waveforms shown in FIG. 2A, 
FIG. 2B and FIG. 2C which, together with FIG. 1, are 
used to better describe and understand the operation of 
our invention. In our device, signal VZa is the signal ap. 
pearing at the output of the pulse width modulator 10 
and is shown in FIG. 2A as a plot of voltage (Vas) 
versus time ( t )  with the waveforms having period T. It 
is this signal Vza that is applied to the input of the R-C 
low-pass network 20. When T,/T is equal to or greater 
than one, the peak-to-peak amplitude of the sawtooth 
voltage V2b appearing across capacitor 20.2 is substan- 
tially constant over the whole range of output modula- 
tion. This sawtooth waveform is superimposed upon a 
waveform proportional to the modulating signal of the 
input pulse train and is, therefore, displaced vertically 
according to the magnitude and sign of the modulating 
signal. The voltage Vzb consists of the sum of a sawtooth 
made up of exponential segments (which become straighter 
as T,/T is increased) and a low-frequency modulation 
signal which, when applied to the input of the switching 
circuit 24 at  terminals 22.1 and 22.2, causes the switch- 
ing circuit 24 to  generate the output signal VZc having a 
greater degree of modulation than the input PWM signal. 
(While the signals of FIG. 2 are shown as being bipolar, 
it should be obvious that they may be unipolar.) 
It is this signal V2b that is applied to the switching 
circuit 24 which may include transistor and vacuum tube 
Schmitt trigger circuits, zero crossing detectors, compara- 
tors, saturated or over-driven amplifiers, two-state cir- 
cuits or devices such as four-layer diodes and triodes, tun- 
nel diodes and negative resistance devices. In brief, any 
of a great number of circuits and devices may be used that 
detect and respond to changes in voltage or current levels 
or changes in polarity. 
Thus, the signal V2b of FIG. 2B that is applied to the 
switching circuit 24 has one polarity (+) for the period 
of time from to-rl. From time tl-tz, the polarity is nega- 
tive (-) while from time trt3, the polarity is positive 
4 
(+). Similarly, from time t3-t4, V2b (FIG. 2B) has a 
negative (-) polarity, while from time t4-t5, it has a 
positive (+) polarity. This is determined by the R-C 
constant (the value of the resistor 20.1 and the capacitor 
20.2). Thus, during those intervals of time that v 2 b  is 
positive, (t4-tlr t2-t3, t4-t5) the output of the switching 
circuit 24, appearing at a pair of terminals 26.1 and 26.2 
and shown as VZc in FIG. 2C will be at level +V. During 
those periods of time that 'kr, is negative (tl-tz, q-h, 
t5-t6) the output level will be level -V, as shown h 
FIG. 2C. In this manner, varying the level about which 
the signal V2b is switched will determine the new modula- 
tion value appearing as signal VzC (FIG. 2C). 
Referring now to FIGS. 3 and 4, there is shown one 
15 form of switching device. It should be understood that 
various other circuits may be used herein and we do not 
wish to be limited solely to the circuit shown. In FIG. 3, 
the input signal V2b is applied to the terminals 22.1 and 
22.2, the latter of which is at some reference potential 
20 herein shown as a ground 24.1. A series limiting resistor 
24.2 is connected between the terminals 22.1 and the base 
of a PNP transistor 24.4. A tunnel diode 24.3 is connected 
between the base and ground with the emitter also con- 
nected to ground. The collector is connected, through a 
25 decoupling resistor 24.5, to a source of potential at volt- 
age level -V, with the output of our device appearing 
at a terminal 26.1, which is also connected to the collector. 
Output terminal 26.2 is connected t o  ground to complete 
the circuit. 
30 What we claim is: 
1. A modulation system comprising: 
means for providing a modulated signal; 
a voltage divider network having first and second im- 
pedances; 
means for applying the modulated signal to  the voltage 
divider network; 
means for deriving a switching signal from across one 
of the impedances; 
switching means operating at  either a first or at a sec- 
ond selected voltage level, and 
means for applying the switching signal to the switch- 
ing means to periodically, alternately vary the volt- 
age from the first selected level to the second se- 
lected level, in accordance with the switching signal. 
2. The modulation system of claim 1, wherein: 
the modulated signal is pulse width modulated hav- 
ing an essentially rectangular, substantially periodic, 
regular waveshape . 
35 
40 
45 
3. The modulation system of claim 2, wherein: 
the first impedance is connected in series with the input 
and the output and with said second impedance in 
shunt between said input and output, and 
the modulated signal is applied across both said imped- 
ances. 
4. The modulation system of claim 3, wherein: 
the voltage divider network is a resistance-capacitance 
the first impedance is a resistor; 
the second impedance is a capacitor, and 
the switching signal is derived from across the capacitor. 
5. The modulation system of claim 4, wherein: 
the time constant of the resistance-capacitance is of the 
order of the period of the modulated signal or 
greater. 
6. A modulation system comprising: 
means for providing an essentially rectangular, sub- 
stantially periodic pulse width modulated signal; 
a resistor-capacitor circuit having the resistor connected 
in series between the input and the output and the 
capacitor in shunt thereto, said circuit having a time 
constant of the order of the period of the modulated 
signal or greater; 
means for  applying the pulse width modulated signal 
across the resistor-capacitor circuit to derive a switch- 
ing signal from across the capacitor; 
50 
55 
integrator circuit; 
60 
65 
70 
75 
3,535,657 
transistor switching means for operating either a t  a 
first or at  a second voltage level and having a base, 
a collector and an emitter; 
a first limiting resistor connected between the common 
junction of the resistor-capacitor circuit and the base 
of the transistor switching means; 
a tunnel diode connected between the base and emitter 
of the transistor switching means; 
the emitter connected to a source of reference potential 
at the second voltage level; 
a source of operating potential a t  the first voltage level; 
a second limiting resistor connected between the source 
of operating potential and the collector of the tran- 
sistor switching means; 
a pair of output terminals; 
one output terminal connected to the collector of the 
the other of the output terminals connected to the 
transistor switching means, and 
source of reference potential. 
eferences Cited 
UNITED STATES PATENTS 
2,987,633 6/1961 .Pallas _ _ _ _ _ _ _ _ _ _ _ _ _ _  307-234 
3,064,208 11/1962 Bullock et al. _ _ _ _ _ _ _ _ _  332-9 
3,191:0165 6/1965 Vargiu _ _ _ _ _ _ _ _ _ _ _  329-106 X 
3,211,926 10/1965 Frysinger _ _ _ _ _ _ _ _  3017-267 X 
3,246,247 4/1966 Grindle. 
3,371,291 2/1968 Forrest et al. _ _ _ _ _ _ _ _  332-9X 
10 OTHER REFERENCES 
Fulcher : “Pulse Generator” August 1965, RCA Tech- 
nical Notes. 
ALFRED L. BRODY, Primary Examiner 15 
U.S. C1. X.R. 
307-246,265; 325-142; 328-58; 329-106 


Pulse-width modulation multiplier Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server