Circuitry for digitally generating an exponentially decaying wave function permits discrete values to be sampled from the exponential waveform for comparison with a binary number of specified accuracy. This exponential-decay generator employs a simple binary counter to count in the sequence of exponential decay

Anderson, T. O.

Hurd, W. J.

English

NASA Technical Reports Server

September 1969	 Brief 69-10439 
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S
NASA TECH BRIEF - 
-it IL	 A 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Simple Quasi-Exponential Slope Generator 
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AB	 ujuu.	 AA 
_ ________ 
A	 •1 
Figure 1 
Circuitry has been devised fOr digitally generating 
an exponentially decaying wave function (such as 
from a discharging capacitor). Its purpose is to per-
mit discrete values to be sampled from the exponen-
tial waveform for comparison with a binary number 
of a specified accuracy. The special-purpose ex-
ponential-decay generator employs a simple binary 
counter designed to count in the sequence of ex-
ponential decay. Although available arbitrary, random, 
and general-purpose digital sequence or function gen-
erators would be applicable, they are of greater com-
plexity than the present circuitry. 
As shown in Figure 1, a standard binary counter, 
A, is regarded as consisting of two sections: a binary 
field counter, AA, and a field-control counter, AB. 
The subfield counter, B, is a simple shift-register
counter. The length of each counter, as well as the 
gating structure, may be increased or decreased as 
required. 
The truth table (not shown) for section AA is divided 
into fields starting at full scale. Each field is half of 
the preceding field. The outputs of simple gating 
structures covering these fields control the input 
clock. Counting from full scale to half scale, full clock 
rate is applied. Counting from half scale to 1/4 
scale, 1/2 clock rate is applied, i.e., one stage of 
section AB is included in the counting chain. Counting 
from 3/4 to 7/8, one-fourth of the clock rate is 
applied, i.e., two stages of AB are included. Counting 
from 7/8 to 15/16, one-eighth of the clock rate is 
applied, i.e., three stages of AB are included, etc. To 
smooth the function further, each field of AA is divided 
(continued ocerleal) 
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into three subfields, the upper 1/4, the center 1/2, 
and the lower 1/4 field. These sub fields are easily 
covered by simple two-term gates, which in turn are 
controlled by the major field gates. For each field, the 
slope is changed from 1/2 for the first 1/4 to 1/3 for 
the center 1/2, and 1/4 for the lower 1/4. This is 
accomplished by an or operation on all upper, center, 
and lower subfields, and controlling an input counter 
with these or terms so as to divide by 2,3, and 4 re-
spectively. Figure 2 shows the actual wave form pro. 
duced.
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Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10439 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: T. 0. Anderson and W. J. Hurd of
Jet Propulsion Laboratory
'"-l1130) 
Time 
Figure 2. 
Brief '6940439	 Category 01


Simple quasi-exponential slope generator

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