Solid-state logarithmic-function generator is compact and provides improved accuracy. Generator includes a stable multivibrator feeding into RC circuit. Resulting exponentially decaying voltage is compared with input signal. Generator output is proportional to time required for exponential voltage to decay from preset reference level to level of input signal

Caron, P. R.

English

NASA Technical Reports Server

February 1975	 874-10285 
NASA TECH
. BRIEF 
• NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Logarithmic-Function Generator 
Input Signal 
VA 	 VB
T^ ultivibrato vc M^^ 
Multivibrator 
Astab le 	 Limiter	
Low-Pass 
I	 I 
Exponential- 
Voltage	 Filter 
Generator	 E 
.	 The problem: 
Typical logarithmic-function generators are either 
electromechanical or purely electronic. The electro-
mechanical type utilizes a logarithmic potentiometer 
in combination with a servomechanism. Its dis-
advantages are large size, high power consumption, 
and poor frequency response. The purely electronic 
type using solid-state components, loses accuracy with 
expanded dynamic range. 
The solution: 
A new solid-state logarithmic-function generator is 
compact and provides improved accuracy. 
How it's done: 
- The generator (Figure 1) includes an astable multi-
vibrator feeding into an RC circuit. The resulting 
exponentially decaying voltage is compared with an 
input signal. The generator output is proportional to 
the time required for the exponential voltage to decay 
from the preset reference level to the level of the input 
signal. 
The time constant of the RC differentiator circuit is 
selected according to the desired dynamic range of the 
converter. Diode limiter D ensures a zero voltage 
across the differentiator at the beginning of each 
cycle.
VA	
VOl]  
VB VS
 
VCI 
VD V02______  
0 t 1 	 T T+t1 
Figure 1. Logarithmic-Function Generator 
Both YB and a dc voltage VS proportional to the 
input signal are fed to a differential comparator. The 
comparator has a differentiated output which triggers 
a bistable multivibrator. Positive pulses at times T = 
0, T, etc. cause the multivibrator positive transition 
(leading edge). The negative pulses of VC at times 
t = t 1 , T+t 1 , etc. are used to reset the multivibrator 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19740000285 2020-03-17T14:19:09+00:00Z
Buffer
Amplifier \ 
Input obtained by averaging and adding output signals VDI 
and VD2 from the last two branches. When the 
system input VS falls in the range of 
•1 
0.01 Vol (VS (0.1 Vol 
1OO/	 \ '1 
I	 I yB 
Voltage	 Voltage	 I	 Voltage 
Comparator	 Comparator I	 Comparator 
VC3	 IVC2	 IVci 
Multivibrator I	 I Multivibrator I	 I Multivibrator 
V03	 1V02
txponenti I Adder 	 Voltage 
Fitter I	 Astable 
I	 Multivibrator 
Output 
Figure 2. Logarithmic-Function Generator

With Wider Dynamié Range 
(negative edge). The resulting pulse width is 
proportional to t and is a measure of log (VOi/VS). 
A comparable circuit with a wider dynamic range is 
shown in Figure 2. Its principle is similar to that of 
Figure 1; however, it employs parallel branches (three 
are shown as an example). In addition, it uses an 
input buffer amplifier, two additional voltage 
comparators and multivibrators, and an adder. 
In operation, the reference branch generates an 
output pulse VD1 as in Figure 1. The other two 
branches are amplified by 10 and 100, respectively. 
The system output is again proportional to log 
(VOi /VS) However, in this case, the output voltage is
the output of the channel which includes amplifier Al 
is greater than Vol. Thus voltage VD3 makes no 
contribution to the system output. The system output 
becomes proportional to 
log 10 + log (V0i/1OVS) = log (Vol /VS) 
On the other hand, when VS falls in the range of 
0.001 Vol (VS (0.01 Vol 
the system output becomes proportional to 
log 10 + log 10+ log (Vi/100VS) = log (Vol /VS) 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP74-10285 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,648,043). Inquiries concerning nonex-
clusive or exclusive license for its commercial develop-
ment should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Paul R. Caron

Electronics Research Center

(ERC-10267)
. 
S 
B74-10285	 Category: 02 (Electronics Systems)


Logarithmic-function generator

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server