A ganged potentiometer with the interesting property of a constant 10k ohm or greater series impedance for all rotations of the shaft was rediscovered. The device provided a versatile passive mixer circuit when used with most signal sources and can be used as a variable series input summing resistor in operational amplifier networks. The potentiometer gave simple solutions to missing problems with a single control knob

Burhans, R. W.

English

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(NASA-CR-140850) GANGED SERIES N75- 12209POTENTIOMETER MIXER NETWORKS (Ohio Univ.)
5 p HC $3.25 CSCL 09A
Unclas
G3/33 04253
TECHNICAL MEMORANDUM (NASA) 16
GANGED SERIES POTENTIOMETER MIXER NETWORKS
A ganged potentiometer witha single linear section
and two opposite log tapered sections is rediscovered
for providing a simple series resistor control element
for mixing of audio frequency signals. An application
is for bench evaluation of detector signaI to noise
ratios with Omega receivers.
by
Ralph W. Burhans
Avionics Engineering Center
Department of Electrical Engineering
Ohio University
Athens, Ohio 45701
December, 1974 7' 1" 7~
Supported by
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia
Grant NGR 36-009-017
https://ntrs.nasa.gov/search.jsp?R=19750004137 2020-03-23T03:16:54+00:00Z
PREFACE
A brief Project Note submitted for publication in the Journal of the Audio
Engineering Society and originally intended for use in audio systems, also has application
as a laboratory measurement method for Omega or VLF receiver bench testing. A preprint
of the paper is reproduced here for the benefit of those concerned with evaluation of Omega
receiver front-end systems. The device consists of a unique combination of linear and log
tapered potentiometers which provide a simple series resistor attenuator for mixing of low
frequency signals.
TABLE OF CONTENTS
Page
I. INTRODUCTION 1
II. CIRCUIT 1
III. OPERATION 1
IV. USES 1
V. ACKNOWLEDGEMENTS 3
I. INTRODUCTION
During the course of looking for a rapid means of making signal to noise ratio
measurements with a signal source and noise generator, some surplus 3 gang potentiometers:
were found which have the interesting property of a constant 10k ohm or greater series
impedance for all rotations of the shaft. The device provides a versatile passive mixer
circuit when used with most signal sources and can be used as a variable series input sum-
ming resistor in operational amplifier networks. While mixer networks are very well known
art, the particular potentiometer "rediscovered" here may not be so well known and it can
provide simple solutions to mixing problems with a single control knob.
II. CIRCUIT
A diagram of the 3 gang potentiometer circuit is illustrated in Figure 1. One
section consists of a linear 10k ohm pot. An end section is a combination of a linear variable
resistor for the first half of rotation to 5k ohms and a clockwise log taper to 1 megohm for the
last half of rotation. The other end section is reversed to provide a counterclockwise log
taper from 1 meg to 5k for the first half of rotation and a 5k linear pot for the last-half of :
rotation.
The mechanical arrangement of the particular potentiometer used is illustrated
in Figure 2. It should be possible to provide a similar ganged pot from the snap-on variable
resistor sections available from other potentiometer manufacturers.
Ill. OPERATION
The net result of using this potentiometer is a variable series resistor element as
shown in Figure 3. The resistance from A to C is constant at 10k ohms for the first half of
rotation, 0 to 50%, and the resistance from B to C is constant at 10k ohms for the last half
of rotation from 50% to 100%. Correspondingly, the resistance B to C decreases exponen-
tially to 10k ohms for the first half of rotation and the resistance A to C increases exponen-
tially for the last half of rotation. Thus a two input series mixing device to a single output
can provide a range of attenuation of ±40db with a Odb center position and a single control
knob.
IV. USES
Figure 4 illustrates some alternate ways of using the network as a variable series
element with operational amplifiers. In some cases the operational amplifier may not be
necessary if the output of the mixer is used only for relative level adjustments such as in
blending for tape recording from audio signal sources. The single output mixer circuit of
A C B
!meg CW log taper, 10k linear taper, imeg COW log taper
Figure 1. Circuit of Potentiometer.
C
TOP VIEW
A.B type J
A (380 0006 00)
B
shaft o4 O
Figure 2. Mechanical Arrangement of Potentiometer.
-2-
Figure 4 has been useful in making relative signal to noise ratio measurements from low
output impedance signal sources where a single control knob can be calibrated in terms of
the inputs for S/N ratios of +40db. Thus if the signal generator is at the A input, rotation
of the shaft to A will provide a +40db S/N output at C, when the noise source is connected
to B with the same RMS input levels from the respective signal and noise sources. Corres-
pondingly a 50% rotation of the control shaft would provide a Odb S/N output and a 100%
rotation to B would provide a - 40db S/N output at C. Combinations of these devices are
useful in mix-up, mix-down, and laboratory measurements over the frequency range to
500kHz.
V. ACKNOWLEDGEMENTS
This project was in part supported by NASA Grant NGR 36-009-017.
-3-
EQUIVALENT CIRCUIT ROTATION FUNCTION
10k
A O--AA All A, A constant 10k
rneg C A B No B, B attenuated Imeg
B O--"A Cv BW-1c
10k
O 
- A =-B, A constant 10k
S Ok C A -B B constant,10k
B
A lme g C AAll B, B constant l0k
10k C no A, A attenuated Imeg
Figure 3. Series Control Operation.
10k
Al
ClGB
B1
A2
C2 C- B2
----o -- -
DUAL OUTPUT 1IXNLER
10k
AO
CC
SIilGLE, OUTPUT NIXER
10k
C 0 -
SINGLE INPUT - OUTPUT SPLITTER
Figure 4. Series Network Uses.
-4-


Ganged series potentiometer mixer networks

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