Integrated-circuit multiplexed telemetry systems for biomedical application

Ko, W. H.

Thompson, W. L.

English

NASA Technical Reports Server

I. Abs t rac t  
The purpose of this project is to study and apply advanchd electronic  
technology to the development of integrated circuit ,  multiplexed te lemet ry  
sys t ems  f o r  bio-medical applications. 
and be capable of te lemeter ing a wide range of physiological signals.  
to  ten channels of signals will be t ransmit ted simultaneously. 
These sys tems should be implantable 
Three 
Severa l  multiplexing telemetering sys tems have been studied. A 
scheme has been developed which minimizes  power drain,  uses  s imple 
c i rcu i t ry ,  and i s  applicable to hybrid integrated circui t  construction. Multi-  
plexing and de-multiplexing circui t ry  for  a th ree -  channel sys tem have been 
constructed and bench tested.  
A study was made  of osci l la tors  for  the R F  c a r r i e r  of this multiplexing 
sys t em including crys ta l  controlled types.  
t ro l l ed  FM osci l la tors  do not offer a wide enough information bandwidth for 
this multiplexing system unless elaborate and extensive c i rcu i t ry  is  used. 
Since extensive c i rcu i t ry  i s  difficult to fabr icate  i n  hybrid integrated circui t  
f o r m ,  these c rys t a l  controlled FM osci l la tors  a r e  not applicable to this 
It was found that c rys t a l  con- 
pro jec t .  A single t rans is tor ,  non crys ta l -  controlled, osci l la tor  frequency 
modulated by varacaps  and an amplitude-modulated c rys t a l  controlled 
osc i l la tor  offer simple circuitry and sufficient bandwidth. 
wil l  be evaluated and compared further.  
These two types 
. 
https://ntrs.nasa.gov/search.jsp?R=19670003592 2020-03-16T16:52:12+00:00Z
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Possible  power supply methods for  the t ransmi t te r  were  studied. 
The resu l t s  indicate that nickel-cadmium rechargeable  ba t te r ies ,  charged 
by an  external  R F  source  will be the m o s t  desirable  method. 
induction charging system was built and i n  bench t e s t s  was used to charge 
a nickel-cadmium bat tery pack of the capacity required for the t ransmi t te r  
c i rcu i t ry  . 
An R F  
11. - P r o g r e s s  Made f rom March  to Septeiiiber, 1906 
A t ime-division PAM-FM multiplexing sys tem has  been selected for 
development because of c i rcui t  simplicity and minimum power requirements .  
The individual channels a r e  sampled i n  sequence i n  the t ransmi t te r  and the 
amplitude samples  a r e  t ransmit ted in  sequence over the R F  c a r r i e r .  
rece iver  de -multiplexing circui t ry ,  i n  synchronism with the t ransmi t te r  
c i rcui t ry ,  s epa ra t e s  the samples to  their respect ive channels for  recording. 
The 
A. Transmi t te r  Circui t ry  
A three channel sys tem is being built to evaluate the design. The 
block diagram for  the t ransmi t te r  is  given i n  figure 1. 
proven, the sys tem will  be extended to a l a r g e r  number of channels] the 
number being determined by the requirements  of the application of the 
p a r  ti cula r s y s tem.  
After the design i s  
The sam2ling gates  are driven by a ring counter. These gates  a r e  
opera ted  by switching on and off the power supply to differential amplif iers  
i n  each channel. Figure 2 gives the circui t  diagram for a single stage of 
- 3 -  
the ring ounter. Note that the circuit  tse o nl t r ans i s to r s  nd es i s to  r s. 
Also, since the circui t  i s  operated i n  e i ther  saturation o r  cutoff, allowable 
component tolerances can easily be plus 10070 o r  minus 50700f their  
nominal value. 
be easi ly  reduced to hybrid integrated circui t  form.  
, 
These design c r i te r ia  were  se t  up s o  that the circui t  could 
The ring counter uses  direct  coupled logic to control the sequence i n  
At any given t ime only the switching on and off of stages around the ring. 
one s tage i n  the ring is  turned on. 
around the ring, and the individual s tages  a r e  on i n  sequence. 
The clock pulse shifts the r'on'' condition 
To understand the operation of the ring counter,  consider the memory  
unit of the individual stage a s  given i n  figure 3. This unit is bistable and 
the r ing coucter consis ts  of these bistable units coupled in  a closed ring. 
Suppose stage number 3 is on. 
to the inputs of stage 3 being on and the a r r i v a l  of a clock pulse, and turns  
s tage  4 on when these conditions occur  together. 
as a single input and gate and turns  off i t s  stage when the following stage 
t u r n s  on. 
f r o m  stage 3 to stage 4. 
Trans is tor  Q in  stage 4 ac ts  a s  a n  and gate 3 
Trans is tor  Q functions 
4 
Thus the clock pulse has  effe'cted the shifting of the "on" condition 
Q5 senses  cu r ren t  i n  the stage and turns  on the external  load. 
Q2, and Q3. 1' Res i s to r ,  R bypasses  the effect of leakage cu r ren t s  i n  Q 
The remaining r e s i s t o r s  a r e  for cu r ren t  limiting. 
1'  
The clock used to dr ive the ring counter i s  an  astable multivibrator.  
Should the ring counter not function, another c i rcui t  will sense this  and 
. -  
I ,  
-4- 
r e se t  the ring counter. This automatic r e se t  feature  i s  necessary  so 
the ring counter will s t a r t  properly when the power supply is turned on 
a s  well as correct ing any malfunction during operation. 
vers ion of the ring counter i s  under evaluation and has given reliable 
A breadboarded 
opera tion. 
B. Receiver De-Multiplexing Circuitry 
A de-multiplexing gate has  been developed. This gate samples  the 
composite P A M  waveform during the t ime alloted for the individual 
channel. It charges  a capacitor which holds its charge,  within one percent ,  
until the next sample for  the channel a r r ives .  
s t re tches  the sample pulse;  and by putting m o r e  energy into a following 
low-pass f i l t e r ,  provides amplification. 
This action, i n  effect, 
The c i rcu i t  for this gate is  given 
in  figure 4. 
The differential comparator (Fairchi ld  y A  710) col-npares the input to 
the voltage on the capacitor during the channel's sample period, and a.djusts 
the capacitor 's  voltage accordingly. The circui t  tested was able to faith- 
fully reproduce the input for  signals of - t 6 volts and up to one-half of the 
sampling frequency . 
, 
The de-multiplexing gates a r e  driven by a ring counter si-milar to that 
i n  the t ransmi t te r .  The clock is a n  astable multivibrator,  synchronized by 
pulses  obtained each t ime the composite waveform changes channel. The 
sync pulses a r e  obtained by differentiating the composite PAM signal and 
passing the pulses thus obtained through a full-wave rect i f ier .  The ring 
- 5 -  
counter is r e se t  each t ime the f rame sync pulse occurs .  
synchronizing actions keep the de -multiplexing gates i n  synchronism with 
the multiplexing gates. 
These 
C. - Frequency-Modulated Osci l la tors  
A study of frequency-modulated osci l la tors  has  been made to 
evaluate types with possible application to  this system. 
c a r r i e r  frequency i s  highly desirable ,  crystal-control led osci l la tors  were  
studied. 
varying the external  capacitance presentec! to the c rys ta l .  
studying severa l  c i rcui ts  of this  type showed that i n  o rde r  to obtain 
deviations of 0. 0170, very critical. adjustments were  necessary.  
bandwidths obtained were  too small  for  the multi-channel systen-1. 
Since a stable 
It is possible to va ry  the frequency of a c rys t a l  oscil lator by 
The resu l t s  of 
Also, 
Phase  modulation of a c rys ta l  osci l la tor  was a l so  studied. This 
method would requi re  too extensive c i rcu i t ry  for the proposed integrated 
c i rcu i t  construction. 
phase-locked loop i n  the t ransmi t te r  to  stabil ize the c a r r i e r  frequency. 
This  method hasn't shown promise since it is difficult to obtain sufiicient 
isolat ion between the reference  oscil lator and the voltage controlled 
osci l la tor .  
bandwidth. 
Another possibility investigated was the use  of a 
There  a r e  a l so  problems i n  obtaining sufficient information 
In the course  of these investigations i t  was noted that, for  the 
deviation ra t ios  necessary  for the requi red  bandwidth for  the mult i -  charmel 
- 6 -  
system, the FM improvement over AM was very smal l  o r  non-existent. 
For  this reason,  and fo r  the c a r r i e r  stability offered by a crys ta l -  
controlled osci l la tor ,  an  amplitude-modulated c rys ta l -  controlled osci l la tor  
was built and tested. This PAM-AM sys tem gave good resu l t s  and had 
m o r e  than adequate bandwidth. 
Non- c rys ta l -  controlled t rans is tor ized  os  cil lator s , frequency - 
modulated by varacaps ,  were  also constructed and tested.  The PAM-FM 
circui ts  so tes ted gave good resul ts  with the exception that the frequency 
of oscillation was influenced by such fac tors  a s  power supply variations 
and by the proximity of external  objects. 
varying effects for this c i rcui t  in  integrated c i rcu i t  packaging. 
effects should be minimized by the miniature  packaging and a power 
amplifier s tage following the oscil lator.  
It remains  to  t e s t  these frequency 
Proximity 
The crystal-control led PAM-i4M circui t  and the non- c rys ta l -  controlled 
PAM-FM ci rcu i t  will be fur ther  evaluated and compared on the bas i s  of 
per formance  and e a s e  of construction. 
1 
An M. S .  thes i s  resul ted from this study of osci l la tors  and modulation. 
D. Power  Supply 
It is  planned that  the power supply will be rechargeable nickel- 
cadmium bat ter ies .  
coupling to a n  implanted pickup and a charging-control circuit .  
The recharging will be performed by an  R F  field 
Rechargeable bat ter ies  were studied and charge-discharge curves 
w e r e  obtained experimentally f o r  continuous cycling over a period of 
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seve ra l  weeks for  a pa i r  of batteries.  
Methods of charging were  studied. It i s  desirable  to charge the 
ba t te r ies  to full capacity i n  a s  short  a t ime a s  possible.  
constant voltage and constant cur ren t  charging, the charging t ime 
necessa ry  was reduced f r o m  the manufacturer ' s  recommended 14 hours  
to 8 hour s  o r  l e s s .  
By combining 
R F  powering was studied and the necessary  coupling coils were  
constructed and tested.  The charging circui t ,  including pickup coil  and 
constant voltage plus constant cur ren t  source,  was built and used to 
successful ly  cha.rge a 6 volt, 150  m a .  hr.  nickel-cadmium bat tery pack 
i n  an 8 hour period. 
l a r g e  for  a n  implanted unit SO that fur ther  work is  necessary  in this a r e a .  
The dimensions of the present  pickup coil a r e  too 
Exploratory investigations were  a l so  macle on the possibility of using 
a biological battery.  Platinum-black and s i lver-chlor ide electrodes were  
used  as anodes while zinc, steel ,  and aluminum were  used as cathodes. 
Ringer 's  solution was used a s  the electrolyte.  
the maximum power density available is  200 microwatts  p e r  square  
cent imeter  for  the exp e rim ental s e tup s inv e s ti g a t  e d . 
The resu l t s  indicated that 
2 
This study on power supply methods resu l t s  in an  M. S. thesis .  
111. Current  P ro jec t s  
A. Signal Conditioners 
Development i s  being ca r r i ed  out on preamplif iers  for  e lec t r ica l  
s igna l  channels fo r  EMG, EKG, and EEG. Resistance controlled osci l la tors  
-a -  
a r e  being studied for  use i n  the temperature  ( thermis tor  pickup) channels 
and for the channels using s t r a in  gage sensors .  
B. Construction Faci l i t ies  
Equipment is  being se t  up for construction of the t ransmi t te r  i n  
hybrid integrated circui t  form.  
dol lars  worth of equipment was purchased f rom NASA and other funds for 
the construction and testing of the multiple-channel system. 
During June to  August, around 36, 000 
The t r ansmi t t e r  c i rcu i t ry  wi l l  be fabricated on ce ramic  substrates .  
Conductive pat terns  will  be s i lk-screened onto the ceramic and then f i red  
into the ceramic .  
Chip r e s i s t o r s  and t r a n s i s t o r s  wi l l  make  up about 3/4 of the total  corn- 
ponents i n  the t ransmi t te r .  
be used. 
The components will be bonded onto these pat terns .  
Some monolithic integrated c i rcu i t s  will a l so  
A s i lk-screen  outfit has  been p rocur red  and will be put into operation 
i n  the next few weeks. 
interconnecting pa t te rns  onto the ce ramic  subs t ra tes .  It can a l so  be used 
to deposit thick f i l m  m a t e r i a l s  f o r  res i s tances  and possibly capacitances. 
The deposited ma te r i a l s  will be f i red  into the ce ramic  using one of our 
tube furnaces .  
This will be  used to lay down conductive pads and 
An older  model  Kulicke and Soffa (K and S) wi re  bonder has  been 
modified and will  be used to bond the t rans is tor  and r e s i s to r  chips to the 
subs t r a t e  pat terns .  Connections will be made  to the chips using a K and S 
-9 -  
ultrasonic wire  bonder and 1 m i l  gold or aluminum wire .  
probing sys tem has  been obtained for making DC measurements  on the 
chips and c i rcu i t s  during the fabrication process .  
A K and S 
, 
The bonding and probing machines a r e  on hand and will be installed 
in  our  clean room a r e a  very  soon. 
By using these fabrication techniques in  the construction of the 
t r ansmi t t e r ,  the s ize  of a three-channel unit, excluding power supply, 
will be about 1 inch square by 1 /4  to 1 / 2  inch thick. 
channels will add about 1 / 4  inch to the thickness. 
Adding 6 m o r e  
IV. - Estimated Schedule 
During October the fabrication facil i t ies will be put into operation. 
Construction of some of the t ransmi t te r  c i rcui t ry  will be under way. A 
breadboard of a th ree  channel system should be completed and tes ted in  
November. 
r eady  for  implanted evaluation. 
By the end of the year a three  channel t ransmi t te r  should be 
Following the evaluation of this system, the design will be extended 
to seven channels. 
to compensate for  changes in  the t ransmi t te r  due to variations i n  its power 
A l s o  refinements will be added to the rece iver  circuitry 
supply. 
-10- 
Reference s 
1. Stevens, Grady, An Examination of T rans i s to r  F-M Transmi t te rs  -
Suitable for Multiplex Bio-Telemetry,  M. S. Thesis,  Case Institute 
of Technology, November 1966. 
2. Noel, Bruce,  Power Supply Problems i n  a Biomedical Te lemet ry  
System, M. S .  Thesis,  Case Institute of Technology, November 1966. 
k 
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To Sense 
*5 
stage 
Sense 
P 
Output 
L 
T- +v 
R1 
--Q5 
*l 
R3 
Q2 w 
T o  
Load 
Sense *output 
Clock Re set  
F igure  2. Ring Counter Stage 
Input 
t t V  
Input 
Figure-3. M e m o r y  Uni t  in  Ring Counter Stage 

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POWER S U P P L Y  PROBLEMS I N  A 
BIOMEDICAL TELEMETRY SYSTEM 
Bruce W.  Noel 
November 1966 
Case Institute of Technology 
ABSTRACT 
Exploratory experiments on galvanic cel ls  for  application to 
2 
body powering a r e  described. Power densit ies as high a s  3 m w / c m  
w e r e  obtained from these cells. 
m e n t  with published data. 
pairs i n  an  electrolyte showed that the open-circuit  term-inai. volt- 
The resu l t s  a r e  in  general  ag ree -  
Experiments with var ious electrode 
ages  were  sensit ive to changes i n  oxygen concentration i n  the elec-  
t rolytes .  Poss ib le  biomedical applications a r e  suggested for this  
phenomenon. 
Evaluation t e s t s  on nickel-cadmium bat te r ies  ir,dicate that the 
batteries will  per form at leas t  as well as claimed by the manufac- 
- t u r e r .  Voltage regulation of these ba t te r ies  during dischazge is 
good over  a relatively small segment of the discharge character is t ic .  
A power supply sys tem for  a n  implantable biomedical te lemet ry  
sys tem was designed. The system couples externally-generated 
r-f power to an  implant battery-charging circui t  by induction. It 
is shown that the charging circui t  is  capable of recharging nickel- 
cadmium bat te r ies  i n  a s  little as  one-half the t ime required for 
constant - cur ren t  charging alone. 
AN EXAMINATION O F  TRANSISTOR F-M 
TRANSMITTERS SUITABLE FOR MULTIPLEX 
BIO- TELEh4ETRY 
Grady H. Stevens 
November 1966 
Case Institute of Technology 
ABSTRACT 
Microminiature  te lemetry t ransmi t te rs  have been developed 
i n  the pas t  which use tunnel diodes to produce high frequency 
oscillations. These devices have inherent power l imitations which 
r e s t r i c t  the t ransmiss ion  range to a few feet. This study i s  a n  
examination of t rans is tor  c i rcui t ry  which might possibly alleviate 
this  problem, while retaining the low power level and wideband FM 
charac te r i s t ics .  These new circui ts  will be  used on the multiple 
channel integrated circui t  t e lemet ry  system. 
Three c i rcu i t s  a r e  considered. The first c i rcui t  introdnced 
is a single t rans is tor  c i rcui t  which is capable of d i rec t  frequency 
modulation. The two remaining circui ts  possess  a c rys ta l  con- 
t rol led c a r r i e r .  As compared with tunnel diode osci l la tors ,  only 
the first c i rcui t  p romises  superior  performance a t  low power 
levels.  The two other circuits have super ior  performance a t  much 
higher power levels.  
c 
c 
An analysis  of the f i r s t  circuit ,  the d i rec t  irequency modu- 
lated t ransmi t te r ,  is  presented. The resu l t  includes a derivation 
of the tuning charac te r i s t ic ,  the deviation sensitivity, and the 
effects of square  law distortion. 
The two remaining circui ts  consist  of a n  indirect  frequency 
modulated t ransmi t te r  and a phase-locked frequency modulated 
t r ansmi t t e r .  A brief t reatment  of the indirect  frequency modu- 
la ted t ransmi t te r  is given. 
lock loop and its application to a phase-locked frequency modulated 
Also, a discussion of the basic  phase 
t r ansmi t t e r  i s  presented. 
The phase-locked frequency modulated t ransmi t te r  is analyzed 
and equations a r e  derived for  reducing the distortion caused by the 
locking action of the loop. The case  of a phase lock loop with a 
low pass  f i l ter  is a l so  analyzed. 
Finally a discussion of the possible applications of these 
circui ts  is  presented  and two d i rec t  frequency modulated t r ans -  
mitters a r e  included a s  examples.  


Investigation of Implantable Multichannel Biotelemetry Systems Semiannual Report, Mar. 1 - Aug. 31, 1966

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