Design and performance of miniaturized portable heart rate and electrocardiographic monitoring system for prolonged space flight

Ko, W. H.

Mabrouk, S.

Ramseth, D.

Yon, E. T.

English

NASA Technical Reports Server

Semiannual  Repor t  VI I I  
F e b r u a r y  1969 - September  1969 
NASA Contract  No. NGR-36-003-079 
Solid S ta te  E lec t ron ics  Labo ra to ry  
Engineer ing Design Cente r  
Case  Wes te rn  R e s e r v e  Univers i ty  
W. H. KO 
E. T. Yon 
D. R a m s e t h  
S. Mabrouk 
https://ntrs.nasa.gov/search.jsp?R=19700002906 2020-03-12T04:45:23+00:00Z
I .  Abstract  
The major  purpose of this project is to design and fabricate miniatur-  
ized, portable external  hea r t  ra te  and EGG monitoring s ystems for  prolonged 
space mis  sions . 
The hear t  ra te  sys tem has been designed and is  being evaluated in 
breadboard form.  All e lectr ical  specifications for  the s.ystem have been m e t  
o r  exceeded in the breadboard model,  The amplifier,  oscil lator and power 
amplifier of the ra te  monitoring sys t em a r e  common to the ECG system, 
therefore,  the only major  subsystem remaining to be designed and constructed 
for  the ECG monitoring sys t em is the subca r r i e r  oscil lator -modulator. 
11. Hear t  Rate System Design 
The sys t em proposed is shown in F igure  1. The overal l  sys t em speci-  
fications a r e  a s  follows. 
7 A.  Input impedance > 10 R 
B. Microvolt sensit ivity 
C. Transmiss ion  distance: 10 feet  re l iabi l i ty  
( R F  field s t rength > 20p v) 
D.  Volume < 114 inch5 (not  including battery) with 
maximum dimension 5 2" 
E. Power consumptionless  than 751~. watt 
F. Weight - not including battery - l0gms. 
At this t ime,  a breadboard sys t em has been designed and is being 
evaluated. The applicable sys tem specifications have been met  o r  exceeded 
in this breadboard model. The t e s t  resul ts  obtained a r e  as  follows: 

9 A. Input impedance 2 x 1 0 ~ ~  to 10 R 
B. Sensitivity 100pv - RMS 
C .  Transmiss ion  distance > 10 feet - ( a t  10 feet,  the 
field s t rength is  35 - 5OPv) 
D. Power 4OP watts f r o m  a 2 . 4  - 2. 8 volt source .  
It is  expected that the r e s t  of the sys t em specifications (volume and 
weight) will be satisfied by the finished prototype. 
Detailed circui t  and design information will be given in the following 
sections.  
111. Input Amplifier 
The input amplifier consists of two differential amplifier s tages fol-  
lowed b.y th ree  single stage amplif iers .  The amplifier c i rcui t  d iagram i s  
shown in F igure  2. Common mode negative feedback i s  applied between the 
two differential stages to stabilize the D. C. operating point. The input stage 
uses  junction Fie ld  Effect T rans i s to r s  with low l / f  noise charac ter i s t ics  
for  high input impedance. The operating cur rent  level for  each t rans is tor  is  
0. 9pA. The amplifier charac ter i s t ics  a r e  a s  follows: 
Voltage supply 2.4 - 2. 8 volts 
D. C .  cur ren t   PA 
Voltage gain 5000 (unloaded) 
Bandwidth 90Hz 
Noise ( r e f e r r e d  to input) 10pV. RMS 
Output voltage 2v peak-to-peak 
maximum 

Operating temperature 25 - 50°c 
In its present  form,  the amplifier requires  one initial balance adjust- 
ment  and exhibits some stability problems with variation in supply voltage. 
This  unit i s  present ly being modified using frequency lag compensation tech- 
niques to overcome stability problems.  
IV.  0 s  cil lator and Power Amplifier 
The oscil lator and power amplifier a r e  shown in F igure  3. The osci l -  
la tor  used is  a s tandard Colpitts oscil lator tuned to 67MHz. Attempts will be 
made  to use  a c rys ta l  controlled oscil lator for  the prototype unit. The proto- 
type model frequency will depend upon the availability of miniaturized crys ta l s ,  
but will probably be 120MHz. 
The power amplifier is  a c lass  C tuned doubler. R-C  coupling with 
the oscil lator is used in an effort  to minimize the final unit volume. The out- 
put tank i s  tuned to 134MHz but will be raised to 240MHz for  the prototype. 
The total  power required for  the oscil lator-amplifier is  10 milliwatts fo r  con- 
tinuous operation. The unit i s  operated in pulse mode by the application of a 
positive triggering potential to the base of the 2N3663 in the oscil lator s e c -  
tion. F o r  the pulse mode of operation used in this unit, the average power 
was decreased  to 15 microwatts.  At this level the field strength,  measured 
at a dipole placed 10 feet f r o m  the t ransmi t te r ,  was 35 - 50 pvol ts .  
V .  Pulse Modulator 
The modulator used in this s ystem is a monostable multivibrator 

using complcmentary devices. Triggering of the n~ul t iv ibra tor  is accom- 
plished by the 2N3117 t rans is tor .  initially a l l  t rans is tors  a r e  cut off and s tay 
off until a positive pulse is applied to the base of the 2N3117. When a suffi- 
cient pulse is applied, this t rans is tor  will conduct for  the t ime the pulse is 
present .  The collector cur rent  produces a sufficient drop ac ross  the 68KQ 
res i s to r  to force  the 2N3638 into conduction, which causes the 2N2923 to con- 
duct. At some t ime,  se t  by the collector-base coupling circui t  t ime constant, 
the 2N2923 will cease to conduct, allowing the 2N3638 to stop conduction. The 
pulse width a t  the output of the multivibrator has been se t  to be 1. 25 mil l i -  
seconds. This was chosen since i t  is shor t  enough, cp nsidering the normal  
hea r t  ra te ,  to allow smal l  duty cycle operation yet long enough to allow the 
oscil lator and power amplifier to  set t le  to a steady s tate  operation af ter  being 
switched on. 



Solid State Electronics Laboratory Semiannual report, Feb. - Sep. 1969

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