BIOTELEMETRY TRANSMITTER DESIGN METHODS 
V. G. Ellerbruch,* F. C. Fitchen' and R. W. Sawrey' 
When telemetry transmitters are designed for opera- 
tion in the UHF frequency range, classical lumped- 
parameter theory cannot be easily applied. Simple 
passive circuit components, like resistors, appear as com- 
plex impedances, and active devices, such as transistors, 
exhibit frequency sensitive gains and phase angles. 
These deviations from the ideal complicate transmitter 
design. 
Crystals, at these frequencies, cause a special prob- 
lem because of their narrow bandwidths and the ever 
present capacity. Also, crystals cannot be cut to place 
their fundamental frequency in the UHF range; those 
I that are used operate on overtones. Since many over- 
I tones exist that are quite close together, the circuit 
must be designed to select the correct frequency. It is 
true that the drive level shquld not be an important fac- 
tor when designing with crystals, as long as a maximum 
power level is not exceeded; however, for practical low 
I energy circuits such as those being used in biotelemetry 
I systems the drive level must be considered. Data is 
! given showing how crystal impedance depends on drive 
level. 
This paper describes design methods that do not re- 
quire complete lumped-parameter component models. 
Transmission line techniques are used with the elements 
in the design represented by reflection coefficients or 
s-parameters. The measured data are plotted on imped- 
ance diagrams which are used as aids in the design 
process. 
I j A design example is used to illustrate the methods and 
i data are given to show the effects of encapsulating ma- 
terial. In general, encapsulating materials cause fre- 
quency shifts which are shown to be functions of curing 
time and coating thickness. 
For FM transmitter design, one can obtain the de- 
I sired frequency deviation by using purely resistive 
ji transducers. By using care, the circuit does not have to 
be overly complicated to realize such a design. 
1 
II INTRODUCTION 
I Designing telemetry transmitters can be a 
frustrating experience, even for the seasoned 
designer. There are several reasons why trans- 
mitter design presents special challenges. 
i 
;l| * Electrical Engineering Department, South Dakota State Univer- 
•j sity, Brookings, South Dakota. 
j| ** PMC Corporation, Minneapolis, Minnesota. 
The feeling of being overv^^helmed sometimes 
sets in when a designer realizes the stringent 
requirements of his system; limited physical 
size, hostile environments, minimum expendable 
energy, long operational life and economic feasi- 
bility. This last condition is not the least to be 
considered because it forces the use of standard 
tolerance, off-the-shelf components. 
It is usually impossible to purchase stock 
electrical components where the manufacturer 
gives all the pertinent data. Perhaps the manu- 
facturer can supply such information but the 
cost of doing so is passed along to the user, 
and may be prohibitive. 
This paper describes a design method, that 
relies upon use of a set of easily measured 
s-parameters. The method does not require that 
one have available a complete analytical descrip- 
tion for each component as a function of fre- 
quency or biasing, but by making careful meas- 
urements and employing graphical techniques 
a satisfactory design is achievable. The method 
allows for close examination of limiting factors 
such as battery drain, environmental effects, 
component tolerances and the like. 
BASIC THEORY 
The development of very high frequency 
semiconductor devices has stimulated consider- 
able use of the scattering parameters. These 
parameters are commonly known as s-param- 
eters. The use of s-parameters has generally 
been confined to high frequency (greater than 
100 MHz) work; however, such a limitation is 
not essential as s-parameters have been used 
for circuit design at frequencies as low as 10 
MHz. The use of s-parameters for 2-port net- 
works has some distinct advantages over other 
parameters. 
An important reason why s-parameters are 
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