1108 
MONITORING 
Typical active network terminations for 
transmitters might include the following: an 
antenna, amplifier, a biological specimen such 
as fluid or body tissue. Each termination, what- 
ever it might be, has its own peculiar electrical 
properties that must be known before it can 
successfully be coupled to the active network. 
Knowing the properties of the port termina- 
tions in terms of the s-parameters gives the de- 
signer the specific information he must have for 
matching these terminations to the active net- 
work for achieving maximum power transfer. 
In many practical designs the terminations are 
known and cannot be modified so that the active 
network must be designed accordingly. A fixed 
input or output termination doesn't present an 
unusual design problem if there is, in the end, 
a physically realizable solution. 
Of all the variables one can list for considera- 
tion of input and output terminations a few are 
of extreme importance. The frequency response 
and termination impedances are among those 
features that are often not considered in sufii- 
cient detail during the design of a biotransmit- 
ter. It is difficult, if not impossible, to obtain the 
data required for making an accurate analysis 
of the electrical characteristics of biological 
systems. Sometimes, the task is further com- 
plicated by the time varying nature of the 
biological system. In spite of all the drawbacks 
and indeterminants, measurements must be 
made and used to integrate the input and output 
terminations together with the active network 
into a compatible, reliable, and useful design. 
As much witchcraft as possible must be elimi- 
nated. 
Input data can and do come from a variety 
of sources and each has its own individual 
characteristics that must be considered. ECG, 
EEG, myograms, pressures and temperatures 
are physiological functions that are monitored 
in great detail by a variety of techniques and 
electronic equipment. There are also any num- 
ber of important measurements being made 
that are of keen interest to only a few individ- 
uals ; specialized equipment is fabricated and 
standard procedures and equipment are not de- 
veloped and available. 
Electronic systems are designed to meet spe- 
cific objectives and one must never lose sight 
of his objective during the design process. For 
example, a system may be designed as a signal 
conditioner where some filtering or amplifica- 
tion is required. In other instances it might be 
that data acquisition and transmission is the 
goal. It is the latter case that this paper dis- 
cusses, and specific illustrative design example 
is included. 
INDIVIDUAL COMPONENT MEASUREMENTS 
It is easy to purchase resistors that do not 
have a reactive component at UHF frequencies, 
but when they are installed in a circuit, care 
must be taken to keep their lead lengths short. 
Figure 5 shows the impedance for a 50 n re- 
sistor that has 2.5 cm leads. For the same com- 
ponent with short leads the resistance appears 
as 50 n. 
Inductors and capacitors, like resistors, have 
values that are functions of lead lengths. The 
inductors used in the designs discussed in this 
paper are of the molded f errite core type ; they 
are not hand-wound. Hand-winding is generally 
not satisfactory because of the variability from 
one coil to the next. The molded inductors are 
manufactured within a tolerance of 30 % which 
is acceptable. 
When several identical circuits are being fab- 
ricated it is best to use a printed circuit board 
if at all possible. Then, even if the components 
do not exhibit "pure values" because of layout, 
at least the contributions due to stray react- 
ances will be repeatable. 
One can spend much time studying the dif- 
ferent variations in the s-parameters for tran- 
sistors. Kim ^ has done such a study. Some data 
is given here to help the reader visualize tran- 
sistor reflection coefficients as functions of fre- 
quency. 
The 4 s-parameters for a UHF transistor are 
plotted in Figure 6 ; the A curves. The operating 
point for these data was Vce = .7 V and Ic = 
.1 mA. 
Data were taken (Figure 6, B curves) for 
the same frequency range, only the collector bias 
current was changed to Ic = .5 mA. The Sai 
parameter has undergone the greatest change 
because it is now greater than 1 and is not 
plotted. 
