1116 
MONITORING 
ever, some general rules help. Tuning of the 
tank circuit in the Hartley oscillator may not 
change the active circuit contour significantly 
but does change the frequency relationship of 
the contour. For example, a change in tank tun- 
ing moves point A, Figure 12, which corre- 
sponds to a frequency of 151.118 MHz to posi- 
tion B. In this fashion retuning gives the option 
of moving a particular frequency to a desired 
point on the contour. 
There is another way of significantly control- 
ling the contour and that is by adding shunt ele- 
ments across the terminals of the active device. 
A shunt inductor rotates the contour clockv^^ise 
along paths of constant conductance. A shunt 
capacitor rotates the contour counter-clockvv^ise 
along the same paths. This operation is depicted 
by the dashed curve in Figure 12. 
The method of retuning and adding shunt 
elements allow^s for a degree of maneuverability 
of the active circuit contour. This control is not 
as v^^ell defined as one v^ould like because as 
components are added, or resonant frequencies 
changed, the quality factors (Q) controlling the 
circuit also change. These non-ideal components 
cause deviations from theoretically "nice" re- 
sults, but with a little experience it is not diffi- 
cult to become adept at "manipulating con- 
tours." 
All that remains now is to manipulate the 
active circuit contour until it is tangent to the 
crystal contour at a particular point. The in- 
ductor Ls, as shown in the complete circuit dia- 
gram of Figure 11, situates the contours so they 
are tangent. Since the crystal curve is a func- 
tion of frequency, every point on the curve cor- 
responds to a unique frequency; thus, the tan- 
gent point relates directly to the frequency of 
oscillation, 151.118 MHz. Note that this fre- 
quency does not necessarily have to be the se- 
ries resonant frequency of the crystal. Capacitor 
Cs in the schematic diagram is a dc blocking 
element. 
Certainly, it would be unfair to leave the im- 
pression that components must be lumped to- 
gether into only two p&rts, active and passive, 
before a design can be completed. From the fact 
that the s-parameters, or any other convertible 
set of parameters, are exactly known, a system 
can be mathematically designed that will func- 
tion ; however, so far there isn't any set of de- 
sign rules that automatically leads one through 
the design process. Certain experience, even 
though based on classical circuit design tech- 
niques, is still required. 
MODULATION CIRCUITS 
There are two active circuit contours that 
show how FM (frequency modulation) can be 
obtained by using a purely resistive or a purely 
reactive transducer. 
Figure 13 shows the two cases. Case I il- 
lustrates an active circuit contour the frequency 
of which can be controlled by a resistance. A 
change from point A, 70 ohms to point B, 100 
ohms forces a change in frequency of 10 MHz. 
By design, a contour such as Case 11 of the same 
figure can be obtained where a change of 10 
MHz is caused by a reactive change of from 10 
ohms at point C to 50 ohms at point D. 
This design method requires that one know 
the relationship of the transducer reflection co- 
efficient over the frequency range of interest, 
and then, for a specified Af of the modulator, 
design the active circuit. The design method 
for FM systems is not much different from that 
discussed in the preceding example. 
ACKNOWLEDGEMENTS 
A practical data acquisition system has been 
designed where the methods presented in this 
paper were used. The project was a joint en- 
deavor between the Electrical Engineering and 
the Wildlife and Fisheries Sciences Depart- 
menta (S-521) at South Dakota State Univer- 
sity. Much of the electronic research was sup- 
ported by NSF Grant GK-21034. 
REFERENCES 
1. BodwaV, George. Two port flow analysis using gen- 
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Vol. 10, No. 6, May 1967. 
2. Weinert, Fritz. Scattering paranjeters speed de- 
sign of high-frequency transistor circuits. Electron- 
ics, September 6, 1966. 
3. Froehner, Wi«:.iam. Quick amplifier design with 
scattering parameters. Electronics, October 16, 1967. 
4. Anderson, Richard. S-parameter techniques for 
faster, more accurate network design. Hewlett-Pack- 
ard Journal, Vol. 18, No. 6, February 1969. 
5. Kim, Young D. Scattering parameters of VHF 
