1174 
MONITORING 
Figure 5. — Photomicrograph of exciter-oscillator chip before die scribing and wire bonding. 
Command Receiver Circuit 
The final circuit used in the implantable sys- 
tem is the command receiver. Its schematic dia- 
gram is illustrated in Figure 10. This receiver 
is a tuned radio frequency (TRF) receiver 
vi^hich operates on a single radio frequency. 
Consequently, there is no need for a local oscil- 
lator or an IF amplifier both of v^^hich vi^ould 
require discrete inductors and capacitors. Fre- 
quency selectivity is accomplished largely by 
means of a resonant ferrite rod antenna, al- 
though a discrete ceramic filter element betv^^een 
the RF and detector gain cells can be used to 
improve selectivity. Essentially, the receiver 
consists of three stages: an RF amplifier gain 
cell, an AM detector gain cell and an audio 
amplifier gain cell. The function of each cell 
is determined by the collector load resistances 
and coupling capacitances associated with it. 
All resistors are high value (125 Kn — 1.5Mn) 
base pinch resistors; diode biasing is used to 
conserve power and avoid excessively high 
value resistors. The receiver design greatly ex- 
ploits the constant temperature environment 
within an animal in its use of these later two 
techniques. 
The most remarkable feature of the command 
receiver is its ultra low power drain of 13 fjiW. 
This feature is essential since the receiver must 
operate continuously. Because receiver power 
drain varies monotonically with radio fre- 
quency, a relatively low radio frequency (500 
kHz) is used. The modulation frequency is 3 
kHz ; this permits a further reduction in power 
drain since much of the receiver gain can be 
achieved at audio rather than radio frequencies. 
The overall conversion gain is 70 dB and the 
sensitivity is 100 (jlV. The receiver drives a 
