Passive telemetry system enables the monitoring of vital biological functions from living organisms, without external connections or power sources. The FM system, using a phase locked loop technique, keeps the information frequency and powering frequencies separate

Visscher, J.

English

NASA Technical Reports Server

11F 
T2
August 1969	 Brief 69-10312 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information-,Springfield, Virginia 22151. 
New Passive Telemetry System 
Fig. 1

Basic Implant Schematic 
A conceptual design of a new passive telemetry 
system has been described which enables the moni-
toring of vital biological functions from living organ-
isms. This system concept has particular merit for 
applications in numerous areas such as permanent 
monitor of physiological variables without attached 
wires, medical research tool for studying animal life, 
and remote monitoring of stress or strain as in ma-
chinery, production processes or material studies. 
This passive telemetry system offers a new ap-
proach for extracting information from a body with-
out external connections or power sources. Electro-
magnetic energy from externally located transmitters 
is imparted to a specially designed circuit module 
implanted in the body. The implanted module utilizes
the propagated energy to produce an rf carrier for 
reradiation which is modulated (either AM or FM) 
by parametric changes within the sensor. 
The basic operating principle of the passive tele-
metry system may be understood by referring to the 
implant schematic shown in Figure 1. TI, T2 and 
T3 are miniature transformers wound on open-"ended 
bobbins of ferrite materials. TI and T2 are pre-
tuned to separate frequencies of external transmitters. 
The common secondary loop, composed of TI, T2, 
T3 and Dl, is a mixing circuit where sum, differelice 
and harmonic frequencies fl and f2 are generated. 
The secondary of T3 is tuned to the sum frequency 
f1
 + f2 which is amplitude and phase modulated by 
the parametric reactance of the sensor element. An 
(continued overleaf) 
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Receiving Channel	 Amphfier I 
Meier	 F	 i	 &	 lnforma Discriminator Out-Put 
Generator Of Phase 
Locked Local Oscil-	 I 
ators Signals For 	 I 
Receiver	 I 
Transmitter
Antenna System	
rOf	 I	
Clock 
Device 
Impla nted 	
HGernatO	 L	 '  Network	 Tr nsmitter II IT;;;^ 
Channel Si9nai Generator Of 
Transmitter I	
j	
Phase Locking And
Transmitter I
al 
____
_____ 
Channel Sign 
Channel Phase Locked To 
Positron A Open Loop	
o B	 Generator	
Frequency Shifting	 Received Signal 
Condition	
A	
Transmitter I 
___________ 
Freed Phase 
Position	 B Closed Loop	 .5 
 
Condition (FM Operation) 	 Channel Signall
	
Signals
Fig. 2
Phase Locked Loop Telemetry System 
external receiver-antenna tuned to the sum frequency 
picks up the reradiated signal which contains the 
information to be telemetered. 	 - 
Initial development efforts utilized an amplitude 
modulated system but the highly distance-dependent 
characteristics requiring complex automatic gain con-
trols resulted in an investigation of a frequency modu-
lation system. The FM system has several inherent 
advantages. The information frequency is nonhar-
monically related to the powering frequencies by 
using a phase locked loop technique. There is a 
virtual elimination of single frequency "cross talk" 
between power transmitter ,
 and information receiver 
in a passive telemetry system utilizing a completely 
passive circuit implant. Accurate calibration is pos-
sible due to the fact that the system is independent 
of amplitude variations and noise such as that norm-
ally encountered in AM systems. Also, an improved 
signal-to-noise ratio can be obtained using frequency 
modulation. 
The logic for the various modes of operation of 
the passive telemetry system is illustrated in the 
block diagram of Figure 2. In the AM version the 
information is contained in amplitude modulations 
of the reradiated sum frequency where. f 1 and f2 re-
main fixed and signal fluctuations result from the 
drifting off-resonance of the tuned circuit as the 
sensor parameter changes. 
'In the FM concept, only one of the two transmitter 
frequencies (f1 and f2) is fixed. The other is caused
to track the instantaneous resonant frequency of the 
sensor circuit so that the sum frequency is always 
equal to this resonant frequency. This tracking is 
accomplished by means of a phase locked loop which 
is closed through the implant. Changes in phase in 
the reradiated sum frequency of the implant module 
are fed back to the frequency control circuits which 
cause one of the transmitter power frequencies to 
vary in such a direction as to keep the phase of the 
received signal constant. The system concept has 
been developed to a breadboard stage where phase 
modulation has been demonstrated. 
Note: 
Documentation is available from: 
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Price $3.00 
Reference: TSP69- 10312 
Patent status: 
No patent action is contemplated by NASA. 
Source: John Visscher of

Fairchild Hiller Corp.

Republic Aviation Division

under contract to

NASA Headquarters 
(HQN-10214) 
Brief 69-10312	 Category 01 


New passive telemetry system

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