Telemetry system utilizing miniature, single-channel, crystal-controlled transmitter is described suitable for biomedical applications. Receiver used in conjunction with transmitter is narrowband superheterodyne FM receiver with crystal control in both conversion stages

Fryer, T. B.

Westbrook, R. M.

English

NASA Technical Reports Server

December 1972 B72-10255 
•	 NASA TECH BRIEF 
Ames Research Center
	 4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Narrowband, Crystal-Controlled Biomedical Telemetry System 
A miniature, single-channel, crystal-controlled trans-
mitter has been developed for biomedical applications. 
Maximum operating range is achieved at minimum 
transmitter power by narrowband (± 7 kHz) frequency 
modulation of the RF; since the radiated power is 
limited to meet FCC regulations in the 88- to 108-. 
MHz band (50 V/m at 15 m), only manufacturer's 
type approval is required for the device and the user 
does not need a license. Additionally, maximum op-
erating range is achieved within the FCC power lim-
itations by use of a narrow band that is consistent 
with the required medical information bandwidth. 
Since the transmission bands of commercial FM sta-
tions are relatively widely separated and carefully 
regulated, the biomedical transmitter frequency can 
be located in a guard band between stations in order 
to minimize interference. 
For EKG and EEG applications, where an informa-
tion bandwidth of 0.1 to 150 Hz is adequate, a sub-
carrier with a fixed frequency (approximately 1 kHz) 
is used with differential pulse-width modulation 
(DPWM). This type of sübcarrier can be obtained 
with simple circuitry in both the transmitter and de-
modulator. Moreover, since DPWM is self-aligning, it 
can be handled effectively by magnetic tape recorders 
having relatively high tape speed errors. For EMC 
and other high frequency data signals, the RF can be 
frequency modulated directly without the use of a 
subcarrier. 
The receiver used in conjunction with the bio-
medical transmitter is a narrowband superheterodyne 
FM receiver with crystal control in both conversion 
stages. Double conversion and a ceramic filter in the..
second IF stage provide a selectivity of 6 dB at ± 7 
kHz and 50 dB at ± 15 kHz. The receiver is readily 
constructed by modifying a low-cost mobile monitor 
radio from the 148-174 MHz band to the 88-108 
MHz band. Modification consists of tuning the RF 
amplifier to the lower frequency band and changing 
the first local oscillator from a frequency tripler to a 
doubler. A variable capacitor is added to the crystal 
circuit to allow alignment of the receiver (± 10 kHz). 
The coupling between the RF amplifier, local oscil-
lator, and mixer are also adjusted to the new carrier 
frequency. 
The output from the receiver's FM discriminator is 
the DPWM subcarrier which is applied to the audio 
amplifier and subcarrier demodulator. The demodula-
tor is a limiter which reshapes the direct-coupled 
rectangular waveform from the receiver; the limiting 
action maintains the subcarrier pulse train at uniform 
amplitude, and low-pass filtering of the rectangular 
waveform reproduces the original EEC input. For a 
subcarrier frequency of 1 kHz, a simple low-pass 
filter with 18 dB/octave rolloff at 100 Hz eliminates 
the sampling frequency, but the DC level information 
within the 0- to 100-Hz data bandwidth is retained. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer.
 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 72-10255 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
is
https://ntrs.nasa.gov/search.jsp?R=19720000255 2020-03-17T03:27:45+00:00Z
Patent status: 
NASA has decided not to apply for a patent. 
Source: Richard M. Westbrook and 
Thomas B. Fryer 
Ames Research Center 
(ARC-10708)
0 
. 
S 
B72-10255	 Category 01


Narrowband, crystal-controlled biomedical telemetry system

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