Self-tuning filter automatically adjusts its center frequency to track signal frequency. This permits the use of a filter with a bandwidth smaller than the range of input signal frequencies

Deboo, G. J.

Hedlund, R. C.

English

NASA Technical Reports Server

Circuit Diagram of Self-Tuning Filter 
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July 1970	 Brief 70-10337 
NASA TECH BRIEF 
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 Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
A Self-Tuning Filter 
The problem: 
Bandpass filters are commonly used in electronic 
systems to improve the signal-to-noise ratio. If the fre-
quency of the input signal is variable, as it is from many 
types of vibrating transducers, a wideband filter is 
frequently used to avoid signal attenuation and phase 
shift at the frequency extremes, and the filtering is then 
not as effective in rejecting noise. 
The solution: 
A self-tuning filter that automatically adjusts its 
center frequency to track the signal frequency. This 
technique permits the use of a filter with a bandwidth 
considerably smaller than the range of input signal 
frequencies. 
How its done: 
A resistance-capacitance bandpass filter whose cen-
ter frequency can be varied by means of a single resist-
ive element is used. However, instead of an ordinary
resistor, a field effect transistor (FET) is used so that 
the voltage of the tuning element can be controlled. 
The control voltage is derived from a phase detector 
which compares the phase of the filter input and out-
put. If the phase between the input and output is 180°, 
the filter is tuned to the input frequency. If the phase 
is less than 180°, a correcting voltage is applied to the 
FET to retune the filter until the phase is 180°, at 
which time the filter is again in tune. A circuit dia-
gram of the self-tuning filter is shown in the figure. 
A reference frequency is not required as in lock-in 
amplifier filters. Also, there is no change in phase as 
the signal frequency varies. A self-tuning filter that 
covers the range from 2 kHz to 20 kHz, while main-
taining constant bandwidth and center frequency 
gain, has been built. It can be made to cover other 
decades of frequency by switching capacitors. 
Note: 
Requests for further information may be referred to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B70-10337 
Patent status: 
Inquiries about obtaining rights for the commer-
cial use of this invention may be made to National 
Aeronautics and Space Administration, Code GP, 
Washington, D.C. 20546. 
Source: G. J. Deboo and R. C. Hedlund

Ames Research Center

(A RC- 10264) 
Category 01 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
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A self-tuning filter

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