Filter determines whether time period of incoming signal matches time preset in filter. When signals do not match, high or low frequency deviation reading is displayed digitally

Meers, B. Z., Jr.

English

NASA Technical Reports Server

--	 August 1973	 B73-10112 
NASA TECH BRIEF 
John F Kennedy Space Center
	
_4V 
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. 
Digital Notch Filter 
is
The problem: 
Notch filters, i.e., band rejection filters that produce 
a sharp notch in the frequency response curve of a 
system, have several disadvantages. Even digital gate 
filters, the currently most accurate notch filters, are 
frequently subject to false resonance indications. Among 
the alternative filters, inductance-capacitance resonance 
circuits and resistance-capacitance resonance circuits 
tend to be unstable and electronic counters are compli-
cated, expensive, and usually inconveniently large. 
The solution: 
A digital notch filter determines whether the time 
period of an incoming signal matches a time preset in the 
filter. When the signals do not match, the high or low 
frequency deviation reading is displayed digitally. 
How it's done: 
The filter consists of five parts: 
(1) A pulse timing circuit that generates a pulse at the 
beginning of each cycle of the filter input signal, 
as well as several other timing pulses, including a 
preset timing pulse, that are sent to various logic 
circuits in the filter. 
(2) A pulse encoder logic circuit which encodes fre-
quency deviations for input into the pulse decoder. 
(3) A pulse overlap and resonance detection circuit 
to detect high or low frequencies, establish the 
bandwidth definition of the filter, and feed logic 
pulses containing this information into the pulse 
decoder. 
(4) A pulse decoder and signal output that allows 
only signals that fall within the filter bandpass to 
be fed out. Signals representing a high or low 
frequency are put into the frequency deviation 
circuit. Also included are signal lights indicating 
whether the frequency Is.high, low, or synchro-
nized. 
(5) The frequency deviation circuit provides an analog
signal that is negative if the frequency is too low 
and positive if too high. This is used to drive an 
analog or digital recorder and/or indicator for 
readout of the degree of frequency deviation. 
This filter has numerous advantages when compared 
to currently used filters: 
(a) It is self-starting. 
(b) When it is tuned to one frequency, no other 
frequency will produce a resonance indication. 
(c) The bandwidth can be adjusted over a wide range. 
(d) It has a high Q at audio and subaudio frequencies 
and is not affected by rf interference at its input. 
(e) The filter follows the frequency variations in the 
input signal with great accuracy, since it can 
determine the frequency from only two cycles. 
(f) i It provides resonance indications, both visual and 
pulse, which have many uses. 
(g) The system is simple to construct and requires no 
external frequency standard. 
(h) It is very stable. 
Note: 
Requests for further information may be directed to: 
Technology Utilization Officer 
Kennedy Space Center 
Code AD-PAT 
Kennedy Space Center, Florida 32899 
Reference: B73-10112 
Patent status: 
This is the invention of a NASA employee, and 
U.S. Patent No. 3,566,263 has been issued to him. 
Inquiries concerning license for its commercial develop-
ment may be addressed to the inventor: Mr. Benjamin Z. 
Meers, Jr., 561 Margrave St., Harriman, Tennessee 37748. 
Source: Benjamin Z. Meers, Jr.

Kennedy Space Center

(KSC-10182) 
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https://ntrs.nasa.gov/search.jsp?R=19730000112 2020-03-17T06:46:14+00:00Z


Digital notch filter

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