Digital comparator permits discrimination at accuracy of reference frequency. The compare circuit is a shift register element

Reid, W. J.

English

NASA Technical Reports Server

June 1967	 Brief 67-10151 
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. 
Electronic Frequency Discriminator 
fwLL
r LQ..	 a / a = - f j 0 -
Compare 
r 
b / U r fi0+90 
S
In a conventional frequency discriminator, the 
unknown frequency is compared against a circuit 
tuned to the desired center frequency. In such a dis-
criminator, the accuracy of the discrimination is the 
stability of the discriminator itself. Replacement of 
the conventional discriminator with a digital fre-
quency comparator, using a very stable reference 
frequency, would eliminate stability problems in the 
discriminator and allow discrimination to an accuracy 
of that of the reference frequency. 
The figure shows the method of frequency compari-
son accomplished by use of a digital frequency com-
parator. 
The unknown frequency f is mixed with the refer-
ence fr at 0 degrees to form fa, and with fr at 90 
degrees to form fb in two different mixers. The out-
puts, fa and fb of the two mixers, are the same fre-
quency but are shifted in phase by 90 degrees. The 
phase compare circuit looks at the phase of fb when 
a=()-(P=O degrees. In this case, where fw<fr, the 
frequency fb will lead fa by 90 degrees which will 
cause the output of the phase compare circuit to go to 
a "I" level. 
Implementation of a digital frequency comparator 
with such an operation, requires a difference mixer
capable of mixing two signals so that the difference 
frequency varies from 0 to 1.2 MHz. 
Using a shift register element as a mixer with one 
signal on the shift input and the other signal on the 
steering line, the maximum difference frequency is 
1/2 of the clock frequency, or, in this case, 1.6 MHz 
if the reference frequency of 3.2 MHz is used as the 
clock. The shift register element transfers whatever 
logic condition exists on steering line to the output 
whenever the clock makes a negative transition. 
The phase compare circuit is simply a shift register 
element with the fa frequency on the clock and the fb 
frequency on the steering. The negative transition of fa 
during the "0" level of fb would cause a "0" level to be 
transferred to the output. If fb were at a "1" level 
during the negative transition of fa, a "I" would be 
transferred to the output. The shift register might not 
generate the actual difference frequency at all differ-
ence frequencies occurring in the range of the frequency 
discriminator. However, as the difference frequency 
becomes small compared with the clock frequency, the 
actual difference frequency will be produced. 
The fact that the actual difference frequency is not 
produced does not affect the effectiveness of the 
frequency comparator as both mixers will still produce 
the same frequency. 
-.	
(continued overteat) 
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.
https://ntrs.nasa.gov/search.jsp?R=19670000151 2020-03-11T20:48:13+00:00Z
Note:	 Patent status: 
Inquiries concerning this invention may be di-	 Inquiries about obtaining rights for the commercial 
rected to:	 use of this invention may be made to NASA, Code 
Technology Utilization Officer	 GP, Washington, D.C. 20546. 
Marshall Space Flight Center 	 Source: W. J. Reid 
Huntsville, Alabama 35812	 of Motorola, Inc. 
Reference: B67-10151	 under contract to
Marshall Space Flight Center 
(M-FS-2434) 
Brief 67-10151	 Category 01


Electronic frequency discriminator

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