Technique automatically sets the frequency of narrow band phase-lock loops within automatic lock-in-range. It presets a phase-lock loop to a desired center frequency with a closed loop electronic frequency discriminator and holds the phase-lock loop to that center frequency until lock is achieved

Attwood, S.

Kline, A. J.

English

NASA Technical Reports Server

April 1968	 Brief 68-10130 
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. 
Phase-Lock Loop Frequency Control and the Dropout Problem 
In the tracking of space satellites, receivers featuring 
phase-lock loop circuitry have been used to advantage, 
but usually, manual frequency adjustment is required 
to maintain contact. Additionally, such systems fre-
quently suffer loss of signal (dropout) to a degree that 
reduces such manual frequency adjustment techniques 
to futile and frustrating "hunting" exercises. An 
investigation has been made into these problems and 
concepts derived that should eliminate or greatly 
relieve them. 
A technique has been conceived for automat-
ically setting the frequency of narrow band phase-
lock loops within automatic lock-in range. In the past, 
phase-lock loops have been constrained so as to pro-
vide an automatic lock-in bandwidth sufficiently wide 
to accommodate both the uncertainty factor of the 
incoming frequency and the drift or uncertainty 
characteristics of the phase-lock loop. 
In many applications it is desirable to construct a 
phase-lock loop possessing narrowband filter charac-
teristics, while being capable of tracking or locking to 
a signal varying in frequency by a factor large relative 
to the filter bandwidth of that phase-lock loop. Such is 
the case for spacecraft phase-lock tracking receivers 
operating with a large frequency uncertainty due to 
the high relative velocities and resulting doppler 
effects as well as equipment drifts. 
This concept suggests a means of automatically 
presetting a phase-lock loop to a desired center 
frequency by use of a closed loop electronic frequency 
discriminator, and holding the phase-lock loop to that 
center frequency until lock is achieved. Specifically, it 
uses a two-state digital frequency discriminator and a 
reference signal in a "bang-bang" type nonlinear 
frequency lock loop. In this manner the phase-lock 
loop is preset to, and dithered slightly around, a 
desired center frequency until the phase-lock is
achieved. The phase-lock transfer characteristics of the 
loop are modified only slightly by the additional 
frequency lock circuit, thus providing normal phase-
lock of the signal to override the frequency preset. 
In a typical operation, there is a very good chance 
that the signal will disappear or fade so badly that the 
received signal-to-noise ratio. will be inadequate to 
permit tracking. Should the condition persist, normal 
tracking procedures will not be able to recover the 
signal when it reappears. There are several reasons 
for this: acceleration during the dropout period, 
noise, and imperfect components. The latter is a 
practical matter, but even in the absence of equip-
ment limitations, the problem still cannot be solved 
by conventional means. 
This concept considers the limitations of conven-
tional tracking methods and suggests a method which 
shows promise of being able to allow the system to 
carry through a dropout period of up to 2.5 seconds. 
Although there may be a wide variety of causes of 
received signal dropout, there are two that are apt to 
occur in many operational situations. The first may 
occur during injection into orbit, immediately after 
which accurate tracking is desired. This dropout could 
occur rapidly and exhibit a very large reduction in 
signal strength; however, the accelerations are not 
expected to be inordinate at that time. The second 
cause is multipath, which causes fading. The rate of 
signal decay is probably lower and only under very 
special conditions can the received signal strength 
approach zero. The "down" time is generally less than 
during orbital injection. 
In order to simplify the analysis and to permit 
readier understanding, a simplified carrier loop is 
analyzed. The principles can be applied without modi-
fication in systems that have recently been developed. 
(continued overleaf) 
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.
https://ntrs.nasa.gov/search.jsp?R=19680000130 2020-03-12T05:29:52+00:00Z
Whenever the signal is present, the phase-lock loop 
will track the phase of the received carrier. Depending 
upon the characteristics of the "tracking filter," the 
tracking capability is governed by the signal. spectrum 
of the received phase. For example, if the received 
phase has constant rate, there will be a small phase 
tracking error with a Type I loop and none with a 
Type II loop. In general, one may assume that the 
loop response must be sufficient to track a fairly high 
acceleration for a period of seconds. As a result, under 
most conditions, the loop may be considered to have 
reached "equilibrium" in a short time before a drop-
out might occur. 
When signal dropout occurs, the control signal out 
of the phase detector vanishes but the noise does not, 
but may even increase if the agc falls. The loop is now 
open. 
When the signal strength recovers, the loop will 
relock if the accumulated frequency error is not too 
large. An alternate method of recovery is to sense the
signal loss in the agc circuit, open the loop until the 
system indicates recovered signal strength, whereupon 
the loop is reclosed. Under poor conditions neither 
method is acceptable without appreciable loop sophis-
tication. 
Note: 
Copies of these concepts may be requested from:

Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B68-10130 
Patent status: 
No patent action is contemplated by NASA. 
Source: Stanley Attwood and Arthur J. Kline 
of Motorola, Inc. 
under contract to 
Marshall Space Flight Center 
(MFS-13948, 13950) 
Brief 68-10130	 Category 01


Phase-lock loop frequency control and the dropout problem

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