Nichols, G. B.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: GP 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
March 30, 1971 
TO: USI/Scientific & Technical Information Division 
Attention: Miss Winnie M. Morgan 
Counsel for Patent Matters 
FROM s GP/Office of Assistant General 
SUBJECT: Announcement of NASA-Owned 
U.S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8, 1970, 
the attached NASA-owned U.S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U.S. Patent NO. : 3,399,299 
Corporate Source : Goddard Space Flight Center 
Supplementary 
Corporate Source . 
NASA Patent case NO.: XGS-01118 
Gayle Parker 
Enclosure : 
Copy of Patent 
NASA-HQ 
https://ntrs.nasa.gov/search.jsp?R=19710014186 2020-03-17T03:03:11+00:00Z
6. B. NICHOLS 
APPARATUS FUR FHASE STABILITY DETERMINATlON 
Ap~aratm for processing a high-freque~~y signal to be 
applied to correlation means 9 determine the phase sta- 
biiity thermf. Two auxiliary. os&ators each provides 
a signal having tbe same cbaracterktics as tbe high-fre- 
quency Signal ymkr catsideration, and tbese are inde- 
pendently multrIJlied with tfie high-frequency signal to 
produce two product signals. High frrquency components 
tbe product signal to provide two low 
each having a component introduced 
by the phase instability of the original high frequency 
signal. These two low frequency sigpals, in tm, are 
digirked are are available to be applied io correlation 
apparatus whereby the phase and frequency instaWity 
of the higft-frequecy signal may be determined. 
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10 
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The inventian described herein may be manufactured 
and wed by and for the Government of the United 3o 
States of America for governmental purposes without the 
payment of any royalties thereon or therefor. 
This invention relates generally to frequency measure- 
ment techniques, and more particularly to apparatus for 
determining the ilbsoiutz phase stability of a high fre- 35 
quency signal source by cross-correlation of two inde- 
pendently generated low frequency signals to produce 
an equivalent autocorrelation of the high frequency 
signal. 
obtaio an accurate determination of the phase stability 
of a reference signal such as a sine wave. For example, 
in the design of narrowband, phase-locked receivers, 
knowledge of the phase stabitiy of the receiver reference 
signaf source is required so that optimum receiver band- 48 
pass characteristics can be ctiIized that wiU reject noise 
yet remain locked to the transmitter. 
A particularly WN technique for measwhg slight 
frequency and phase varktions with i' Mgh degree of 
accuracy is th3t of autocorretatian, that is, comparing 80 
a signal with ii& in a mznner to produce a function 
which is the time zverage of tbe product of the signal 
by the same Signal snifted in phas.ase, When the two signals 
are in phase the auiocorrelation function is a maximum, 
and when the p h w  shift is a quarter wave, or any odd 53 
multiple tbereof, the autocorrelation function zpproacbes 
zero. In the instance of a periodic signal source, for 
example, the resulting autocorrelation function contains 
information about the spectral power distribution con- 
tour which contains fwqaency components that produce 60 
t& phase instability, and such components can be cx- 
traded therefrom by further analysis of the autacofrela- 
tion function. 
The preferred aad esiablished techniques fw obtain- 
ing the autocorreia~on function of a signal kclade the = 
.- of a data processing digitid computer of a conven- 
tioaat type. Frogam charts are avdlable for use with 
such mputers to enable the autocorrelation fundon 
& a signal to Zre obtained. Such techniques have the 
providing memory storage d tbs: 70 
the computer may a h  be pm- . 
thc resulting cornlation function. may 
Tbere are many instances where it is desirable to 40 
be trans€ormed from a time base to a &cqilency baSe 
by the use of La Place and Fourier trtsnsf0-s. 
BrieBy, the signal is sampled at discreet iatervais On 
a time base to provide a &gital infomation repres+nta- 
five of the instantaneous magnitude (Vdtage) Of ths 
signal for each sampling. The same signal, shifted 41% 
time, is similarly sampfed. Each samfle of the dgaaI IS 
then multiplied, in turn, by all the smpks of the phast 
shifted signal, with the result of all such rnultipEcafhIs 
added and divided by #e number of samples to grodwx 
a correlation function representing an average p a w e  
densiry. Subsequent frequency analysis of this cemls- 
tion function provides an indication of the phase stability 
of the s i s a l  under investigation. 
It is readily apparent from the foregoing tbat for a 
periadk signal such as a sine wave, a number of samp'hs 
must be taken for each cycle in order to provide satid- 
caUy signiicant results However, at the present state of 
the computer art the rate at which a signal can be digitized, 
stored and processed in a digital computer is limited by 
freqwncy response considerations of the various equip- 
mts involved. Accordingiy, the foregoing Qethnique 
is not practicable for use with oscillators providing refa- 
ence sine waves of 30 megacycles or higher, a w m m  
frequen.y range used in present day phase-locker telem- 
e w  receivers. This moblem has therefore been ap 
priached by transla& the signal to a lower frequeniy 
by use of mixers or product detectors. However, such 
marigemenis require the use of an extremely stable local 
oscillator which must be at least as st&e as the signal 
source under investigation, and since the signal 60wce 
itself mag require sial;.af-the-art stability, incollflusive 
results are obtained For example, accccuracies of one 
part in X O B  are required, and present techniques are based 
on tbe tenebrous assumption that the iocaI oscillator P 
good as, or M) worse than, the signal source under in- 
vestigation. In addi~on, since the signal soare. and tho 
local oscillator have the same characteristics, it is difE- 
cult to identify the desired signal which is to be processed. 
Accordingly, it is an object of the present invention to 
provide apparatus useful in determining the phase stability 
of a high frequency signal source. 
Anothei abject of the present invention i s  to prdde 
an improvement in apparatus for performing the ew'v- 
dent  autocorrelation of a signal by the crors-earrektion 
of.two signals of lower frequency, each havirg B w m n  
frequency component 
A further object of the presznt invention is to provkle 
novel apparatus for delermining the phase stabirity c.f a 
b;gh trequency signal by use of a digital computer adapted 
to perform the cross-correlation of two low frequency sig- 
nals derived from the multiplication of tbe high frequency 
signal and two auxiliary signals of similar charackrish, 
The foregoing and further objects of the present inven- 
tion are attained by providing two auxifirPry sipgnds from 
oscillators having characterhics similar to those of tlm 
oscillator producing the signal of interest under investiga- 
tion. The auxiIiary signals are independently mult ipw 
with the signal of interest, and the high frequency c o m p  
nmts removed therefrom to provide two low frequency 
signals* each contaking a frequency component introduced 
5y the phase instability of the signal sotirce under i n v e  
tigation. These two low frequeilcy signals, in turn, efcl 
d i g i h d  and e i t h r  stored or fed directly &to a digital 
computer. The computer is programmed to corfiputc the 
cros-cormlation funciion of tbe two iower friqueacy 
signals, that is, provide a signal that represents tftr d w  
of time coherenee of tbose componcnta of the two icq 
frequency signals that have the same frcquenoy and phaos, 
'Ws cross-correlarinn function, in fum, ia !ha equivak3 
of the autworrela!!on of tho signal of interest, and p m  
- _  
ence now lio FIGURE 2, the: outpnts of d 
and 14 are represzntatively shown by wave- 
fcmm A, B and C. f t  is to be- noted that eaeh oumt 
is sinusoidal in nature and all are substantially Miare 
15 each vaTying froin the ozher only by its phast ithstabilitp, 
sb. n% ?he output of each oscillator may be expn?ssed 
aS fOlfOWS: 
FIGURE 1 is a schematic block diagram showing a 
preferrd embodiment of apparatus %r carrykg out &e 
present kventian; and 
E I G W  2 illustrates characterisLjc waveforms corm 
spondinz to signals at designated points in the block dia- 
(13 gram of FIGURE 1. Referring now to FIGURE 1, oscihtor 10 represents 
the signal source under investigation and produces &e 80 
signdof interest. Oscillator IO may be any'periodic sig- (3) C==COS w ~ t + # d t )  
nal sou;tce, the absotute phase stability w k h  i s  to be 
determined. Representatively. but not limiting, oscithtor As a Of the Of muitipliers l6 and the 
10 may fie the reference oscillator for the automatic phase fo'owiag dot BQduc' am 2m*ded: 
control loor, af a phase-locked receiver and generates a 2.5 (4) A-B=fcos wlr+hCt)lfcos wgt++&)J 
sine wave 6 Ihe aider of 30 megacycles. 
There are also provided two additional oscillators p f 2  
and 14, to produce two awriiiiary signals having the m e  
cbaracterkties (center frequency, input and output im- 
pedance, etc.) as the signal genercted by oscillator 10. 
Thus, for the purpose of a given investigation, osciltatorp 
12 and 14 may be constructed identically IO oscillator 10. 
n e  output of oscillator 18 is supplied separately to 
each multiplier 16 aad 18. Multiplier 16 further receives 
the output of oscillator 12, and multiplier 18 further re- 
ceives the output of oscillator 24. MuitipIiers 16 and 38 
may be mixers of conventional type, capable of producing 
a product sigoal (the dot product) of signals fed thereto 
to thereby provide an output s i s a l  including the sum and 
merence frequencies of such sigads. 
The high frequency components of the output of mul- 
tipliers I6 and 28, hcluriing rhe center frequency and 
the sum frequencies of oscillators I@, I2 aod 14, are re- 
moved by low pass filten 22 and 2C respectively. Since 
oscillators 32 and 14 Dave the same nomhsll center €re- 
queccy ts osciIlator 10, there remains a difference fre- 
quencjr provided by the ~hase instaSility (or mise disturb 
ances of the outputs of osCi)l2tors, I@, I2 and 14. This is 
a quasi-random simal having a relatively low funda- 
mental frequency. It is to be noted at this point &at the 
resulting pbzse inskbility or noise disturbances of oscil- 
Iators I@, 12 ami 14 are uncomlated because of the in- 
dependeat mixing of the outputs of oscillators 1@ and 12 
and of oscilfators 18 and 34. The frequency compohcnts 
eontn^bued by the pbase instability of oscillator 1il is 
cornmoil to %e output of both row pass filters 22 and 24. 
The output of low pass filter 22 is fed to analog-tsdigital 
converter 26 snd the output of low pass Nter 24 k fed '9 
analog-to-dinital converter 2%. Analos-to-di&al converters 
- 
26 ad 28 Gay be conventional di&h eadem, operable 
to convert an analog inout to a dinitized outmt in a code E 10) T 
format required bf a-digital w&&z. l$pi&y, the 
analog inpnt received fram low pass fiibm 22 and 24 i s  
sampled at discreet &ne intervals, with amlog-ta-di$td 
converters 26 and 28 providkg binary numbers indica 65 
tive of the instantaneous voBage level ~f the two low &e- 
quency signals supplied by low pass filters 22 a d  24. 
The outputs of aaalog-t&djgitirf converters 26 and 2% 
are fed to stbrage devices 32 and 34 (such as a tape 
rtasrder, punch cardsor memory drum), Qr fed directly 70 
to computer 36. Cmputer 36 is a digital data pr~ssing.  
computer of stmdard type (for exampla, an ISM 7030 
compnfcr) prcvgmmaed to opmatc an two applied digittit 
signals to provih the cross-camhtion function thereof. 
T&c frequency distribution of the croar-corrolafion func- 7s 
In fhe mms-correlatiw process h(r )  is 2lmltpBed by 
f&) and 'the average vd& of this product detetmiaed. 
Subsequently, f a ( f )  is shifted ite time by BB a m a n t  AT 
and a new average product determined. As f&) i s  sw- 
cessively shifted by AT and further overage prodprts deter- 
mined, coniponents common to both f z ( f )  and f&) (Ley 
those companents introduced By tbt phase instability of 
oscillator 10) ndd or arc correlated, wbib thasc etot corn 
m n  &e., those introdwed by oscillators 32 End 14, go lo 
zero). nius rbc croes-corrrlniioa of the waveform d 
... .*" . .  .- . - I  
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Apparatus for phase stability determination Patent

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