Measurements were made of the stability of time transfer by the IRIG-B code over a dedicated telephone link on a microwave system. The short and long term Allan Variance was measured on both types of microwave system, one of which is synchronized, the other having free local oscillators. The results promise a time transfer accuracy of 10 microns. The paper also describes a prototype slave clock designed to detect interference in the IRIG-B code to ensure local time is kept during such interference

Bedard, G.

Beland, J.

Bussiere, P.

Label, D.

Missout, G.

English

NASA Technical Reports Server

TIME: TRANSFER BY IRIGB TIME CODE VIA 
DEDICATED TELEPNONE LINK 
G. Missout, J. Bgland, D. Lebel, G .  B6dard and P. Bussisre, 
Institut de recherche d'Hydro-Qu6bec Hydro-Qudbec 
ABSTRACT 
Measurements were made of the stabiIity of time transfer by the 
IRIG-B code over a dedicated telephone link on Hydro-Qu6bec's micro- 
wave system. The short-and long-term Allan Variance was measured on 
both types of microwave system used by Hydro-Qugbec, one of which is 
synchronized, the other having free local oscillators. The results 
promise a time transfer accuracy of 10 ps. The paper also describes 
a prototype slave clock designed to detect interference in the IRIG-B 
code to ensure local time is kept during such interference. 
1. INTRODUCTION 
Hydro-QuEbec's requirements with regard to time dissemination cover 
a wide range. As far as voltage phase angle measurements are concern- 
ed, it calls for an accuracy of 10 ps on a round-the-clock basis, 365 
days a year. The GOES system used for this purpose to date has yielded 
unsatisfactory results and Hydro-QuEbec is therefore exploring the pos- 
sibility now of making maximum use of the IRIG-B system. 
1 
2 
This paper presents the results of measurements performed at two 
points on the Qu6bec utility's power system and describes the measu- 
rement and calculation techniques. The operating principles of the 
slave clock to be used for detection purposes are also described. 
281 
https://ntrs.nasa.gov/search.jsp?R=19820012633 2020-03-21T16:34:26+00:00Z
~~ ~ 
/ CHURCH I L L  
HYDRO-QUEBEC M I C R O W A V E  N E T W O R K  
S I M P L I F I E D  D I C I G R A M  
Figure 1. 
2. STABILITY MEASUREMENTS 
2.1 Hydro-Qugbec microwave system 
Hydro-Qugbec's microwave system, illustrated in Fig. 1, comprises 
two main branches basically: Montrgal-Churchill Falls Via Arnaud sub- 
station, and Montrgal-LG2. The former, i.e. the MontrBal-Churchill- 
Falls branch is the synchronized type. A pilot is transmitted from 
MontrBal and used at the various substations to phase-lock the local 
oscillators of the multiplexer. The Montrgal-LG2 branch, on the other 
hand, is the free-oscillator type in which the pilot serves only to 
generate alarms when the frequency error exceeds a given limit. 
282 
In practice, the synchronized system provides a guarantee that the 
frequency of the multiplexer input and output signals will be identical 
except in the case of loss of synchronization. Furthermore the phase 
between the input and output si-gnals increases or decreases-stepwise 
whenever a momentary l o s s  of synchronization occurs. 
The nonsynchronized system can introduce a maximum error of 1 Hz 
in the transmitted signal, which would therefore be 1 0 0 0 ~ ~  at the input 
and 1OO1Hz at the output. However, in practice this error is more like 
0.1 Hz, which results in a smooth phase drift. 
These characteristics of Hydro-QuGbec's microwave systems mean 
that a corrector has to be used in order to rebuild a usable I R I G B  
signal, Tests were therefore conducted on both types of multiplexer to 
study their performance. For this purpose we used an IRIG-B code ori- 
ginating in MontrGal from a rubidium frequency standard HP 5065-2. 
Measurements were made on the IRIG-B code received and corrected at 
LG-2 and at Arnaud substation. 
2.2 Test setup 
In the test setup at Arnaud Substation (Fig. 2)  we used a time 
code reader and a synchronized generator (Datum 9390). A clock with a 
quartz oscillator was used while an interval counter and a strip chart 
recorder served to measure the accumulated line error between the two 
times scales ( IRIG-B and crystal oscillator). 
I At LG2 (Fig. 3) we used our mobile clock with a rubidium standard 
as reference (FRKH EFFATOM). This unit comprises the IRIG-B decoder, 
the corrector and the interval counter together with a separate re- 
corder. 
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I T I M E  T I M E  
IPPS IRIG E 
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COUNTER I M X ,  I REeDER 
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RECORDER 
Figure 3 
I M O B I L E  C L O C K  
1 0 0 0 H z  
F I L T E R  
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TRIGGER COUNTER 
S H O R T  T E R M  M E A S U R E M E N T  A P P f % R A T U S  
Figure 4 
285 
For t h e  short-term measurements (Fig,4) a computing counter HP 
93608, a band pass  f i l t e r  (4-4 a t  1000 Hz) and a Schmitt t r i g g e r  w e r e  
used. 
2 .3  Experimental r e s u l t s  
Typical curves obtained are presented i n  Pig.5. Some synchroni- 
za t ion  l o s s e s  w e r e  observed a t  Arnaud (F ig . -6) .  The accumulated t i m e  
e r r o r  values  obtained w e r e  d ig i t i zed .  The following equation served 
t o  c a l c u l a t e  t h e  frequency s t a b i l i t y :  
3 
where 
x ( t )  = v a r i a t i o n s  i n  t h e  t i m e  s c a l e  due t o  random 
frequency va r i a t ions .  
Two such curves w e r e  obtained,  one at Arnaud, t h e  o the r  a t  LG2. The 
LG2 curve (Fig. 7) shows a long-term s t a b i l i t y  l i m i t  as w e l l  as the  
combined e f f e c t  of t he  two rubidium references (master and mobile 
c locks) .  This expla ins  t h e  series of po in t s  above the  t y p i c a l  f r e -  
quency s t a b i l i t y  curve of a rubidium frequency standard.  The c i r c l e s  
i nd ica t e  t h e  short-term measurements made a t  IREQ. The curve f o r  Arnaud 
( F i g 8 )  shows the  long-term frequency s t a b i l i t y  l i m i t s  of t h e  reference 
clock. 
It is c l e a r  from these  curves t h a t  a 1 0 - u ~  t r a n s f e r  t i m e  i s  possi-  
b l e ,  al though t h i s  remains t o  be confirmed by longer-term d i s c r e t e  mea- 
surements (over months). I n  Figs .  9 and 10 w e  p l o t t e d  t h e  s t a b i l i t y  
frequency curve i n  t h e  case of a minor synchronization l o s s  and it  can 
be  seen t h a t  a l ready  t h e  s t a b i l i t y  has dropped one order  of magnitude. 
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. I . .  . . . I . .  . . . I . .  . . .). . ...I.. * .  . b . .  . ... 
Sample t i m e  10 SECONDS 
F R A C T I O N A L  FREQUENCY STeBILITY O F  THE 
I R I G  S CODE RECEIVED A T  LG 2 
Figure 7 
289 
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F R A € T I O N A L  F R E Q U E N C Y  S T A e I L I T Y  O F  T H E  
lRIG 5 CODE R E C E I V E D  A T  CIRNRUD 
IF4 P R E S E N C E  O F  N O I S E  
Figure 10 
292 
However, t h i s  can b e  put  t o  good use ,  as w i l l  b e  expla ined  i n  t h e  next  
s ec t ion.  
The curves presented  i n  Figs .  6 and 9 show t h a t  a convent ional  
slave c lock  t r a c k s  t h e  input -s igna l  d r i f t  w i t h  a r e l a t i v e l y  s h o r t  t i m e  
cons tan t  (= 5 min o r  less). This  is ind i spensab le  because t h e  IRIG-B 
code r eade r  must be  s u f f i c i e n t l y  a g i l e  t o  fo l low t h e  inpu t  s i g n a l ;  
o therwise  i t  r i s k s  l o s i n g  t h e  s i g n a l  and w i l l  have t o  begin  sweeping 
t o  r e l o c a t e  i t .  The coupled genera tor  t h e r e f o r e  has  t o  fo l low t h e  read- 
er . W e  are now i n  t h e  process  of  developing a u n i t  i n  which t h e  read- 
er and t h e  genera tor  are c o n t r o l l e d  s e p a r a t e l y  by t h e  s a m e  micropro- 
cessor .  
F igure  11 p r e s e n t s  a block diagram of t h e  pro to type  slave clock.  
The t i m e  code r eade r  t r a c k s  t h e  input  code, e l imina t ing  t h e  short- term 
no i se  (up t o  a few t e n s  of seconds) .  The programmable d i v i d e r  A d r i v e s  
t h e  t i m e  code reader .  
On t h e  o t h e r  s i d e  w e  have a.separate generator ,  which is co r rec t ed  
another  programmable counter  B, and a u n i t  f o r  measuring t h e  t i m e  with 
d i f f e r e n c e  between t h e  r eade r  and t h e  genera tor .  
This  approach enables  us  t o  main ta in  a r e l a t i v e l y  s h o r t  t i m e  cons- 
t a n t  (a few tens  of seconds) f o r  t h e  r eade r ,  and consequently keep a 
check on any code d r i f t  whi le  checking i t s  cons i s t ency ,  without  needing 
t o  s h i f t  t h e  genera tor  i n  t i m e .  The genera tor  can i n  f a c t  be  co r rec t ed  
wi th  a t i m e  cons tan t  of between a few hours  and over  one day i f  w e  u se  
a rubidium s tandard .  
Furthermore w e  can make n o t e  of c o r r e c t i o n s  t o  t h e  d i v i d e r  B so 
t h a t  w e  have an i d e a  of t h e  d r i f t  of t h e  frequency s tandard ,  which can 
be  s y s t e m a t i c a l l y  co r rec t ed  whenever t h e  inpu t  s i g n a l  d i sappears .  I n  
t h i s  case w e  have a ze ro  o f f s e t  frequency equ iva len t  at t h e  o u t s e t .  I f  
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a rubidium frequency standard is used, the corrections can be transmit- 
ted, an excellent means of remotely studying the clock 
performance without having to add an outside reference clock. 
thus providing 
Furthermore, the AT interval counter allows us to calculate the 
stability of the IRIG-B code as received and read, thus providing a 
reliable means of knowing whether or not we are in the presence of 
noise so as to ignore the input in the generator correction process. 
This approach is far more sensitive than the method presently employed, 
which consists in checking whether or not the input signal is present. 
A photograph of the prototype clock is shown in Fig. 12. 
SLAVE CLOCK PROTOTYPE 
Figure 12 
295 
CONCLUSION 
In this paper we have examined the stability of a time code dis- 
seminated on a microwave system. The measurements obtained reveal a 
possibility of time transfer with an accuracy of 10 us. The new pro- 
totype slave clock used should allow us to give a reliable time code 
despite interference in the microwave network by calculating the fre- 
quency stability of the signal received. This unit will also allow 
remote transmission of corrections to the generator and thus provides 
a possible means of control and measurement. 
REFERENCES 
(1) PTTI Application at Hydro-Qusbec, G. Missout, J. Bgland, G. BGdard, 
p. 377-385, 12th PTTI. 1980. 
(2) Time Dissemination in the Hydro-QuGbec Network, G. Missout, 
W. Lefranqois, L. Laroche, p.343, 11th PTTI, 1979. 
(3) The NBS Atomic Time Scale: Generation, Stability, Accuracy and 
Accessibility, D. Allan, J. Gray, H. Machlam, NBS Monograph 
140, Chap. 9, p.210. 
296 


Time transfer by IRIG-B time code via dedicated telephone link

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