During warmup, crystal oscillators often show a frequency offset as large as 1 part in 10 to the 5th power. If timing information is transferred to the oscillator and then the oscillator is allowed to warmup, a timing error greater than 1 millisecond will occur. For many applications, it is unsuitable to wait for the oscillator to warmup. For medium accuracy timing requirements where overall accuracies in the order of 1 millisecond are required, a no warmup crystal concept was developed. The concept utilizes two crystal oscillator, used sequentially to avoid using a crystal oscillator for timing much higher frequency accuracy once warmed up. The accuracy achieved with practical TCXOs at initial start over a range of temperatures is discussed. A second design utilizing two oven controlled oscillators is also discussed

Phillips, D. H.

English

NASA Technical Reports Server

No Warmup Crystal Oscillator 
David H. Phillips, Naval Research Laboratory, Washington, D.C. 
ABSTRACT 
During warmup, crystal oscillators often show a frequency offset 
as large as 1 part in lo5. 
the oscillator and then the oscillator is allowed to warmup, a timing 
error greater than 1 millisecond will occur. 
it is unsuitable to wait for the oscillator to warmup. 
If timing information is transferred to 
For many applications, 
For medium accuracy timing requirements where overall accuracies 
in the order of 1 millisecond are required, a no warmup crystal 
concept has been developed. 
tors, which are used sequentially to avoid using a crystal oscillator 
for timing during the trauma of warmup. 
One oscillator may be a TCXO which preserves timing information 
during the warmup of the second crystal oscillator, which has a much 
higher frequency accuracy once warmed up. 
This paper will show the accuracy achieved with practical TCXOs 
at initial start over a range of temperatures. A second design 
utilizing two oven controlled oscillators will also be discussed. 
TYPICAL WARMUP FOR AT CUT CRYSTAL 
The concept utilizes two crystal oscilla- 
Figure 1 shows ;he warmup curve for a typical AT cut crystal 
oscillator with oven control. 
room temperature (23°C) to crystal oscil lator oven control temperature 
of 85OC. 
(nominal frequency) this oscillator has already gained approximately 
11 milliseconds. During the next 5 minutes the oscillator loses 
approximately 60 microseconds. The negative overshoot due to stress 
is far too small to compensate for the large positive time error 
accumulation during oven warmup. 
the crystal oscillator to warmup and it is necessary to have ?ess 
than 1 millisecond time error, another approach is required. 
This particular warmup was done from 
If one observes the point at which the curve crosses zero 
If one cannot wait 20 minutes for 
TCXO TURNON 
Figure 2 shows the phase difference between a TCXO (temperature 
compensated crystal oscillator) and a cesium beam standard from 
turnon of the TCXO to the end of the first minute. This shows a very 
regular beat frequency and is virtually the same as the nominal 
accuracy (2 parts in lo7) of the TCXO. 
occurring with turnon of a TCXO. 
There is very little trauma 
83 1 
https://ntrs.nasa.gov/search.jsp?R=19820012661 2020-03-21T08:34:11+00:00Z
TIME ERROR OF CRYSTAL OSCILLATOR AND TCXO COMBINED 
Figure 3 shows the theoretical  time e r ro r  accumulated i f  a clock 
i s  s tar ted w i t h  a TCXO osc i l l a to r  a t  time To and switched to  an oven 
controlled crystal  o sc i l l a to r  a t  time TI .  
time error  accumulated i s  determined, f, is the nominal frequency of 
1 MHz, f l  i s  the TCXO’s frequency, f 2  i s  the oven controlled osci l la-  
t o r ’ s  frequency and AT is  the accumulated time error.  
T2 i s  the time a t  which the 
In practice precision frequency sources are  not readily available 
to  determine when the oven control led crystal  o sc i l l a to r  goes through 
zero (nominal frequency). 
switching i s  the oven cutback of the oven controlled crystal  osc i l la -  
tor .  An experiment was performed a t  Naval Research Laboratory ( N R L )  
u s i n g  a TCXO w i t h  switching c i r cu i t ry  which allowed the TCXO t o  drive 
the clock when no other standard frequency was available. 
One source of information available fo r  
EQU I PMENT 
Figure 4 shows a p r i n t e d  c i r c u i t  card w i t h  a cornrnerci’al TCXO and 
the switching c i rcu i t ry  required to  switch quickly enough t o  avoid 
losing more than one microsecond dur ing  swi tch ing .  
Figure 5 shows the modular construction of a n  AN/URq-23 developed 
by NRL and contai’ntng a double proportional oven controlled crystal  
o sc i l l a to r  w i t h  a 5 MHz, f i f t h  overtone AT cu t  crystal .  
card shown i n  Figure 4.  To the l e f t  of the clocks is the AN/URQ-23 
shown i n  Figure 5. All of the equipment i s  i n  a temperature chamber. 
EXPERIMENT 
Seven experimental data runs were conducted a t  sevefi different  
temperatures ranging from -18% t o  55OC. In between da ta  runs, the 
equfpment was qllowed to  equalize a t  the new temperature before power 
on. Prior to  the f i r s t  d a t a  run a l l  clock oscil7ators were set to  
nominal frequency. Synchronization of the clocks w i t h  NRL’s master 
clock occurred w i t h i n  10 seconds o f  power on for each run. 
experimental runs, T1 (switching time) was chosen as the time when 
cutback of the inner oyen of the AN/URQr23 occurred. 
Figure 6 shows six SG 1157/U clocks w i t h  each containing the TCXO 
In these 
Figure 7 shows the datq of t h e  f i r s t  r u n  a t  - 1 8 O C .  This tempera- 
ture was below the spec limit f o r  the TCXOs and so the data fo r  the 
two clocks shown yield a larger than normal time er ror  accumulated, 
b u t  which i s  s t i l l  l e s s  t h a n  1 millisecond. The vertical  l ine  a t  40 
minutes shows the switching time Tl (oven cutback of AN/URQ-23). 
83 2 
Figure 8 shows the data of the second run at -8OC. The data for 
clocks 1, 2, 3 and 5 are shown here and each yield an accumulated 
time error of less than 0.25 milliseconds. 
minutes shows the switching time TI. 
The vertical line at 25 
time 
time 
Figure 9 shows the data of the third run at 7°C. 
T1 is shown by the vertical line at 18 minutes. The accumulated 
error for each clock is less than 0.25 millisecond. Clock #3 
shows a negative time accumulation and a change in slope is noted when 
the switch over to the oven controlled oscillator occurs at time T1 
After switching, the positive slope of the oven controlled oscillator 
is observable in all the clocks and this slope becomes less as the 
stress on the crystal is relieved. 
Switching 
Figure 10 shows the data of the fourth run at 23°C (room tempera- 
ture). Switching time T1 is shown by the vertical line at 20 minutes. 
This plot shows data for two hours after power on. The oven con- 
trolled oscillator by itself over its entire warmup curve yielded an 
accumulated time error of -10 milliseconds while the ensemble of TCXOs 
and oven controlled crystal oscillator each yielded an accumulated 
time error of 0.1 millisecond or better. 
Figure 11 shows the data of the fifth run at 27°C. Switching 
This plot shows time T1 is shown by the vertical line at 22 minutes. 
the data for two hours after power on and the accumulated time error 
is less than 0.25 millisecond for each clock. 
Figure 12 shows the data of the sixth run at 43OC. 
Clocks 1, 3, and 5 each yielded an accumu- 
Switching 
time T1 is shown by the vertical line at 22 minutes. This plot shows 
the data for 30 minutes. 
lated time error of less than 0.2 millisecond. 
The seventh and final run of data at 55°C is shown in Figure 13. 
This shows the performance of the TCXOs at 55'C for 30 minutes. The 
switching time TI would have occurred at 14 minutes. 
controlled oscillator warms up much more quickly at this temperature 
and with less overshoot. 
The oven 
IMPROVEMENT WITH SC CUT CRYSTAL 
When a crystal is warmed up very rapidly internal stresses are 
set up which cause a frequency perturbation. The h e  of a stress 
compensated crystal such as the SC cut improves the performance, but 
even with it there will be a minimum warmup time. 
Figure 14 shows the SC cut crystal in fast warmup showing less 
frequency dependence on temperature than the AT cut and n J stress 
833 
related overshoot. 
TCXO i s  used as an initial oscillator the optimum warmup time may in 
practice be several minutes to achieve the minimum accumulated time 
error. 
The warmup could be as short as 90 seconds. If a 
ALTERNATIVE TECHNIQUE 
oscillators is shown in Figure 15. The oscillators shown have a self 
tuning capability so that an external frequency may be utilized to 
assure accurate initial frequency. In step 1 when oscillator A is 
holding, it maintains the external frequency while oscillator B is 
warming up. 
It requires only a second for the serial time code to be shifted 
into the shift register and shifted into the counter driven by 
oscillator A. 
until oscil ator B is warmed up. Oscillator B now becomes the master 
and in step 2 it holds the time while oscillator A warms up. In step 
3, when osc llator A has warmed up, it also becomes a master and two 
independent clocks are now available for intercomparison. 
CONCLUSIONS 
An alternative technique using two oven controlled crystal 
The counter of oscillator B is continually updated 
The experimental results show that the no warmup concept has an 
order of magnitude increase in timing accuracy for situations re- 
quiring instant turnon and deployment. 
more stringent than the simple model tested, then the alternative 
suggestions should be uti1 ized. 
If timing requirements are 
ACKNOWLEDGEMENTS 
The author would like to acknowledge the contributions of 
Joseph J. O'Neill and Ruth E. Phillips. 
REFERENCES 
[l] Robert Burgoon and Robert L. Wilsen, Proc. 33rd Annual 
Symposium on Frequency Control , 1979, "Performance Results 
of an Oscillator Using the SC Cut Crystal" p 408 
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PROCEEDINGS 
1. IO* 
OF THE 
33rd ANN AL SYMPOSIU ON FREQUENCY CO 
1979 
PERFORMANCE RESULTS OF AN 
OSCILLATOR USING THE SC CUT CRYSTAL 
Robert Burgoon and Robert L .  Wilson 
Hewlett-Packard Company 
Santa Clara, Ca. 
OVEN M E R  - 19 I WATTS 
X CUT CRVSTAL 
10 1- 110 m 4 s  yo T ~ M E ~ S E C ~ D S ~  
Figure 3 Oscillator Warmup for an HP 10544 Oscillator wlth an SC Cut Crystal 
., . IC.* 
"" t FREWENCYCHANGE vs TEMPERATURE 
\ 81 CUT 
Fig. 14 SC Cut Performance 
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No warmup crystal oscillator

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