GW Instek GFC-8010H User Manual
Page 9
FREQUENCY COUNTER
USER MANUAL
15
High Accuracy Measurement
The accuracy of the time base oscillator is almost completely determined
by the characteristics of the crystal oscillator. The specifications of the
time base are:
Oscillation frequency
10MHz
Aging rate:
1×10
-6
/month
Temperature stability
5Ч10
-6
(25±5℃)
±2Ч10
-5
(calibration ambient temperature
0~40℃)
The temperature characteristics for the crystal oscillator used in this
instrument shown in Fig. 8, in which you can see the temperature
coefficient is as large as 25℃.
Fig. 8 Temperature characteristics of the crystal oscillator
0℃
25℃
33℃
60℃
Accuracy (temperature coefficient)
Temperature
4x10
-5
2x10
-5
2x10
-5
FREQUENCY COUNTER
USER MANUAL
16
The temperature stability of the crystal oscillator is: 2×10
-5
(temperature 0~60℃)
with 25℃ as reference. The temperature of 0~60℃ is given here because the
internal temperature rise is approximately 20 ℃ and the 0~40 ℃ ambient
temperature is because the crystal oscillator is similar to it. If the temperature of
the crystal oscillator is assumed to be 25℃ and set to 10MHz that is over the
worst case temperature stability of 2×10
-5
(0~60℃) temperature range and since
the frequency is 10MHz, a variation of only (10Ч10
6
)Ч(2Ч10
-5
) =2Ч10
2
=200Hz is
possible. In actual use, worst case conditions are produced in two circumstances:
1) Frequency calibration is performed as soon as the switch is set to ON at an
ambient temperature of 0℃ and measurement is performed after ample time
has elapsed after the switch is turned ON at an ambient temperature of 40℃.
2) Calibration is performed after ample time has elapsed after the switch has been
turned ON at an ambient temperature of 40℃ and measurement is performed as
soon as the switch is turned ON at an ambient temperature of 0℃.
Under these worst case conditions, the guarantee accuracy is 4×10
-5
(calibration
temperature:0~40℃) and becomes 0.004%.
Fig. 9 Example of crystal oscillator rise characteristics
0
5
30min
60min
90min
Time elapsed after
Switch on
X10
-6
Switch ON
Ambient temperature 23℃