TIME BASE CALIBRATION
It
is
important to note that the precision of the time base calibration directly affects overall counter ac-
curacy. Reasons for recalibration, and the procedures to be used, should be thoroughly understood before
attempting any readjustment.
The fractional error in the frequency indicated by the counter,
is
equal to the negative of the fractional
frequency error of the Time Base Oscillator with respect to
its
true value. That is:
A
fs
-
Aft
-
--
s
ft
where fS is the true frequency of the measured signal, and ft
is
the true frequency of the Time Base Oscil-
lator. Thus, the inaccuracy associated with a frequency measurement
is
directly related to the quality of
the Time Base Oscillator, and a measure of the precision with which it was originally adjusted.
TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR (TCXO)
The standard Time Base Oscillator used in the counter
is
a TCXO (A1 16). The range of the actual measured
frequencies of this oscillator will differ by no more than 2 parts in
lo6
if the temperature
is
slowly varied
from
0 to +50 degrees C.
With a stable input frequency, the measurement indicated by the counter will fluctuate proportionally
to the TCXO drift. To center this fluctuation on the true value of the measured signal, each TCXO has
imprinted on its side the frequency setting required at +25 degrees C.
At approximate room temperature
(+25 degrees C.), the slope of the frequency vs. temperature curve
is
normally no worse thank1 X 10-7 parts per degree C. When the counter is used in an ordinary laboratory
environment, the TCXO may be set as close to 10,000,000 Hz as desired. In this environment, a peak-to-
peak temperature variation of 5' C. will result in a measured signal error of no more than
f
2.5 X 10-7
parts. This signal error
is
due to the temperature characteristics of the Time Base Oscillator.
The natural aging characteristics of the crystal in the Time Base Oscillator can also cause inaccurate signal
measurements. Aging refers to the long term, irreversible change in frequency (generally in the positive
direction) which
all quartz oscillators experience. The magnitude of this frequency fluctuation in the
TCXO
is
less than 3 X parts per month as specified. This may improve to at least 1 X
parts per
year when in continuous operation.
Error due to aging adds directly to error due to temperature. The number of times the counter requires
recalibration depends on the environment in which the counter operates, and upon the level of accuracy
required.
For example, if the counter
is
subjected to the full operating temperature range one month after proper
initial adjustments, the inaccuracy could vary from
+1.3 X
parts to -0.7 X
10'~ parts.
Scans by ArtekMedia © 2007
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