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Phone: 310.955.5300
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3.2. Deciding the CL value
CL means Load Capacitance; that is the capacitance
of the circuit (the capacitance of a circuit between the two
terminals of a crystal). The CL is specified by the circuit.
Please do not specify the values that may cause a shift in
frequency. The relationship between CL and frequency is
shown in Figure 3.1: When the CL required is 'a' pF, the
crystal will oscillate within a fine tolerance under an 'a'pF
CL circuit. If the actual CL is low, the frequency will be
high and if the CL is high, the frequency will be low.
Therefore in order to get an accurate frequency, the CL
value for the crystal device and circuit must match.
Again, the CL value of the Crystal device means 'a crystal
for a XXpF CL circuit'. It is as important as the frequency.
However it is impossible to measure the actual CL.
Therefore, an approximate value is calculated using the following formula:
CG
×
CD
CL
≅
+CS [ pF ]
CG
+
CD
The Cs (Stray Capacitance) is usually 2pF if the board is a dual surface board. If using a multi-layer board, the value will be
higher. For example, if both Cg and Cd are 10pF, the CL will be 7pF, as the Cs is 2pF.
Next, confirm the CL accuracy using a crystal with a Known CL. If there is still a difference in frequency, there are the
following ways to correct it:
i. Change Cg and Cd
ii. Use a different CL crystal
Please evaluate all the parameters if the circuit board has been modified. The value usually changes even if the circuit is the
same. We recommend that the easiest way to confirm the values, is to confirm the actual oscillation frequency with SEIKO
EPSON's sample parts which have data attached.
4. Measurement procedure
4.1.Measurerment of the Drive Level
The procedure of measuring the drive leve1 is to
measure the current which runs through the crystal.
Then calculate with Ohm's formula as follows:
P(D.L.)=i
2
R [W]
'i' must be the actual value and 'R' must be the
specified CI maximum. The way to measure 'i' is
different for AT crystals and Tuning fork crystals.
4.1.1. Measurement of the Drive Level for AT crystals
In order to measure the current as accurately as
possible, use a current probe and an oscilloscope.
The current probe is similar to the CT (Current
Transformer) which measures a current that runs through
a lead using a magnetic flux according to the right screw rule. The signal goes to the oscilloscope via an amplifier. The current
can be read from the scope.
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