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2.4. Characteristics of each parameter of the oscillation circuit
The circuit parameters which referred to Figure 1 have the characteristics as follows:
Rf: Used for a bias for the inverter closed loop. If the value is small, it will cause an overtone oscillation due to the
small phase difference between the inverter input and output.
Ri: Used to bias the crystal. Generally, a small Rd will cause a high drive level. Cg, Cd: It is a basic part of the closed
loop on the crystal side. Generally, a large capacitance will cause a high drive level. Please try to keep the same
value for Cg and Cd. This is ideal for stable operation.
3.Circuit evaluation
The results of the circuit evaluation wil1 decide the circuit parameters and characteristics for suitable operation.
3.1. Deciding the best parameters
To decide the best parameters, please use the following guidelines:
i. Drive Level: Must be below 1µW (20kHz to 165kHz)
Must be below 300µW. 100µW Typ. (5.5MHz to 24MHz)
ii. Oscillation Allowance: At least 5 times CI maximum value
(Negative Resistance) (ex., if the crystal is C - 002RX, in this case at least –250kΩ)
In the case when CL is low, the required oscillation allowance is
at least twice Re*
1
(Effective Resistance). Three (3) times Re is
recommended.
Re*
1
: The Re (Effective Resistance) can be calculated by the
following formula:
C
0
2
Re = R
1
1+ [Ω]
C
L
iii. Oscillation start up voltage: It must start up with in 2.5V if the Vdd=5.0V.
iv. Oscillation start up time: Within 3 seconds (20kHz to 165kHz)
(General standard value) Within 4m seconds (5.5MHz to 24MHz)
v. Oscillation frequency: The value must be matched to the intended circuit frequency.
The characteristics iii., iv. and v.
can be measured while checking i.
and ii. So, to decide the
parameter's accuray always use
i. (drive level) and ii. (oscillation
allowance). Using graphed data
will help for confirmation of
characteristics. Use drive level
and oscillation allowance as Y
axes, Cg and Cd as X axis.
According to the graph, there is a
range of suitable characteristics.
Please choose a common value
of capacitance for the parts
within the acceptable range.
(134 Seiten)
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