User Manual

Jennic
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Jennic
© Jennic 2008 JN-DS-JN5139 v1.5 73
This can be used to give an equation for the required transconductance .
21
2
2121
2
]
)
(
[
4
TT
TTTTSm
m
C
C
C
C
C
C
C
R
g
×
×
+
+
×
≥
ω
Example: Using typical parameters of m R =40 Ω , S C =1pF and 1T C = 2T C =18pF ( for a load capacitance of 9pF), the
equation above gives the required transconductance ( m
g
) as 647uA/V. The JN5139 has a typical value for
transconductance of 1.25mA/V
The example and equation illustrate the trade-off that exists between the load capacitance and crystal ESR. For
example, a crystal with a higher load capacitance can be used, but the value of max. ESR that can be tolerated is
reduced. Also note, that the circuit sensitivity to external capacitance [ C
1
, C
2
] is a square law.
Below is measurement data showing the variation of the crystal oscillator amplifier transconductance with
temperature and supply voltage, notice how small the variation is. Circuit techniques have been used to apply
compensation, such that the user need only design for nominal conditions.
Crystal Oscillator Transconductance Versus Temperature
(VDD=3V)
1.245
1.25
1.255
1.26
1.265
1.27
1.275
1.28
1.285
-40 -20 0 20 40 60 80 100
Temperature (C)
Transconductance (mA/V)
Crystal Oscillator Transconductance Versus Supply Voltage
(Temp=25C)
1.2
1.22
1.24
1.26
1.28
1.3
1.32
2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6
Supply Voltage (VDD)
Transconductance (mA/V)