bc41b143a07 ETC-unknow, bc41b143a07 Datasheet - Page 58

• Manufacturer Part Number: bc41b143a07
• Description: Single Chip Bluetooth
• Manufacturer: ETC-unknow

9.3.4
The crystal and its load capacitors should be viewed as a transimpedance element, whereby a current applied to one
terminal generates a voltage at the other. The transconductance amplifier in BlueCore4-ROM uses the voltage at its
input, XTAL_IN, to generate a current at its output, XTAL_OUT. Therefore, the circuit will oscillate if the
transconductance, transimpedance product is greater than unity. For sufficient oscillation amplitude, the product
should be greater than three. The transconductance required for oscillation is defined by the relationship shown in
Equation 9.8:
BlueCore4-ROM guarantees a transconductance value of at least 2mA/V at maximum drive level.
Notes:
9.3.5
An alternative representation of the crystal and its load capacitors is a frequency dependent resistive element. The
driver amplifier may be considered as a circuit that provides negative resistance. For oscillation, the value of the
negative resistance must be greater than that of the crystal circuit equivalent resistance. Although the BlueCore4-ROM
crystal driver circuit is based on a transimpedance amplifier, an equivalent negative resistance may be calculated for
it with the following formula in Equation 9.9:
This formula shows the negative resistance of the BlueCore4-ROM driver as a function of its drive strength.
The value of the driver negative resistance may be easily measured by placing an additional resistance in series with
the crystal. The maximum value of this resistor (oscillation occurs) is the equivalent negative resistance of the
oscillator.
CS-101564-DSP12
More drive strength is required for higher frequency crystals, higher loss crystals (larger R
loading.
Optimum drive level is attained when the level at XTAL_IN is approximately 1V pk-pk. The drive level is
determined by the crystal driver transconductance, by setting PSKEY_XTAL_LVL (0x241).
Transconductance Driver Model
Negative Resistance Model
R
neg
g
m
Equation 9.8: Transconductance Required for Oscillation
>
>
g
3
m
(
2
(
Equation 9.9: Equivalent Negative Resistance
π
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F
π
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)
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2
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R
2
m
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(
C
(
C
0
0
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C
Production Information
C
int
© CSR plc 2003-2007
int
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Device Terminal Descriptions
)
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Page 58 of 97