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Clock Generation
5.4.3
Transconductance Driver Model
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 BlueCore5‑Multimedia External 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
g m
> 3
(2πF x
)
2
R m
((C
0
+ C int
)(C
C t1 t1
+ C
(C t2 t2
+ C
+ C trim
) trim
) + C t1
(C t2
+ C trim
))
Equation 5.5: Transconductance Required for Oscillation
BlueCore5‑Multimedia External guarantees a transconductance value of at least 2mA/V at maximum drive level.
Note:
More drive strength is required for higher frequency crystals, higher loss crystals (larger R m capacitance loading
) or higher
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.
5.4.4
Negative Resistance Model
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
BlueCore5‑Multimedia External crystal driver circuit is based on a transimpedance amplifier, an equivalent negative
resistance can be calculated for it using Equation 5.6.
R neg
> g m
(2πF x
)
2
(C
0
+ C int
)((C
C t1
(C t2 t1
+ C
+ C t2 trim
)
+ C trim
) + C t1
(C t2
+ C trim
))
2
Equation 5.6: Equivalent Negative Resistance
This formula shows the negative resistance of the BlueCore5‑Multimedia External 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.
5.4.5
Crystal PS Key Settings
The BlueCore5‑Multimedia External firmware automatically controls the drive level on the crystal circuit to achieve optimum input swing. The PSKEY_XTAL_TARGET_AMPLITUDE is used by the firmware to servo the required amplitude of crystal oscillation. Refer to the software build release note for a detailed description.
BlueCore5‑Multimedia External should be configured to operate with the chosen reference frequency.
CS-121064-DSP4
Production Information
This material is subject to CSR's non-disclosure agreement
© Cambridge Silicon Radio Limited 2006 - 2010
Page 31 of 104
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