BC352239A Datasheet, PDF (54/116 Pages) List of Unclassifed Manufacturers – BlueCore 3-Multimedia External

Device Terminal Descriptions
with ageing and temperature. A crystal with an ageing and temperature drift specification of better than
±15ppm is required.
8.3.4 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 BlueCore3-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 3. The transconductance required for oscillation is defined by the
following relationship:
g_m
>
(2πF_x )²R_m((C₀
3(C_t1 +C_trim )(C_t2 + C_trim )
+ C_int )(C_t1 + C_t2 + 2C_trim ) + (C_t1 + C_trim )(C_t2
+ C_trim ))²
Equation 8.7: Transconductance Required for Oscillation
BlueCore3-Multimedia External guarantees a transconductance value of at least 2mA/V at maximum drive level.
Notes:
More drive strength is required for higher frequency crystals, higher loss crystals (larger Rm) or higher
capacitance 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 the Persistent Store KEY_XTAL_LVL (0x241).
8.3.5 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
BlueCore3-Multimedia External crystal driver circuit is based on a transimpedance amplifier, an equivalent
negative resistance may be calculated for it with the following formula in Equation 8.8:
R_neg
>
g_m(2πF_x )²(C₀
3(C_t1 +C_trim )(C_t2 + C_trim )
+ C_int )((C_t1 + C_t2 + 2C_trim ) + (C_t1 + C_trim )(C_t2
+ C_trim ))²
Equation 8.8: Equivalent Negative Resistance
This formula shows the negative resistance of the BlueCore3-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.
Frequency
Initial Tolerance
Pullability
Minimum
Typical
8MHz
-
-
16MHz
±25ppm
±20ppm/pF
Table 8.6: Crystal Oscillator Specification
Maximum
32MHz
-
-
8.3.6 Crystal PS Key Settings
See Table 8.5.
BC352239A-ds-001Pc
© Cambridge Silicon Radio Limited 2004
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