BC41B143A-DS-001PE Datasheet, PDF (59/102 Pages) List of Unclassifed Manufacturers – BlueCpre 4-ROM Single Chip Bluetooth v2.0 System with EDR

Device Terminal Descriptions
10.3.4 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 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 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 10.7: Transconductance Required for Oscillation
BlueCore4-ROM 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 R_m) 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).
10.3.5 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
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 10.8:
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 10.8: Equivalent Negative Resistance
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.
Frequency
Initial Tolerance
Pullability
Min
Typ
8MHz
-
-
16MHz
±25ppm
±20ppm/pF
Table 10.4: Oscillator Negative Resistance
Max
32MHz
-
-
10.3.6 Crystal PS Key Settings
See tables in Section 10.2.5.
BC41B143A-ds-001Pe
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Production Information
© Cambridge Silicon Radio Limited 2005
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