BC41B143A-DS-003PC Datasheet, PDF (51/94 Pages) List of Unclassifed Manufacturers – BlueCore 4-ROM Plug-n-Go Single Chip Bluetooth v2.0 + EDR System

Device Terminal Descriptions
10.3.3 Frequency Trim
BlueCore4-ROM Plug-n-Go enables frequency adjustments to be made. This feature is typically used to remove initial
tolerance frequency errors associated with the crystal. Frequency trim is achieved by adjusting the crystal load
capacitance with on-chip trim capacitors, C_trim. The value of C_trim is set by a 6-bit word in the PS Key
PSKEY_ANA_FTRIM (0x1f6). Its value is calculated thus:
C_trim = 110fF × PSKEY_ AN A _ FT R IM
Equation 10.2: Trim Capacitance
There are two C_trim capacitors, which are both connected to ground. When viewed from the crystal terminals, they
appear in series so each least significant bit (LSB) increment of frequency trim presents a load across the crystal of
55fF.
The frequency trim is described by Equation 10.3.
∆(F_X ) _{= pullabiliyt × 55×10−3 (ppm/ LSB)}
F_X
Equation 10.3: Frequency Trim
Where Fx is the crystal frequency and pullability is a crystal parameter with units of ppm/pF. Total trim range is 63
times the value above.
If not specified, the pullability of a crystal may be calculated from its motional capacitance with Equation 10.4.
∂(F_X
∂(F_X
)
)
=
F_X•
C_m
4(C_I + C₀ )²
Equation 10.4: Pullability
Where:
C₀ = Crystal self capacitance (shunt capacitance)
C_m = Crystal motional capacitance (series branch capacitance in crystal model). See Figure 10.6.
Note:
It is a Bluetooth requirement that the frequency is always within ±20ppm. The trim range should be sufficient to
pull the crystal within ±5ppm of the exact frequency. This leaves a margin of ±15ppm for frequency drift with ageing
and temperature. A crystal with an ageing and temperature drift specification of better than ±15ppm is required.
BC41B143A-ds-003Pc
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© Cambridge Silicon Radio Limited 2005
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