MC72000 Freescale Semiconductor, MC72000 Datasheet - Page 45

• Manufacturer Part Number: MC72000
• Description: Integrated Bluetooth Radio
• Manufacturer: Freescale Semiconductor

6.5 Crystal Oscillator
The crystal oscillator provides the reference for the data clock PLL and main PLL. It can be configured as
a Colpitts type (negative resistance) oscillator and utilize an external parallel resonant crystal or may be
driven from an external source. The oscillator circuit has an on-chip capacitor trim network that provides
the capability to compensate for crystal and/or load capacitor tolerances. This allows the use of relatively
inexpensive crystals with as much as 50 ppm tolerance. The oscillator also provides three bias current
modes. Xtal Enable (R11/0) enables/disables the bias current and Xtal Boost Enable (R11/4) enables/
disables a high current mode. Refer to the Reference Oscillator Electrical Characteristics in Table 11 for
the available current modes. Table 23 gives examples of parallel trim capacitances that can be programmed
to register map location Xtal Trim (R6/14-10). Typical stray capacitance is on the order of 1.0 pF.
To drive the oscillator with an external source, program the Xtal Enable (R11/0) to zero and AC-couple the
external signal into the oscillator base with a 15 to 100 pF capacitor. It is also recommended to set Radio
Xtal Trim (R6/14-10) to zero to reduce the load on the external source. The external source may be either a
sine or a square as long as it is AC-coupled and keeps the requirements for phase noise, amplitude, and
duty cycle in Table 11. Additional characteristic data is shown in the following figures.
MOTOROLA
8.0
6.0
4.0
16
14
12
10
Figure 21. Oscillator Open Loop Gain vs.
0
See Figure 4, T
Open Loop Gain Measurements @ -10 dBm
Crystal Load Capacitance = 13 pF
Electronic Trim (R6/14-10) = 16 (decimal)
0.5
XTAL Trim (R6/14-10) Setting
1.0
CRYSTAL CAPACITOR RATIO (C5/C8)
Capacitor Ratio
A
= 25°C
1.5
(MSB to left)
Table 23. Examples of Programmable XTAL Trim Capacitances
2.0
00000
00100
10000
10101
11111
Freescale Semiconductor, Inc.
2.5
For More Information On This Product,
MC72000 Advance Information Data Sheet
3.0
3.5
Go to: www.freescale.com
4.0
Preliminary
4.5
Figure 22. Oscillator Negative Resistance vs.
-100
-150
-200
-250
-300
-350
Electronic Parallel Crystal Trim Capacitance
-50
0
0
Electronic Parallel Crystal Trim (C
(Crystal Boost Enable R11/4) = 0
4.0
ELECTRONIC PARALLEL CRYSTAL TRIM (R6/14-10)
25°C
8.0
See Figure 4, T
Curve Measured with 13 MHz Crystal Reference
Crystal Load Capacitance Ratio = 1.8 (C15/C16)
T
A
Radio Functional Description
= -20°C
1.2 pF
4.8 pF
6.3 pF
9.3 pF
(C
0 pF
12
PT
)
16
A
= 25°C
20
85°C
24
PT
)
28
45
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