MC72000 Freescale Semiconductor, MC72000 Datasheet - Page 38

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

Radio Functional Description
6.4.2 Main Synthesizer Operation
The internal local oscillator (LO) of the MC72000 is derived from the external reference frequency by
means of a 3-accumulator, fractional-N synthesizer. The external low pass filter (C14/R1 of Figure 3) has a
corner frequency of approximately 140 kHz. The external low pass filter requires two or three passive
components, dependent on the reference frequency.
f
I is the integer portion of the fractional synthesizer.
R is the numerator portion of the fractional synthesizer.
f
LO is the desired local oscillator frequency then,
I = (LO/f
R = REM(LO/f
where the INT function is the integer portion of the result and REM is the remainder portion of the result.
An example is shown below:
f
LO = 2.441 GHz
f
then,
I = INT(2.441 GHz/13 MHz - 157.5 kHz/13 MHz) - 3 = 184
R = REM(2.441 GHz/13 MHz - 157.5 kHz/13 MHz) x 2
Accuracy to at least 10 decimal places is suggested.
6.4.3 Transmit ROM Operation
The MC72000 uses a look-up table (LUT or Transmit ROM) to shape incoming transmit data bits and
produce a Gaussian filtered mask with BT=0.5. The value of the current data bit, along with knowledge of
the previous two bits, determines a unique trajectory for shaping. Only four unique trajectories are required
to implement this filter, and due to the symmetrical nature of the Gaussian response, these trajectories can
be reduced to a single quadrant. Furthermore, without compromising accuracy, this table can be reduced to
only 11 values.
The output of the LUT is fed to the accumulators of the fractional synthesizer. The seven MSBs are
eventually fed to the second port of the main VCO during transmit operation (see Figure 17). For receive
operation, the output of the LUT is constantly held to the value contained in R1C1.
These 11 trajectory constants are listed in Table 18 (see also Figure 94, Radio Register Map). The actual
value to place in the LUT is calculated as:
LUT RxCx
This number is then rounded and converted to binary:
LUT RxC
where the INT function is the integer portion of the result.
As an example for calculating the LUT value for R4C2 and f
LUT R
38
dev
ref
dev
ref
External is the external reference frequency.
External = 13 MHz
is the nominal transmit ROM frequency deviation (typically 157500 Hz).
= 157500 Hz
4
C
ref
2b10
xb2
b10
External - f
= INT((LUT RxCx
= (157.5 kHz/13.0 MHz) x 2
= (f
ref
External - f
dev
/f
dev
ref
External) x 2
/f
ref
Freescale Semiconductor, Inc.
dev
External) - 3
For More Information On This Product,
MC72000 Advance Information Data Sheet
/f
b10
ref
External) x 2
+2)/4)
16
Go to: www.freescale.com
x (RxCx constant)
16
x 0.5229292198 = 415.2
Preliminary
16
16
= 49618
10
ref
External = 13 MHz:
10
MOTOROLA