VCO
N
=
PLL DIV x P
f
f
25
VCO x RDIV
REF
NFRAC = floor
NINT x 2
x PLL DIV
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f
f
VCO x RDIV
REF
NINT = floor
x PLL DIV
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f
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PFD
RDIV =
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f
40
SLWS245B – MAY 2014 – REVISED FEBRUARY 2017
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Copyright © 2014–2017, Texas Instruments Incorporated
c.
RDIV, NINT, NFRAC, PRSC_SEL
The remaing PLL parameters are calculated according to the following equations:
The DMP division ratio (P/P+1) can be set to 4/5 or 8/9 through the PRSC_SEL bit. To allow proper
fractional operation, set PRSC_SEL according to:
PRSC_SEL = 0, (P/P+1) = 4/5 for 20
≤
NINT < 72 in integer mode or 23
≤
NINT < 75 in fractional mode.
PRSC_SEL = 1, (P/P+1) = 8/9 for NINT
≥
72 in integer mode or NINT
≥
75 in fractional mode.
The PRSC_SEL limit at NINT < 75 applies to Fractional mode with third-order modulation. In Integer mode,
the PRSC_SEL = 8/9 should be used with NINT as low as 72. The divider block accounts for either value of
PRSC_SEL without requiring NINT or NFRAC to be adjusted. Then, calculate the maximum input frequency
(f
N
) to the digital divider. Use the lower of the possible prescaler divide settings, P = (4,8), as shown by
(8)
Verify that the frequency into the digital divider, f
N
, is less than or equal to 375 MHz. If f
N
exceeds 375 MHz,
choose a larger value for PLL_DIV_SEL and recalculate f
PFD
, RDIV, NINT, NFRAC, and PRSC_SEL.
8.4.2 Setup Example for Integer Mode
Suppose the following operating characteristics fractional example are desired for Integer mode operation:
•
f
REF
= 61.44 MHz (reference input frequency)
•
Step at RF = 2.56 MHz (RF channel spacing)
•
f
RF
= 1799.68 MHz (RF frequency)
The VCO range is 2050 MHz to 4100 MHz. Therefore:
•
LO DIV = 2 (LO_DIV_SEL = 1)
•
f
VCO
= LO DIV × 1799.68 MHz = 3599.36 MHz
In order to keep the frequency of the prescaler below 3000 MHz:
•
PLL_DIV = 2 (PLL_DIV_SEL = 1)
The desired stepsize at RF is 2.56 MHz, so:
•
f
PFD
= 2.56 MHz
•
f
VCO
, stepsize = PLL_DIV × f
PFD
= 5.12 MHz
Using the reference frequency along with the required f
PFD
gives:
•
RDIV = 24
•
NINT = 703
NINT
≥
75; therefore, select the 8/9 prescaler.
f
N
= 3599.36 MHz/(2 × 8) = 224.96 MHz < 375 MHz
This example shows that Integer mode operation gives sufficient resolution for the required stepsize.
8.4.3 Integer and Fractional Mode Selection
The PLL is designed to operate in either Integer mode or Fractional mode. If the desired local oscillator (LO)
frequency is an integer multiple of f
PFD
, then select integer mode otherwise select fractional mode. In Integer
mode, the feedback divider ratio is an integer, and the fraction is zero. Thus, bits corresponding to the fractional
control in integer mode are
don’t care
and fractional divider functionality is disabled.