REFERENCE INPUT SECTION
The reference input stage is shown in Figure 11. SW1 and SW2
are normally closed switches. SW3 is normally open. When
power-down is initiated, SW3 is closed and SW1 and SW2 are
opened. This ensures that there is no loading of the REFIN pin
TO R COUNTER
Figure 11. Reference Input Stage
RF INPUT STAGE
The RF input stage is shown in Figure 12. It is followed by a
2-stage limiting amplifier to generate the current-mode logic
(CML) clock levels needed for the prescaler.
Figure 12. RF Input Stage
RF INT DIVIDER
The RF INT counter allows a division ratio in the PLL feedback
counter. Division ratios from 23 to 4095 are allowed.
INT, FRAC, MOD, AND R RELATIONSHIP
The INT, FRAC, and MOD values, in conjunction with the
R counter, make it possible to generate output frequencies that
are spaced by fractions of the phase frequency detector (PFD).
See the RF Synthesizer: A Worked Example section for more
information. The RF VCO frequency (RFOUT) equation is
RFOUT = FPFD × (INT + (FRAC/MOD))
where RFOUT is the output frequency of external voltage
controlled oscillator (VCO).
FPFD = REFIN × [(1 + D)/(R × (1 + T))]
REFIN is the reference input frequency.
D is the REFIN doubler bit.
T is the REFIN divide-by-2 bit (0 or 1).
R is the preset divide ratio of binary 5-bit programmable
reference counter (1 to 32).
INT is the preset divide ratio of binary 12-bit counter
(23 to 4095).
MOD is the preset fractional modulus (2 to 4095).
FRAC is the numerator of the fractional division (0 to MOD−1).
RF N DIVIDER
N = INT + FRAC/MOD
Figure 13. RF INT Divider
RF R COUNTER
The 5-bit RF R counter allows the input reference frequency
(REFIN) to be divided down to produce the reference clock to
the PFD. Division ratios from 1 to 32 are allowed.
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