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HFA3783 Datasheet, PDF (17/34 Pages) Intersil Corporation – I/Q Modulator/Demodulator and Synthesizer | |||
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HFA3783
REF_IN
PIN 14
R COUNTER
R
TO
DC OFFSET
CAL
V
ISOURCE
N COUNTER
RESET
A
DUAL MODULUS
CONTROL
P/P+1
B
PRESCALER
RESET
ISINK
LO_IN+
PIN 27
OR 26
CP_D0
PIN 22
VCONTROL
VCO
VCO
[P*B+A]
REF
R
FIGURE 4. PLL SIMPLIFIED BLOCK DIAGRAM
1/2VCC
FIGURE 5. CHARGE PUMP OUTPUT FOR TWO SLIGHTLY DIFFERENT FREQUENCY SIGNALS
PLL
The HFA3783 includes a classical architecture Phase Lock
Loop circuit with a three wire serial control interface to be
used with an external VCO. Figure 4 depicts a simpliï¬ed
block diagram of the PLL. It consists of a programmable âRâ
counter used to divide down the frequency of a very stable
reference signal up to 50MHz to a phase comparator. A
couple of counters (âAâ and âBâ) with a front end prescaler
(âP or P+1â), with dual modulus control, divides down the
frequency of an external VCO signal to the same phase
comparator. The comparator controls a charge pump circuit
and an external loop ï¬lter closes the loop for VCO control.
The VCO frequency dividing chain works with a dual
modulus control as follows: At the beginning of a count
cycle, and if the A counter is programmed with a value
greater than zero, the prescaler is set to a division ratio of
(P+1) where P can take programmable values of 16 or 32.
Notice that the prescaler output signal is always fed
simultaneously to both A and B counters. Upon ï¬lling
counter A, the prescaler division ratio becomes P and the B
counter continues on its own with A in standby. This process
is known as âpulse swallowingâ. The expression B-A (counts)
is the remainder of counts carried out by the B counter after
A is full. Both A and B counters are reset at the end of the
counting cycle when B ï¬lls up. As a result, the total count or
division ratio used for the VCO signal is A*(P+1) + (B-A)*P
which simpliï¬es to [P*B+A]. (A and B counters are referred
as the âNâ counter).
The Charge Pump (current source/sink) has 4
programmable current settings. This variation allows the
user to change the reference frequency for different
objectives without changing the loop ï¬lter components. The
user can program the charge pump sign based on the
direction of increase or decrease of the VCO frequency. The
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