ADF4252
RF Phase Detector Polarity
Lock Detect
DB7 in the ADF4252 sets the RF phase detector polarity.
When the VCO characteristics are positive, this should be set to
1. When they are negative, it should be set to 0.
The digital lock detect output goes high if there are 40 successive
PFD cycles with an input error of less than 15 ns. It stays high
until a new channel is programmed or until the error at the PFD
input exceeds 30 ns for one or more cycles. If the loop bandwidth
is narrow compared to the PFD frequency, the error at the PFD
inputs may drop below 15 ns for 40 cycles around a cycle slip; thus
the digital lock detect may go falsely high for a short period until
the error again exceeds 30 ns. In this case the digital lock detect is
reliable only as a “loss of lock” indicator.
RF Charge Pump Current Setting
DB9 and DB10 set the RF charge pump current setting. This
should be set to whatever charge pump current the loop filter
has been designed with (see Table V).
RF Test Modes
These bits should be set to 0, 0, 0 for normal operation.
IF N DIVIDER REGISTER
(Address R4)
With R4[2, 1, 0] set to [1, 0, 0], the on-chip IF N divider register
will be programmed. Table VII shows the input data format for
programming this register.
MASTER REGISTER
(Address R3)
With R3[2, 1, 0] set to 0, 1, 1, the on-chip master register will be
programmed. Table VI shows the input data format for program-
ming the master register.
IF CP Gain
RF and IF Counter Reset
When set to 1, this bit changes the IF charge pump current
setting to its maximum value. When the bit is set to 0, the
charge pump current reverts back to its previous state.
DB3 is the counter reset bit for the ADF4252. When this is 1,
both the RF and IF R, INT, and MOD counters are held in reset.
For normal operation, this bit should be 0. Upon power-up, the
DB3 bit needs to be disabled, the INT counter resumes counting
in “close” alignment with the R counter. (The maximum error is
one prescaler cycle).
IF Prescaler
The dual-modulus prescaler (P/P + 1), along with the IF A and
B counters, determine the overall division ratio, N, to be realized
(N = PB + A) from the IFIN to the IF PFD input. Operating at
CML levels, it takes the clock from the IF input stage and divides it
down to a manageable frequency for the CMOS counters. It is
based on a synchronous 4/5 core. See Equation 2 and Table VII.
Charge Pump Three-State
This bit puts both the RF and IF charge pump into three-state
mode when programmed to a 1. It should be set to 0 for normal
operation.
IF B and A Counter
Power-Down
The IF A and B counters, in conjunction with the dual modulus
prescaler, make it possible to generate output frequencies that
are spaced only by the reference frequency (REFIN) divided
by R. The equation for the IFOUT VCO frequency is given in
Equation 2.
R3[3] on the ADF4252 provides the programmable power-down
mode. Setting this bit to a 1 will perform a power-down on both
the RF and IF sections. Setting this bit to 0 will return the RF
and IF sections to normal operation. While in software power-
down, the part will retain all information in its registers. Only
when supplies are removed will the register contents be lost.
IF R DIVIDER REGISTER
(Address R5)
When a power-down is activated, the following events occur:
1. All active dc current paths are removed.
With R5[2, 1, 0] set to [1, 0, 1], the on-chip IF R divider register
will be programmed. Table VIII shows the input data format for
programming this register.
2. The RF and IF counters are forced to their load state conditions.
3. The RF and IF charge pumps are forced into three-state mode.
4. The digital lock detect circuitry is reset.
IF REFIN Doubler
Setting this bit to 0 feeds the REFIN signal directly to the 15-bit
IF R counter. Setting this bit to 1 multiplies the REFIN
frequency by a factor of 2 before feeding into the 15-bit IF R
counter.
5. The RFIN input and IFIN input are debiased.
6. The oscillator input buffer circuitry is disabled.
7. The input register remains active and capable of loading and
latching data.
15-Bit IF R Counter
The 15-bit IF R counter allows the input reference frequency
(REFIN) to be divided down to produce the reference clock to
the IF phase frequency detector (PFD). Division ratios from
1 to 32767 are allowed.
XO Disable
Setting this bit to 1 disables the REFOUT circuitry. This will
be set to 1 when using an external TCXO, VCXO, or other
reference sources. This will be set to 0 when using the REFIN
and REFOUT pins to form an oscillator circuit.
IF CONTROL REGISTER
(Address R6)
With R6[2, 1, 0] set to [1, 1, 0], the on-chip IF control register
will be programmed. Table IX shows the input data format for
programming this register. Upon initialization, DB15–DB11
should all be set to 0.
MUXOUT Control
The on-chip multiplexer is controlled by R3[10–7] on the
ADF4252. Table VI shows the truth table.
If the user updates the RF control register or the IF control
register, the MUXOUT contents will be lost. To retrieve the
MUXOUT signal, the user must write to the master register.
–22–
REV. B
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