Datasheet

Data Sheet AD9552
Rev. E | Page 13 of 32
THEORY OF OPERATION
07806-006
REFA
XTAL
XTAL
SERIAL
PORT
A2:0
Y5:0
OUT2
FILTER
OUT1
REGISTER BANK
Σ-Δ
MODULATOR
PFD
CHARGE
PUMP
LOCK
DETECT
DETECTOR
PRECONFIGURED
DIVIDER VALUES
3
3
6
N, MOD, FRAC, P
0
, P
1
AD9552
P
0
, P
1
MOD,
FRAC
N
N
1
P
0
P
1
4 OR 5
N = 4N
1
+ N
0
2
2
4 TO 11 1 TO 63
3350MHz TO
4050MHz
VCO
LOCKED
TUNING
CONTROL
2×
Figure 19. Detailed Block Diagram
PRESET FREQUENCY RATIOS
The frequency selection pins (A[2:0] and Y[5:0]) allow the user
to hardwire the device for preset input and output divider values
based on the pin logic states (see Figure 19). The pins decode
ground or open connections as Logic 0 or Logic 1, respectively.
Use the serial I/O port to change the divider values from the
preset values provided by the A[2:0] and Y[5:0] pins.
The A[2:0] pins select one of eight input reference frequencies
(see Table 9). The user supplies the input reference frequency by
connecting a single-ended clock signal to the REF pin or a crystal
resonator across the XTAL pins. If the A[2:0] pins select 10 MHz,
12 MHz, 12.8 MHz, or 16 MHz, the input frequency to the AD9552
doubles internally. Alternatively, if Register 0x1D[2] is set to 1,
the input frequency doubles.
Table 9. Input Reference Frequency Selection Pins
A2 A1 A0 Reference Frequency (MHz)
0 0 0 10.00
0 0 1 12.00
0 1 0 12.80
0 1 1 16.00
1
0
0
19.20
1 0 1 19.44
1 1 0 20.00
1 1 1 26.00
The Y[5:0] pins select the appropriate feedback and output dividers
to synthesize the output frequencies (see Table 10). The output
frequencies provided in Table 10 are exact; that is, the number of
decimal places displayed is sufficient to maintain full precision.
Where a decimal representation is not practical, a fractional
multiplier is used.
The VCO and output frequency shift in frequency by a ratio of the
reference frequency used vs. the frequency specified in Table 9.
Note that the VCO frequency must stay within the minimum and
maximum range specified in Table 1. Typically, the selection of
the VCO frequency band, as well as the gain adjustment, by the
external pin strap occurs as part of the device’s automatic VCO
calibration process, which initiates at power up (or reset). If the
user changes the VCO frequency band via the SPI interface,
however, a forced VCO calibration should be initiated by first
enabling SPI control of the VCO calibration (Register 0x0E[2] = 1)
and then writing a 1 to the calibrate VCO bit (Register 0x0E[7]).