AD9552PCBZ datasheet(13/32 Pages) AD

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AD9552

Rev. C | Page 13 of 32

THEORY OF OPERATION

REFA

XTAL

SERIAL

PORT

A2:0

Y5:0

OUT2

FILTER

OUT1

Σ-Δ

MODULATOR

PFD

CHARGE

PUMP

LOCK

DETECT

DETECTOR

PRECONFIGURED

DIVIDER VALUES

N, MOD, FRAC, P0, P1

AD9552

P0, P1

MOD,

FRAC

4 OR 5

N = 4N1 + N0

4TO 11

1TO 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

Reference Frequency (MHz)

10.00

12.00

12.80

16.00

19.20

19.44

20.00

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]).

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