SA8028W datasheet(8/28 Pages) PHILIPS

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Philips Semiconductors

Product data

SA8028

2.5 GHz sigma delta fractional-N /

760 MHz IF integer frequency synthesizers

2002 Feb 22

1.0

FUNCTIONAL DESCRIPTION

Frequency synthesizers, such as Philips Semiconductors’ SA8028,

are a crucial part of Phase Locked Loops (PLL) for both voice and

data devices used in communications. Five components make up

the basic PLL (see Figure 4). A very stable, low frequency, signal

source (typically a temperature controlled crystal oscillator TCXO_)

is used as a reference to the system. A second signal source

(typically a VCO) is used to generate the desired output frequency.

A phase/frequency detector (PFD) is used to compare the

phase/frequency error between the two signals. A loop filter (LPF)

rejects undesired noise while also integrating the PFD output current

to drive the VCO with the necessary tuning voltage, and a divider in

the feedback path is used to down-convert the VCO output

frequency to the reference frequency for comparison. The SA8028

is a dual synthesizer that integrates programmable dividers,

programmable charge pumps and phase comparators to be

implemented as part of RF and IF PLLs. The RF synthesizer

operates at VCO input frequencies up to 2.5 GHz, while the IF

synthesizer operates at VCO input frequencies up to 760 MHz.

SR02370

PFD

TCXO

LPF

INTEGRATOR

VCO

DIVIDER

∆N(τ)

Figure 4.

PLL block diagram.

1.1

RF Fractional-N divider

The RFin inputs drive a pre-amplifier to provide the clock to the first

divider stage. For single ended operation, the signal should be fed

(AC-coupled) to one of the inputs while the other one is AC grounded.

The pre-amplifier has a high input impedance, dominated by pin and

pad capacitance. The bipolar divider is fully programmable. For

allowable division ratios, see the “characteristics” table.

During each RF divider cycle, one divider output pulse is generated.

The positive edge of this pulse drives the phase comparator, the

negative edge drives the sigma-delta modulator which is of 2nd

order and has an effective resolution of 22 bits. Internally, the

modulator works with 23 fractional bits K<22:0>, but the LSB (bit K0)

is set to ‘1’ internally to avoid limit cycles (cycles of less than

maximum length). This leaves 22 bits (K<22:1>) available for

external programming.

Under these conditions (2nd order modulator, 23 fractional bits,

K0 = ‘1’), all possible sigma-delta sequences are 2*223 divider

cycles long, which is the maximum length. The noise shaping

characteristic is +20 dB/dec for offset frequencies up to approx.

fCOMP/5, which needs to be cancelled by a closed-loop transfer

function of sufficient high order. The output of the sigma-delta

modulator is 2 bits, which are added to the integer RF division ratio

N, such that the momentary division ratios range from

(N–1) to (N+2) in steps of 1.

1.2

IF divider

The IFin input drives a pre-amplifier to provide the clock to the first

divider stage. The pre-amplifier has a high input impedance,

dominated by pin and pad capacitance. The divider consists of a

fully programmable bipolar prescaler followed by a CMOS counter.

The allowable divide ratios are from 128 to 16383 (C-word bits

<21:8>). Table 14 shows all the possible values that can be

programmed into the C-Word for the IF divider.

1.3

Reference divider (see Figure 5)

The IF phase detector’s reference input is an integer ratio of the

reference frequency. The reference divider chain consists of a

bipolar input buffer followed by a CMOS divider and a 3-bit binary

counter (SA register). The allowable divide ratios, R, are from 4 to

1023 (B-word bits <21:12>) when the 3-bit binary counter (C-word

bits <2:0>) is set to all zeros, SA = 000. The 3-bit SA register

determines which of the 5 divider outputs (refer to Table 12) is

selected as the IF phase detector input (see Figure 5). For the RF

synthesizer, the output of the reference input buffer is routed directly

(not reference divider) to the input of the RF phase detector.

SR02294

DIVIDE BY R

SA=”100”

SA=”011”

SA=”010”

SA=”001”

SA=”000”

TO RF PHASE

DETECTOR

TO IF PHASE

DETECTOR

REFERENCE

INPUT BUFFER

REFERENCE

INPUT

Figure 5.

Reference divider.

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