SA9025 데이터 시트보기 (PDF) - Philips Electronics

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SA9025

900 MHz transmit modulator and 2.2 GHz fractional–N synthesizer

Philips Electronics 

Philips Semiconductors

900 MHz transmit modulator and 2.2 GHz

fractional–N synthesizer

Objective specification

SA9025

REFERENCE R

MAIN M

VCO CYCLES

N

N

N+1

N

N+1

DETECTOR

OUTPUT

ACCUMULATOR

CONTENTS

2

4

1

3

0

FRACTIONAL

COMPENSATION

CURRENT

PULSE

WIDTH

MODULATION

mA

OUTPUT ON

PHP, PHI

µA

PULSE LEVEL

MODULATION

Figure 10. Waveforms for NF = 2; Fraction = 0.4

SR01454

Figure 10 shows that for a proper fractional compensation, the area

of the fractional compensation current pulse must be equal to the

area of the charge pump ripple output.

The fractional compensation current is derived from the main charge

pump in that it will follow all the current scaling through external

resistor setting, programming or speedup operation.

For a given pump,

|comp +

|pump

128

x

Fdac

5 x 128

x

FRD

Where:

Icomp is the compensation current, Ipump is the pump current, Fdac

is the fractional DAC value and FRD is the fractional accumulator

value.

The theoretical value for Fdac would then be: 128 for Fmod = 1

(modulo 5) and 80 for Fmod = 0 (modulo 8).

When the serial input A word is loaded, the output circuits are in the

“speedup mode” as long as the STROBE is H, otherwise the

“normal mode” is active.

Lock Detect

The output LOCK maintains a logic ‘1’ when the auxiliary phase

detector ANDed with the main phase detector indicates a lock

condition. The lock condition for the main and auxiliary synthesizers

is defined as a phase difference of less than "1 cycle on the

reference inputs XTAL1,2. The LOCK condition is also fulfilled when

the relative counter is disabled (PDmain = ‘0’ or PDaux = ‘0’) for the

main or auxiliary counter, respectively. Lock indication when PDmain

= PDaux = ‘0’.

Functional Description of Offset Loop, Modulator

and Power Control

Transmit Offset Synthesizer

The transmit offset phase locked loop portion of the SA9025 design

consists of the following functional blocks: reference oscillator,

limiters, phase detector, ÷M, IF VCO and passive loop filter.

Harmonic contents of this signal are attenuated by an LP filter. The

output of the IF VCO is also divided by N and compared with the

reference oscillator in the phase detector.

Reference Oscillator

This Oscillator is used to generate the reference frequency together

with an external crystal and varicap. The output is internally routed

to three buffers and a phase comparator. It is possible to run the

oscillator as an amplifier from an external reference signal (TCXO).

Phase Detector and Charge Pump

The phase comparator is used to compare the output of the divider

with the reference. It provides an output proportional to the phase

difference between the divided down VCO and the reference. This

output is then filtered and used as the control voltage input to the

VCO. The phase detector is a Gilbert multiplier cell type, having a

linear output from 0 to π (π/2 ±π/2), followed by a charge pump. The

charge pump peak output current is programmable to 6.4mA via the

use of an external resistor.

A preliminary design analysis has been performed with the following

loop parameters:

A lock detect signal is provided and ANDed together with lock detect

signals from both the main channel synthesizer and auxiliary

1997 Aug 01

20

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