ML2721 datasheet(8/27 Pages) MICRO-LINEAR

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ML2721

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

FUNCTIONAL DESCRIPTION

INTRODUCTION

The ML2721 enables the design and manufacture of low

cost, high performance digital DSSS cordless telephone

transceivers. It can also be used as a general purpose

900MHz transceiver. Integral to the ML2721 is a low IF

receiver whose LO port is driven from an internal

synthesizer. Included are image rejection IF filters,

limiters, discriminator, data slicers, and baseband lowpass

data filters. It also contains internal voltage regulators to

protect critical circuits from power supply noise and

transmit modulation circuits.

The ML2721 has an internal control interface that

programs the synthesizer, the mode of operation, the

external LNA and PA, and provides a convenient and

flexible interface to various baseband processors. For

power level monitoring an RSSI block is included.

The ML2721 is designed to transmit and receive

1.536Mchips signals in 2.048MHz spaced channels in the

902 to 928MHz ISM band. The 1.536Mchips rate with a

15 bit spreading code gives a 102.4kb/s data rate and

provides a 10dB processing gain.

In the Receive mode the ML2721 is a single conversion

low IF receiver. The IF frequency of 1.024MHz results in

an image response in an adjacent channel. An image

reject mixer gives sufficient rejection in this channel. All

IF filtering and demodulation is performed using active

filtering, centered at 1.024MHz. The demodulator is

followed by a matched bit rate filter and a data slicer. The

sliced data is provided to a baseband chip for de-

spreading.

In the Transmit mode the ML2721 uses the Receive mode

VCO and frequency division, with a driver amplifier

providing typically 0dBm output to feed the power

amplifier. The PLL frequency synthesizer loop is opened

during the transmit slot, and the VCO is directly

modulated by low-pass filtered circuits from the internal

modulation filter.

The frequency generation circuits are an internal VCO at

1.83GHz, dividers, a phase comparator and a charge

pump for a PLL frequency synthesizer. The VCO output is

divided by two to produce accurate quadrature outputs at

915MHz. No external components are need for the VCO.

Other modes that are available include power down, and

receive and transmit calibrate, which are discussed in

further detail.

CIRCUIT BLOCK DESCRIPTIONS

PHASE LOCKED LOOP (PLL) AND VOLTAGE

CONTROLLED OSCILLATOR (VCO)

The PLL synthesizes channel frequencies to a 512kHz

resolution, which is more finely spaced than the

1.536MHz signal bandwidth. Non-overlapping channels

are spaced by 2.048MHz where the IF filter and image

reject mixer give a typical adjacent channel rejection of

25dB. There are twelve non-overlapping channels in the

902 to 928MHz ISM band. See Table 1.

The LO PLL is programmed via a 3-wire serial control bus.

Program words are clocked in on the DATA line (pin 5) by

the CLK (pin 6), and loaded into the dividers or control

circuits when EN (pin 4) is asserted. There is no check for

error in the program words. Once loaded, register contents

are preserved regardless of power conditions. The register

status and operation is independent of the mode of

operation of the PLL.

The reference signal from an external crystal oscillator at

either 6.144MHz or 12.288MHz is fed to a programmable

reference divider. The 1.024MHz reference divider output

is fed to the LO phase frequency detector. The PLL

prescaler input comes from the VCO at 1.83GHz, so the

1.024MHz comparison frequency gives 512kHz frequency

resolution at 902 to 928MHz.

The output of the LO divider is fed to the LO phase/

frequency detector and subsequently to the charge pump.

The dividers and charge pump are disabled during the

active slot to save power.

Table 1. Non-Overlapping Channel Frequencies

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