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ML2721

Low IF Digital Cordless Transceiver

Micro Linear Corporation 

PRELIMINARY

ML2721

CIRCUIT BLOCK DESCRIPTIONS

LO VCO AND TRANSMIT DRIVER

DATA FILTER AND DATA SLICER

The internal LO VCO operates at 1.8 to 1.86GHz, which

is two times the LO frequency of 900 to 930MHz. The

VCO output is divided by 2 to give accurately matched

quadrature signals at 915MHz. In Receive mode the LO is

offset from the wanted signal by +1.024MHz to produce

the low IF. This 1.024MHz shift is produced by

automatically adding an offset of +2 counts to the PLL

divider programming.

In Transmit mode the LO is frequency modulated by the

transmitted data, using a modulation port of the VCO. The

VCO output is divided by 2 and a driver amplifier

typically develops 0dBm at 900 to 930MHz to directly

drive an ML2751 power amplifier.

TRANSMIT DATA FILTER AND MODULATION DRIVER

Logic level NRZ signals at DIN are scaled and filtered by

a 5th-order lowpass filter. The lowpass filter is tuned to

give a 1.35MHz 3dB point to pass the 1.536Mchips

transmit data. The filter data is then fed to the internal

modulation port of the LO tank circuit.

In the transmit closed loop mode the modulation port is

held at its midpoint so that the synthesizer locks to the

center channel frequency. In the transmit open loop mode

the VCO is modulated by Gaussian filtered data via the

VCO modulation port. The modulation driver contains

scaling circuitry to control the FM deviation over the

entire VCO tuning range. This circuit is inactive in the

receive mode.

The FM demodulator is followed by a Gaussian lowpass

filter whose 768kHz cutoff frequency is matched to the

transmitted 1.536Mchip/s waveform. This Gaussian filter is

implemented with similar circuits to the IF filter, and is

shared with the Transmit modulation path. The filter output

can be AC coupled to the slicer because the spreading

code is almost DC balanced. The data slicer signal is

output to the baseband processor for timing recovery and

decoding.

POWER SUPPLY

The ML2721 uses multiple voltage regulators to protect

sensitive internal circuits from power supply noise.

Separate regulators supply the PLL dividers, RF circuits

and IF circuits. Each of these regulators takes its power

from VCC5, and supplies power internally to its respective

RVCCn pin. External capacitors are required at each

RVCCn pin to decouple the outputs of the internal

regulators. The VCO regulator takes its power from the

VCC2 pin which is normally connected to the RVCC6 pin.

An external decoupling capacitor is also used on the

internal bandgap voltage reference to improve the noise

performance of the regulators.

These regulators are effective at supply voltages from

3.0V to 5.0V. As the VCC5 supply voltage drops below

3.0V chip performance gradually degrades, but the

ML2721 transceiver will maintain the link as long as the

supply voltage is greater than 2.7V.

RECEIVE MIXERS AND IF CHAIN

The Receive RF quadrature mixers down-convert the

signal to the 1.024MHz receiver IF. The input of the mixer

is single-ended and matched to 50W by a series inductor.

This gives a good terminating impedance for the

preceding RF filter. The quadrature outputs of the down

converter feed the IF filter. The quadrature mixer and IF

filter together achieve a typical image rejection of 35dB.

A quadrature combiner for the image reject mixer and a

12th-order Gaussian bandpass filter make up the active IF

filters. The active filters provide an accurate Gaussian

characteristic with a 1.408MHz, 3dB bandwidth which

improves both sensitivity and adjacent channel rejection.

The IF amplifiers provide the bulk of the receiver’s gain.

An RSSI signal is generated by using the outputs of the IF

amplifiers. The RSSI signal is conditioned and sent to the

baseband controller. A frequency-to-voltage converter

provides highly linear FM demodulation with good data

recovery from the low IF.

January, 2000 PRELIMINARY DATASHEET

9

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