ML2721 datasheet(10/27 Pages) MICRO-LINEAR

10 / 27 page

ML2721

January, 2000

PRELIMINARY DATASHEET

PRELIMINARY

I.F. FILTER

LIMITING

AMPLIFIERS

ILO

RECEIVER

INPUT

QLO

RSSI

AGC

DATA FILTER

DOUT

RSSI

∫

Figure 1. Receiver Block Diagram

MODES OF OPERATION

OVERVIEW

n STANDBY: All circuits powered down, except the

control interface (Static CMOS)

n RXCAL: Receive Calibration. PLL lock up time and

IF filter alignment, prior to RECEIVE

n RECEIVE: Receiver circuits active

n TXCAL: Transmit Calibration. PLL lock up time prior

to TRANSMIT

n TRANSMIT: PLL open loop, modulated RF output

available from IC

The four operational modes are RXCAL, RECEIVE, TXCAL

and TRANSMIT. They are set by the RXON (pin 2) and

PLLEN (pin 3) control pins. XCEN (pin 1) is the chip

enable/disable pin and can be set for standby operation.

The relationship between the parallel control lines and the

mode of operation of the IC is given in Table 2.

PARALLEL MODE CONTROL

The ML2721 is intended for use in TDD and TDMA radios

in battery powered equipment. To minimize power

consumption it is designed to switch rapidly from a low

power mode (STANDBY) to receive or transmit. The

ML2721 can also make a quick transition from receive to

transmit for TDD operation. Prior to transmitting or

receiving time should be allowed for the PLL to lock up,

and in the case of receive, for the IF filters to be aligned.

The ML2721 operates as a TDD radio on one RF frequency

(transmit or receive). The VCO frequency changes

between receive and transmit functions because of the IF

frequency. This VCO frequency step is automatically

performed.

Table 2. Mode Selection

RECEIVE MODES

RXCAL AND RECEIVE

In RECEIVE mode, the received signal at 900MHz is down

converted, bandpass filtered (IF filter), fed to the

frequency-to-voltage converter and low-pass filtered. The

output of the low-pass filter is fed to the data slicer which

outputs NRZ digital data. An RSSI voltage output

indicates the signal level at the output of the IF filter.

Prior to receive mode the ML2721 should be switched to

RXCAL mode. This is to allow the IF filters to be aligned

and for the PLL to switch frequency and settle to the

required frequency for receiving. Filter alignment, using

the Reference Frequency input signal REF, in the RXCAL

mode sets:

n Discriminator center frequency

n IF filter center frequency and bandwidth

n Receiver data low pass filter bandwidth

n Transmit data low pass filter bandwidth

In receive mode the programmed center frequency sets

the RF channel that will be received. This means the PLL

center frequency is the programmed RF center frequency

plus the IF frequency. The frequency offset required for the

low frequency IF in the RXCAL mode is automatically

calculated and performed.

RECEIVER CIRCUIT OPERATION

The ML2721 receive chain is a low IF receiver using

advanced integrated radio techniques to eliminate

external IF filters and minimize external RF filter

requirements. No external support circuitry is required

other than supply decoupling and input matching. The

precise filtering and demodulation circuits give improved

performance over conventional radio designs using

external filters. See Figure 1.

111

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