ML2712 Datasheet, PDF (7/22 Pages) Micro Linear Corporation – 2.4GHz RF Transceiver

PRELIMINARY
ML2712
FUNCTIONAL DESCRIPTION
INTRODUCTION
The ML2712 2.4GHZ RF Transceiver contains all the RF
circuitry, including the phase lock loop (PLL), and the
active VCO circuits, for a half duplex transceiver.
When combined with the ML2713 it enables the
design of a high performance 2.4GHz half duplex
radio with a fast switching time between transmit
and receive modes. This is ideal for applications
such as frequency hopping radio for the IEEE 802.11
FH standard.
The ML2712 Transceiver has four modes of operation; 1)
Standby 2) Transmit 3) Receive and 4) Sleep. The
operating modes of the ML2712 can be programmed
through a parallel control interface or through a serial
interface. Two serial control interfaces are utilized for
programming the PLLs and on chip A/D.
In STANDBY mode all the PLL and VCO circuits are
enabled while all the transmitter and receiver circuits are
disabled. The use of STANDBY mode is recommended
when the PLLs are locking, after the PLL frequencies have
been reprogrammed, or after the IC has been transferred
out of SLEEP mode.
The transmit circuits include a 2.4GHz RF down converter
for transmit frequency translation and a Wideband Phase
Detector that implements a directly modulated VCO
transmitter radio architecture. The receive section of the
ML2712 inlcudes a 2.4GHz RF mixer that down converts
the received RF frequency to the first IF frequency;
nominally 260MHz. All required frequency generation
circuits are integrated on-chip for the RF conversion
including the 1LO VCO and the PLL and 1LO frequency
doubler. Additionally, a second VCO and PLL provide a
2LO output useable in IF circuits in a dual conversion
radio. An 8 bit D/A & Comparator for an RSSI tracking
A/D are also integrated on-chip.
The two local oscillator signals generated in the ML2712
are the 1LO with at a typical frequency in the region of
2.2GHz and the 2LO, with a typical frequency of
236MHz. Both signals are phase locked by the
independently programmable PLLs to a common external
reference frequency. External tank circuits are required
for the 1LO and 2LO VCOs to determine the operating
frequency ranges. The 1LO signal, generated by doubling
the frequency of the 1LO VCO, is used by both the
transmit and receive mixer circuits to down convert the
2.4GHz RF signals. The half-frequency 1LO VCO eases
tank circuit design and minimizes the VCO pulling when
the the radio switches between transmit and receive
modes. The differential output from the 2LO VCO is
provided for use in radio IF circuits such as the ML2713.
A lock detect output indicates when the PLLs are
frequency locked. Programming of the PLL frequency is
performed via a three wire serial interface.
The ML2712 implements a directly modulated VCO
transmitter architecture. The elements of this architecture
include a Power Amplifier, a transmitter VCO, a
transmitter reference generation circuit and a 1IF
Wideband PLL which locks the Transmitter VCO to this
transmit reference signal. The ML2712 does not integrate
the PA but does provide the Wideband PLL and an RF
down-convert mixer, enabling the transmit reference
signal to be generated at a lower frequency. In
TRANSMIT mode the 2.2GHz (nominal) 1LO signal is
used to down-convert the 2.4GHz external transmit VCO
RF signal. The down converted signal is then phase
locked by the on chip wideband phase detector to an
externally modulated signal (Transmit reference IF) and
output to pins 1IFB and 1IF. The output of the Wideband
phase detector controls the transmit VCO frequency (Tx
VCO external to IC) via an external loop filter. For a
typical application, e.g., IEEE802.11 the symbol rate is
1Msymbol/sec. The lock up time is less than 2msec.
enabling a radio designed with the ML2712 to switch
between transmit and receive modes in less than 2msec.
The ML2712 receiver circuits perform the RF down
conversion. A typical receiver design would include the
ML2712, an external LNA and RF filter, an IF filter and
the ML2713 IF Transceiver. In RECEIVE mode, the
ML2712 uses the 1LO signal to down convert the
received 2.4 GHz band signal to a nominal 260 MHz IF.
The received IF signal is output on 1IF and 1IFO pins. By
multiplexing both the transmit and receive signals on one
set of pins, only a single Surface Acoustic Wave (SAW) IF
channel filter is required in the radio design. A SAW filter
with a nominal Gaussian impulse response can be used to
provide modulation filtering of the transmit reference IF
signal. When in receive mode the A/D and Comparator
provide the analog circuits for a tracking A/D converter,
intended for RSSI digitization or clear channel assessment
for "listen before talk" radios.
In SLEEP mode all circuits, except for the central
interface and programming registers, are powered
down to minimize power consumption.
January, 2000 PRELIMINARY DATASHEET
7