SA1921 Datasheet, PDF (6/36 Pages) NXP Semiconductors – Satellite and cellular dual-band RF front-end

Philips Semiconductors
Satellite and cellular dual-band RF front-end
Product specification
SA1921
OPERATION
The low-band contains both an LNA and mixer that is designed to
operate in the 800 to 1000 MHz frequency range. The high-band
contains an LNA and image-rejection mixer that is designed to
operate in the 1515 to 1600 MHz frequency range with over 30 dB of
rejection over an intermediate frequency (IF) range from 150 to
185 MHz.
Image rejection is achieved in the internal architecture by two RF
mixers in quadrature and two all-pass filters in the I and Q IF
channels that phase shift the IF by 45_ and 135_, respectively. The
two phase shifted IFs are recombined and buffered to produce the
IF output signal.
The LO section consists of an internal phase shifter to provide
quadrature LO signals to the receive mixers. The filters outputs are
buffered before being fed to the receive mixers. The transmit mixer
section consists of a low-noise amplifier, and a down-convert mixer.
In the transmit mode, an internal LO buffer is used to drive the
transmit IF down-convert mixer.
Low-Band Receive Section
The circuit contains a LNA followed by a wide-band mixer. In a
typical application circuit, the LNA output uses an external pull-up
inductor to V_CC and is AC coupled. The mixer IF outputs are
differential. A typical application will load the output buffer with an
inductor across the IF outputs, a pull-up inductor to V_CC and an AC
coupled capacitor to the matching network.
Low-Band Receive Section (Analog Transmit
Mode)
The bias current of the low-band LNA will increase during analog
transmission, which increases its gain compression point and makes
the receiver less sensitive to PA leakage power for an AMPS
application.
High-Band Receive Section
The circuit contains an LNA followed by two high dynamic range
mixers. These are Gilbert cell mixers; the internal architecture is fully
differential. The LO is shifted in phase by 45_ and 135_ and mixes
the amplified RF signal to create I and Q channels. The two I and Q
channels are buffered, phase shifted by 45_ and 135_, respectively,
amplified and recombined internally to realize the image rejection.
The IF output is differential and of the open-collector type. A typical
application will load the output buffer with an inductor across the IF
outputs, a pull-up inductor to V_CC and an AC coupled capacitor to
the matching network.
Control Logic Section
Pins HI/LO, SYN ON, Rx On, Tx On, Strong Signal, control the logic
functions. The HI/LO mode selects between low-band and
high-band operation. The SYN ON mode enables the LO buffers
independent of the other circuitry. When SYN ON is high, all internal
buffers in the LO path of the circuit are turned on, thus minimizing
LO pulling when the remainder of the receive or transmit chain is
powered-up.
The Rx ON mode enables the LO buffers when the device is in the
low-band receive normal, receive strong signal and transmit modes;
the Rx ON mode enables the LO buffers, also, when the device is in
the high-band receive normal, and receive strong signal modes.
The Tx ON mode enables the transmit mixer. The strong signal
mode, when disabled, allows the low- and high-band LNAs to
function normally; and when the strong signal mode is enabled, it
turns-off the low- and high-band LNAs. This is needed when the
input signal is large and needs to be attenuated.
Local Oscillator (LO) Section
The LO input directly drives the two internal all-pass networks to
provide quadrature LO to the receive mixers. A synthesizer-on (SYN
ON) mode is used to power-up all LO input buffers, thus minimizing
the pulling effect on the external VCO when entering receive or
transmit mode.
Transmit Mixer Section
The transmit mixer is used for down-conversion to the transmit IF. Its
inputs are coupled to the transmit RF which is down-converted to a
modulated transmit IF frequency, and phase-locked with the
baseband modulation.
The IF outputs are HIGH impedance (open-collector type). A typical
application will load the output buffer with an inductor across the IF
outputs, a pull-up inductor to V_CC and AC coupled capacitors to the
matching network.
1999 Mar 02
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