OVERVIEW: Receiver Path Section
1.8.1 JANUS Module (RXPath)
The Janus Front End module (U500) contains the antenna switch, the passive SAW filters
and the associated switches, along with the active LNA and the balun circuit. The antenna
switch routes the received signal from the antenna pin input to the receiver block of Janus.
During transmit mode, this switch disconnects the receiver path and connects the antenna
to the transmit path within Janus. The triple preselector SAW filters protect the RF LNA
from strong, out-of band signals. There are three filters (800MHz, 900MHz, and ISM) that
are appropriately selected by the RF switches, for the appropriate band of operation. The
LNA RF amplifier contains three major blocks: 20dB step attenuator, low noise amplifier
and the AGC stage. The low-noise amplifier provides the gain to achieve the necessary
receiver system take over gain and the AGC stage provides continuous attenuation to avoid
overload of the receiver backend. Finally, the balun is incorporated in the module and
facilitates the conversion of the single ended LNA output to a differential signal that is
necessary to feed the input block of the Sledgehammer IC.
1.8.2 Sledgehammer IC (RXPath)
The Sledgehammer IC (U600) contains the frequency synthesizer, down-conversion
mixers, baseband amplification and filtering stages. The main function of the
Sledgehammer IC is to translate the RF input signal in to the two I and Q baseband
differential signals. The signal path has a fixed amount of gain and also contains the DC
offset correction circuitry. The IC contains basic analog anti-aliasing filtering. The IC also
supplies a digital control line to the Janus module that enables the LNA.
1.8.3 Roadrunner IC (RX Path)
The Roadrunner IC (U701) performs the digital conversion, digital filtering, and the AGC
control of the radio. The main function of the IC, from an RX perspective is the Analog to
Digital conversion performed by the Sigma Delta converter. This digitized output signal is
processed by FIR and IIR filtering and then the final output is framed and sent to the
Baseband Processor via the Receive Synchronous Serial Interface Bus. The IC also
controls both the RF step attenuator and the continuous AGC control lines. The RF step
attenuator is enabled via SW and the Roadrunner logic line signals the Janus module to
enable the attenuator when the unit receives a desired signal stronger than -50dBm. The
continuous AGC functionality is controlled by an analog voltage line which feeds the
Janus module. This voltage ranges from approximately 1.3875 V to 2.5 V and increases
linearly for signals greater than ~-60dBm at the antenna input.
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