OVERVIEW: Transmitter Path Section
The JANUS transmitter module includes the following:
• Center tapped RF balun for RFPA differential input
• High Power Linear RFPA
• RFPA Load line switch selection mode.
• Reduced quiescent current selection mode.
• Directional coupler.
• Low pass harmonic filter.
The DSP performs VSELP data compression and generates digital I & Q words to be
transmitted to the ROADRUNNER. The signal then is sent at a rate of 48 K samples per
second. The ROADRUNNER provides the serial clock to the DSP and a frame sync pulse
to tell the DSP to send a sample. Each sample is sent as a 16-bit I word followed by a
16-bit Q word and then some meaningless fill bits.
The I word and the Q word are then converted to an analog differential pair by the
ROADRUNNER and amplified. The ROADRUNNER also sends a 4.2 MHz reference
clock signal to the SLEDGEHAMMER, together when a DMCS and TXE signal are
received from the DSP. The TXE transition starts the TX sequence with
SLEDGEHAMMER, which entails the training sequences as well as enabling the antenna
switch and PA.
DMCS signals the start of data being sent from DSP to ROADRUNNER. Once the
transmit burst is finalized, the TX shutdown is done by disabling TXE and DMCS.
1.7.1 ROADRUNNER
The ROADRUNNER IC is designed as a derivative of several other ICs previously used in
iDEN products. The ROADRUNNER IC includes the following blocks:
RX:
• ADC
• Digital Filtering
TX:
• Digital interface (the transmitter receives inputs from the SSI. The SSI has a single
data line).
• Pulse Shaping FIR (A programmable waveform generator "FIR" filter provides
interpolation of the pre shaped input words).
• TX DAC (The transmit DACs have eleven bit resolution and are capable of supporting
8.4 Mbits). Fine step TX attenuation (cutback).
Smoothing filters (programmable RC smoothing filters limit the amount of far out
quantization noise and images due to aliasing. The filters incorporate a dc offset correction
block).
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