User's Manual

Doc. No.
Rev.
Date
1.00
2011-12-08
Damm Cellular Systems A/S, Denmark
TETRAFLEX
®
V7.5 MANUAL - TR412 Transceiver Description
TetraFlex® 7.5 Manual 4-18
4.4 TR412 TRANSCEIVER DESCRIPTION
4.4.1 Introduction
The TR412 Transceiver is a complete Tetra Carrier Unit containing a dual diversity receiver,
a highly linear transmitter and all the necessary computer power to handle the lower
protocols and control functions. The TR412 is an upgraded plug-in replacement for the
TR411.
The TR412 is able to run different modulation and protocol types depending on the actual
software running on the DSP and controller. The standard TETRA version also support
analogue PM modulation for testing purposes, to allow complete testing of a site with
analogue test equipment.
4.4.2 Receiver
The receiver contains two completely independent signal paths for dual diversity reception as
standard.
Each RX signal comes from the Receiver Multi-coupler system into the TR412 via the TNC
connector on the front plate, and is amplified with about 17dB in the RF amplifier, followed of
a 3-pole BPF.
The signal now enters 1
st
mixer, where it is converted down to 1
st
IF of 45MHz. 1
st
mixer is a
high-level double balanced mixer running with an injection level of +17dBm to obtain a high
dynamic range.
The 1
st
mixer is followed of a 4-pole 45MHz crystal filter, an IF amplifier with about 30dB gain
and a second 2-pole crystal filter. The crystal filters are of linear phase type to avoid distortion
of the digital modulation.
From the output of the second crystal filter the signal enters 2
nd
mixer, where it is converted
down to 2
nd
IF of 144kHz. The signal is then amplified and balanced with op-amps before it
goes from the RX Front End Board to the Main Board, where it is converted to digital.
The ADC samples the 2
nd
IF signal of 144kHz directly with a sampling frequency of 576kHz
and a resolution of 16bit. The digital signal is going to the DSP on a serial interface.
In the DSP the signal is first down-converted to base-band and is then passing the channel
definition filter, which determines the actual bandwidth of the receiver. In TETRA mode the
filter is the root-raised cosine filter.
Afterwards frame synchronization, diversity combination and demodulation takes place.
The Local Oscillators for the RX is generated on the RX FS board, locked to a 12.8MHz
reference coming from the Main Board.
1
st
LO is a Voltage Controlled Oscillator running either 45MHz below or above the actual
receiver frequency. The VCO is phase locked to the 12.8MHz reference with a fractional-N