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 4-19
PLL circuit, controlled through a serial interface from the Micro-Controller. The VCO signal is
amplified to about +23dBm and brought to the RX Front End via the attached cable. Here it is
first filtered in a 3-pole ceramic filter before it is split into two +17dBm injection signals for the
two 1
st
mixers.
2
nd
LO is a Voltage Controlled X-tal Oscillator running on 44.856MHz and this is also phase-
locked to the 12.8MHz reference. The signal is amplifier to about +13dBm and brought in
another cable to the RX Front End, where it is split into two +7dBm injection signals for the
two 2
nd
mixers.
4.4.3 Transmitter
The transmitter of the TR412 is build around a Cartesian loop to meet the demanding TETRA
linearity requirements. The TX signal is generated directly on the transmit frequency to
optimize spurious and wide-band noise performance. The transmitter is able to deliver
60Wpep at the output connector of the TR412, equal to 25W TETRA.
The carrier frequency is generated of a Voltage Controlled Oscillator on the actual TX
frequency. The VCO is phase locked to the 12.8MHz reference coming from the Main Board
with a fractional-N PLL controlled through a serial interface from the MC. The VCO is running
independent of whether the TX is keyed.
The output signal from the VCO is amplified to about +23dBm and split into two signals, a
+11dBm signal for the I/Q modulator in the forward leg and a +21dBm signal for the I/Q de-
modulator in the feedback leg.
The I/Q modulation signals are generated directly of the DSP and send on a serial interface
to the dual 16-bit DAC with associated low pass filters. The I/Q signals acts as reference
signals for the Cartesian feed-back loop.
After the comparator comes a variable phase circuitry to adjust for the Cartesian loop phase
and the loop filter before the modulation is applied to the I/Q modulator. A linear amplifier
increases the output level from the Exciter to +14dBm pep.
The Power Amplifier consists of one LDMOS driver and one output transistor, delivering 60W
pep output power with good linearity. An attenuator in the input is used to fine adjust the total
gain of the Power Amplifier. Both transistors are running in class AB for an optimum
compromise between linearity and efficiency and are supplied with +26V DC.
After the PA four directional couplers are provided. A forward and a reverse coupler are used
for the Power Meter to allow the display of forward and reflected power. One directional
coupler is used to sample the output signal for the RFTL switch, and the last directional
coupler is used to take a sample of the output signal for the Cartesian loop feedback.
The feedback signal is brought back to the Exciter in a thin cable. After passing an attenuator
to adjust the level it enters the I/Q demodulator, where the signal is converted back to base-
band. After the base-band feedback amplifier the signal is compared with the I/Q modulation
reference signal from the two DAC.