VS 2001
Technical Handbook
Technical Description
Page 36 Marconi Proprietary information
P/N: 779-0373/02
Revision 02
The radio has a single synthesiser. This simultaneously produces the local
oscillators for both RX and TX in such a way that the reception of a RX frequency
can take place at the same time as the transmission of a TX frequency spaced
from the RX frequency by a 10 MHz duplex step. In this way the radio switches
from the reception on a downlink slot to the transmission on an uplink slot on the
same physical TETRA channel (and vice versa) in zero time. Obviously, when the
radio is allocated by the network to a physical channel of another TETRA radio
channel, the radio requires a switching time (typically less than 7 ms in a 10 MHz
band) in order to switch the radio to the new pair of uplink and downlink
frequencies. Note that the radio, not having a duplexer nor the necessary
insulation between RX and TX, cannot receive and transmit simultaneously and
thus remains of the half-duplex frequency type.
2.3.2.1
Functional description
The block diagram of the R/T card is shown in Fig. 2.9. The description of the
individual blocks that made up this card, is given below.
The received signal enters the R/T card through the J13 connector and than it is
sent to the front-end stage.
The front-end stage complies with the normal criteria for conventional tuneable
receivers. Frequency selectivity is provided by a tuneable π LC cell that is placed
before the active element (BJT) and by another two analogous cells after the same
BJT.
These three cells, which use Varicaps, are tuned through VT1, VT2 and VT3
voltage signals generated by a series of 12 bit DACs.
The total gain is G = 11 ± 1 dB in the centre of the band and its variation within the
band is ± 1.5 dB.
The front-end output is connected to a mixer together with the signal coming from
the VCO (SD3) local oscillator to carry out the first IF (Intermediate Frequency)
conversion at Fif1 (see Tab. 2.4).
Note that in reception a 3-conversion scheme has been adopted. The SD3 local
oscillator, set at Flo1 (see Tab. 2.4), is a VCO (Voltage Controlled Oscillator) is
controlled by the Dual PLL SA7025 (see Fig. 2.9). The signal supplied by the VCO
SD3 is obtained from the Temperature Compensated Oscillator (TCXO) SD1 (see
Fig. 2.9).
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