Codan Radio MT-4E Analog and P25 Digital - User Guide - Manual (Page 30)

USER GUIDE | MT-4E ANALOG & P25 DIGITAL RADIO SYSTEMS
Chapter 4: MT-4E Radio System Block Diagrams Page 22
BASE STATION BLOCK DIAGRAM
Figure 4-2 shows a block diagram of an MT-4E analog controlled base station system using a CI-BC-4E
base controller.
The incoming FM or C4FM transmission is routed through the RF Preselector where the RF signal:
• is fi ltered with a high selectivity multiple pole, helical resonator or ceramic fi lter
• is amplifi ed through a low noise amplifi er
• is filtered again by a low pass, high pass or band pass filter
• is mixed, with a local oscillator supplied by the synthesizer
• produces an IF frequency of 21.4 MHz (136 - 520 MHz) or 73.35 MHz (768 - 960 MHz) that is output
to the Receiver Mainboard.
The Receiver Mainboard processes the low level 21.4 MHz or 73.35 MHz IF signal from the RF
Preselector through selective crystal fi ltering and IF amplifi cation. The signal is then passed through the
IQ Demodulator and Digitizing Stage for demodulation to two quadrature-related baseband outputs.
These outputs are represented in a digital stream which is passed to the UDB Board, where DSP
techniques are used to further process the incoming sampled signal to detect and extract P25 digital
voice signals and analog voice signals and is then routed through either the analog or P25 digital
settings programmed into the receiver.
Analog signal settings include:
• CTCSS or DCS decoding
• de-emphasis or fl at audio selection
• wide or narrowband selection
P25 digital settings include NAC and/or TGID decoding.
The digitally sampled signal is then converted back to baseband and then routed out of the receiver as
analog audio out the subrack / motherboard to the base controller. If a P25 digital signal is sent out as
baseband analog audio, the signal must fi rst be de-vocoded before it can be converted to baseband
analog audio. MT-4E receivers may have an optional AES / DES-OFB decryptor module.
The analog audio is then routed through the base controller (CI-BC-4E Base Controller) and is sent
to the auxiliary audio output for connection to an external device such as a tone remote adapter. The
auxiliary audio input from the external device is routed through the base controller and is sent to the
transmitter. Internal audio levels between the receiver, base controller and transmitter are typically set
at -8.0 dBm for 60% of maximum modulation. Independent audio level controls in the base controller
allow the auxiliary input and output levels to be adjusted separately (default is 0 dBm for 60% maximum
modulation). For external control, the CI-BC-4E also has optically isolated COR and PTT inputs and
outputs for connecting external equipment with high voltage control signals such as E&M.
The transmitter will accept the incoming analog audio, convert it to a digitally sampled signal, and
then route it through the analog or P25 digital settings programmed into the transmitter. Analog signal
settings include:
• CTCSS or DCS encoding
• pre-emphasis or fl at audio selection
• wide or narrowband selection
P25 digital settings include NAC, TGID and Unit / Source ID encoding. MT-4E transmitters may have
an optional AES / DES-OFB encryptor module. The signal is then converted back to a baseband signal
and is then modulated as either an FM or C4FM transmission based on the analog or digital software
settings, or the front panel switch. The modulated carrier is then amplifi ed by the RF Power Amplifi er
sub-module.