User Manual

DE
DANIELS
ELECTRONICS
VT-4R150 VHF Project 25 Digital Transmitter Instruction Manual 2-1
2 THEORY OF OPERATION
2.1 General
An VT-4R150 Transmitter is constructed using two primary modules: the MT-4 Digital Transmitter
Main Board and the VT-4 Amplifier Board. The Main Board supports two plug in modules: the
Digital Signal Processor Board and the Transceiver RF Board. The Digital Signal Processor
Board also supports another module, the Microprocessor Board.
The MT-4 Transmitter Main Board accepts a baseband audio signal, performs some level
adjustment, and applies it to the Digital Signal Processor Board. The microprocessor on the
Digital Signal Processor Board samples the audio signal and converts the digitized audio to a
modulation signal. When the transmitter is required to produce an analog FM modulated carrier,
pre-emphasis and limiting functions are applied using DSP techniques, and the resulting
modulation signal is converted back to baseband and applied to the VCO on the Transceiver RF
Board, thus producing an analog modulated carrier. When P25 Digital transmit mode is required,
the sampled audio is passed through a vocoder process in the DSP, which compresses the voice
signal, and adds error correction information to produce a 9600 bps data stream. The data stream
is applied to a quadrature modulator in the DSP and the resulting modulation data is converted to
an analog waveform which is applied to the VCO on the Transceiver RF Board, thus producing a
digitally modulated carrier.
The Microprocessor board provides an interface between the Digital Signal Processor Board and
the user controls such as PTT inputs, front panel switches, and hang timer selection jumpers. It is
responsible for generating the optional “kerchunk” noise which can be added to the audio at the
end of a transmission to simulate the burst of noise heard when a traditiona l analog receiver
squelches. This can be a useful aid to repeater users, who are used to listening for the noise as an
indicator that they’ve successfully keyed up the repeater.
The VT-4 Amplifier provides the final stage of RF amplification and filterin g for the MT-4
Transmitter. The amplifier has inputs for the RF signal from the Transceiver RF Board, and for
DC power. It has outputs for the amplified RF signal and for forward and reverse power alarms.
Two power options are available, with each power option being available in two frequency bands.
Hardware support for fully compliant P25 operation is provided in the form of a high speed serial
I/O connection at the front panel RJ-45 connector. Future firmware upgrades will allow the
transmitter to accept data and voice signals in digital form from a MT-4 P25 Compliant Receiver
for fully digital repeat capability.
2.2 Power Supply
Regulated +9.5VDC enters the transmitter module from the M-3 subrack at P1-B6 & Z6. It is
distributed through diodes D3 and D4 to the Transceiver RF Board, and to voltage regulator s U1
and U2. U1 produces a clean +8Vdc supply for the analog audio circuitry. U2 produces a
regulated +5Vdc supply to drive the high speed serial LVDS serial driver and the microprocessor
board.