DE
DANIELS
ELECTRONICS
2-2
VT-4R150 VHF Project 25 Digital Transmitter Instruction Manual
2.3
High Speed Serial Interconnect
Fully compliant Project 25 operation will be supported by the transmitter via a firmware upgrade.
At that time, IC U4 will be enabled. It provides a high speed bi-direction LVDS (Low Voltage
Differential Signaling) port on front panel jack J1. Each signal path uses a balanced current loop to
achieve high serial bit rates with low noise. These signaling paths will provide a means for digital
and analog RF signals to be passed from receiver to transmitter for repeating.
2.4
Audio Circuits
Op amp U3a serves a mixer for audio signals from the microphone connector MIC1, unbalanced
audio1 input, and the “kerchunk” audio signal generated by the Microprocessor Board. Level
adjustment is provided by potentiometer R11. Separate control of the microphone input level and
DC biasing is provided by potentiometer R9.
2.5
PTT Circuitry
The transmitter may be keyed by either grounding the PTT* input at P1-B10 & Z10, by moving
the front panel power switch to the “KEY TX” position, or by pulling the ANALOG_COR* or
DIGITAL_COR* inputs low. These inputs are monitored by microprocessor U6 and used in
conjunction with the position of the front panel “MIC MODE” switch to generate a sequence of
signals to the Digital Signal Processor Board which will cause it to key up in either P25 Digital or
Analog mode.
2.6
Microprocessor Board
The microprocessor board contains U6, a 68HC11E2 microcontroller. U6 is responsible for
monitoring all sources of Push-To-Talk information, the Bank Select Input, and the Mic Mode
front panel switch. It generates bank select and PTT signals to the Digital Signal Processor Board,
the “kerchunk” audio signal, and drives the front panel LEDs to indicate that the transmitter is
active in either P25 Digital or Analog mode. U6 also monitors the state of the Hang Time select
jumpers to determine whether the transmitter should remain keyed after a source of PTT
disappears, and whether a “kerchunk” noise should be inserted at the end of the transmission.
2.7
Channel and Bank Selection
Four channel select lines CSEL0-3 are named on the M-3 motherboard, and are brought into the
receiver module by pins on connector P1, allowing selection of 16 different channels. These
signals normally float high (+5V) in the receiver, but are typically pulled low by jumpers on the M-
3 motherboard to select channel 1 by default. In addition, a bank select input is provided to switch
between Bank A and B, each of which has 16 channels. In the Interim mode of operation, the
bank select input has no effect. A convention has been adopted to use Bank A channels as analog
channels, and Bank B channels as P25 Digital channels. Using this convention, it is possible to
have an MT-4 Interim Receiver connected to the transmitter to form a repeater that repeats an
incoming voice signal in the same mode in which it is received. The LEDs on the transmitter front
panel will correctly indicate Analog or Digital operation as long as this convention is followed. In
addition, the Mic Mode switch on the front panel will operate correctly, selecting Bank A for
analog channels, and Bank B for digital channels, as long as this convention is followed.