R.V.R. Elettronica PTRL NV/3, User Manual

Page: 32 / 56

28 / 52 User Manual Rev. 1.0 - 29/11/05
PTRL NV/3 & RXRL NV/3
The top right portion of the diagram shows the connections and signals at the control
bus connector connected to the CPU. The digital control lines are brought to the
lowermost pins of this connectors, whereas the analogue lines are brought to the
uppermost pins. The power supply lines also depart from this connector; however,
only the +12.5V and -12.5V lines are used in this board and the connected boards.
Let us now take a look at sheet two of the diagram. The lower left corner shows the
digitally controlled three-channel attenuator IC8. It controls the right, left and auxiliary
channels separately. The external multiplex signal is processed in the right channel
and then relayed without filtering and preemphasis. Each channel is monitored by 3
amplifiers IC7a, IC8a and IC12a. The output of the first two amplifiers drives
preemphasis stages; the time constant of preemphasis stages may be digitally set
to 0.50 and 75 µs via the analogue ports of IC9. The limiting stage is made up of
diodes D8 and D9 that act as clippers. Limiter threshold Vl and -Vl can be
set by changing the reference voltage of the limiter driven by the CPU via IC13a.
RT4, if fitted, dictates a maximum value for the limiting threshold.
The signal is sent to the input sections of the stereo coder circuit, if fitted (internally).
At the same time, the signal running along the right channel path is sent to a low-pass
filter which is made up of the circuit section built around IC10 and IC11 that attenuates
any frequencies higher than 15kHz for the mono section.
IC14 selects the non-preemphasized signal coming from the input section via R124
or from the preemphasis and filtering section via R128 or from the stereo coder via
R131. IC12b amplifies the selected signal and mixes it with the signal from the auxiliary
channel. If needed, diode D17 provides further limitation for the final total signal. This
signal is sent to the FM modulator circuit via buffer IC12c and adjusted to the level
required by RT6. A separate section of IC12 amplifies the modulation signal separately
and sends it to the modulation monitor output connector.
IC15 deserialises the commands sent by the CPU to control transmission channels
with IC14 and preemphasis action with IC9. Two output lines of IC15 are used to
drive the "Alarm" and "On the air" remote output lines.
8.1.3 Stereo Coder Module
The coder circuit uses an 8-stage sequential switching technique that ensures
excellent performance coupled with simple circuit design. With this technique, the
first harmonics associated with the switching process are the 7th and 9th harmonics
(266 and 342 kHz); this simplifies the layout of the low-pass filter on the multiplex
signal that eliminates these harmonics.
Above 15 kHz, the signal is filtered by the two precision low-pass filters built around
IC1 ¸ IC4. The signal is then amplified by IC3d and IC4c and applied to the coder
circuit IC8.