FM Transmitter
Sistemas Electrónicos S.A EM 100 DIG
Technical Manual - v1.2 - November 2005 8
Figure 1-1: EM-100 DIG DIGITALLY-CONTROLLED F.M. TRANSMITTER.
As imposed by various national standards, these transmitters incorporate sophisticated low-pass
audio filters on mono and stereo channels,and a sharp acting modulation limiter, which is usually set
at a peak deviation slightly higher than 75kHz. Its intervention may nevertheless be avoided,if
required,pre-setting its threshold at a deviation higher than 150kHz. Output frequency is phase-locked
to a temperature-compensated crystal oscillator,which ensures superior precision and stability. A
very low noise,low distortion VCO produces a harmonic-free,spurious-free signal. A lock control circuit
inhibits the presence of power on the output until the apparatus is on the right frequency,when
turning on.
To reach a further lowering of noise threshold, the low-frequency inputs are fitted with balanced input
circuitry.The input level is precisely adjustable over a broad range,by means of a 0.5dB stepwise
variable attenuators.The transmitter has an auxiliary input,specifically designed for RDS and SCA
encoders. A modulation sample output permits to control other transmitters or STL's with the same
internally processed high-quality mpx signal.
The alphanumeric display permits easy and accurate metering,adjustment and continuous
monitoring of modulation levels,power,operation and internal parameters.All these information may
be externally available on the same RS-232 I/O bus that may be used to remotely control the
transmitter.
Above Figure 1-1 shows us the external view of Transmitter's cabinet,whose control panel has been
simplified to a maximum, being Microcontroller in charge of practically to select and make all
tests and adjustments of all parameters that are relevant to the normal Equipment's performance.
Due to this fact, Front Panel has only a few control elements, since with only the four push buttons
pertaining to Microcontroller (like those used to control movement of cursor in a Computer) and two
keys, these virtually controlling all processes & parameters having place within Equipment.
Microcontroller uses a Liquid-Crystal dot matrix as Alphanumeric Display unit in order to watch
parameter's values, as it's asked for it, as can be seen in the corresponding Chapter in this Manual.
In addition to the serial I/O port, some signals (RF power, ON THE AIR status, Disable line) are available
on a parallel I/O remote socket for easy interfacing with others analog controllers or supervisory
systems. A top-quality stereo encoder may be factory installed as option and even retrofitted in the
field in a second time, requiring minimum technical skill. The powerful internal software and
monitoring functions recognizes its presence and enables its functions.
The RF power amplifier employs a broadband design and has a lot of of reserve: the output power is
feedback-controlled for increased stability still higher than nominal level. High reflected power is
limited to prevent output stage degradation;direct power is accordingly continuously reduced so as
not to exceed the reflected power safety level. A sturdy telecom-grade high efficiency switch-mode
power supply allows operation in a very wide and noisy mains environment.
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