FM Transmitter
Sistemas Electrónicos S.A EM 250 COMPACT DIG
Technical Manual - v1.1 - February 2006 8
Fig. 1-1: THE EM-250 COMPACT 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 lower the noise threshold further, 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 Modulator / Exciter'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 and 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 section 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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