T.E.M. A07B500S Manual page 13

60–70dB of attenuation, even with 0.1dB of amplitude linearity up to 15KHz, unavoidably
creates a
phase distortion on the analogue signal that an experienced ear can perceive. Right or left signals
can be combined to generate monophonic transmission (should you only have one of the two
signals, it will be necessary to externally put in parallel the two inputs); in case of stereophonic
transmission, the two channels are fed inside the stereo code board.
The mono signal or the stereo one, thus obtained, is combined with the other three possible
input audio signals: an external Multiplex signal and two SCA signals, one of which can be the
RDS one, which can be synchronized with a 19KHz one connected on the IN/OUT rear connector.
The composite signal can enter the AGC board, which has the task to check the its amplitude
and consequently to keep the modulation at the correct value, or it can follow its path and enter into
the frequency modulator after having passed through a limiter circuit (CLIPPER). This circuit must
became active just in cases of faults of previous circuits or in case of mistake in the setting of the
low frequency input nominal levels; this is to avoid to interfere with the adjacent channels. For not
activating this "fuse", which produces remarkable distortions on the modulation, it is necessary to
take all proper cautions, that is the use of external compressor-limiters or by inserting the internal
AGC circuit which protects a lot against damages and drifts.
The oscillator, directly modulated by the composite signal, covers the whole FM band and
it' s synthesized in steps of 10KHz . The reference frequency is obtained by a 10MHz crystal ,whilst
the output frequency is set by the main microcontroller. The oscillator phase noise is very good and
it is in compliance with ETSI 300-384 regulations (<-145dBc for a shifting of 1MHz from the
carrier). The modulation linearity is typically contained within 0.1dB without complicated
corrections.
The RF final power circuit is wide band and it provides 500W RF output controlled with
high accuracy; directional coupler has a directivity higher than 25dB on the whole band and an error
which is lower than 0.2dB, it is also compensated in temperature and totally shielded.
The power supply is of a switching type and it gives the four essential voltages, all obtained
with this technique. The mechanical position of the power supply and the final circuits of RF power
allow to obtain a vent flu just for cooling of the involved circuits, obtaining this way a really
remarkable efficiency of that function. In normal running conditions, when the transmitter is
working in a full power at environmental temperature, the radiator temperature is lower than 50°C,
whereas the other circuits temperature does not exceed 30°C. No components are involved with the
air flow, so it isn' t requested a filter on the aspiration fan, which replacing is rather simple. The
power supply is completely shielded both for internal circuits and for its unavoidable emissions
toward the outside.
Data displaying and setting is obtained by a board which is placed directly on the front panel
containing a microprocessor, memory, keyboard, LCD display. The displaying area is wide so
allowing to display and set needed data in a very easy way, thus making the transmitter-user
technician interaction extremely user friendly.
It' s possible to protect the transmitter input and output parameter settings with a password,
while all measurements can be done by whoever without interfering on its operation.
Pag. 13
A07B500S manual