Mba Board; Agc Board - T.E.M. A07B500S Manual

5.3 MBP board
The central board has the task to distribute the power supplies and the input and output signals;
moreover, the audio filters and the peak-to-peak detector for the different modulation level
measurements are implemented in it.
Both the left and the right channel signals coming from the AUDIO-IN board through the J7
connector, pass through an elliptic filter made of precision active components; the bandwidth at
0.1dB is 15KHz and the attenuation over 19KHz is higher than 40dB; no adjustment is provided,
the resistances have a precision of 0.1% and the capacitors are selected and high quality type. U3,
U4, U5 and U6 make the left channel filter, the right one is symmetrical.
Another elliptic filter of an lower order clean the MULTIPLEX signal by removing the spurious
signals created by the switching over 600KHz; however this is a typical L-C placed between the
two sections of U1. The two further stages formed of U2 make a phase equalizer (RT2) and a
amplitude equalizer (RT3) to compensate the DMPX board errors and the previous filter.
The operational amplifier U12 generates the composite signal by adding all the signals; the output
of the first section can either enter in the automatic gain control optional board or, in its absence, it
enters the U12 second section which acts as a clipper using the saturation and the interdiction of the
operational amplifier output circuit. The threshold value is regulated by RT6, this output of this
stage enters directly into the frequency modulator placed on the SINTD board.
The U17 switch selects the audio signal to be measured which the level is detected by a peak-to-
peak detector made by U13, U14, U15. Through the U18 switch, controlled by the DLCD board,
the measurement can be of peak or envelope, in accordance to the peak measurements or
modulation power.
A circuit made of U22 and U23 disables the output power in case of external command (CE) or
synthesizer fault. This function is performed through software also and this circuit represents a
security guarantee for such an important function.
5.4

AGC board

The task of this board is to guarantee the maximum allowed modulation where is not sure that the
input audio signal has a fixed value. This option can be added to the transmitter at any time and,
when it' s present, the Z3 jumper placed on the MBA board must be set to ON. Its adjustment, when
enabled, is 6dB around the nominal value and it uses 32 gain variation steps of 0.3dB each approx.
The operation is quite simple: a wide band amplifier (U6) has the gain which depends on the R2 -
... - R33 resistive value; these are switched by U2, U3, U4 and U5, they are controlled at their time
by the microcontroller U1. The AGCO output audio signal is detected by U7, U8 and U9 and the
peak-to-peak value is measured by the microcontroller, which consequently decides which gain
must be given to the amplifier.
The intervention time of the gain variations is not constant, but it' s for the input signal value; the
gain variation algorithm versus the time is complex in order not to distort the signal, anyhow it' s
possible to say that, when the signal has a level equal to half of the nominal one, in a couple of
Pag. 37
A07B500S manual