T.E.M. A07B500S, Manual

Page: 35 / 131

A07B500S manual
Pag. 35
CIRCUITS DESCRIPTION
5.1 AUDIO-IN board
The AUDIO-IN board has the task to interface the input audio signals with the modulator. Level
adjustments are performed on them, as well as pre-emphasis insertion and input impedance
selection. The outputs, going through a flat-cable to the mother board, are raised to a high level and
made balanced in order not to be interfered with the transformer flow dispersion.
The LEFT and RIGHT signals available on the connectors placed on the rear panel enter,
after a first RF noise filter, respectively into U6 and U1. By the U11 switch and the Z1 jumper
accessible at the back, the input impedance can be selected (600
?
or 10k
?
). A similar function is
performed by the jumpers Z3, Z4, Z5, Z6, which allow to select the input nominal value level; on
the two channels MONO examined, the switches U13 and U12 change the gain by switching three
resistances or a trimmer to put the input level to 0, 4.1, 6dBm or by RT3 and RT4 to a level between
–6 and +12dBm. The signal is then the pre-emphatized; the value 50µs or 75µs is chosen by the
jumper Z2, while the possible inserting is controlled by the front keyboard. Through U3 and U4 the
LEFT and RIGHT channels output is made differential.
The MULTIPLEX external signal path is simpler. On it, it’s only adjusted the level at the
nominal value by U16, still controlled by Z3, Z4, Z5, Z6. U9 adds up the MPX signal with the two
SCA signals and generates the balanced output signal.
Normally, on the SCA signals it’s difficult to establish an input nominal level since their
contribution to the frequency deviation is variable and depends both on the number of subcarriers
between 53 and 100KHz and on the difference about MONO or STEREO transmission. In any case,
the total deviation of all the subcarriers (19KHz, SCA1, SCA2) must not exceed 10% of the
maximum nominal deviation, which in most cases is ±75KHz. If the transmitter is monophonic and
only the RDS signal placed in one of the two SCA inputs is present, the deviation level of the
transmitted data can reach ±7,5KHz; whereas if the transmitter is stereophonic and besides the RDS
signal also a lower quality audio channel on a subcarrier is present, for example at 76KHz, the total
of each subcarrier deviations can’t exceed ±7,5KHz. The stereo driving carrier will deviate ±4KHz,
the RDS signal and the other audio channel will have to deviate, for example, ±1,75KHz.
For this reason, it has been preferred to make the SCA channel levels independent between the
nominal input one of the audio channels. The adjustment is obtained by RT1 and RT2 trimmers
always placed on the rear panel.
All the set levels are showed in the display and the choice to adopt a parameters manual setting
related to the input signals level has been preferred to an easier keyboard setting to avoid a non
standard levels setting which makes the servicing or the transmitter replacement problematical. The
audio signal level errors must not be cleared on the transmitter, but at a former stage. Normally,
every broadcasting station fixes a nominal level for all signals and all the adopted equipment must
respect this sole value. As higher is this value, as higher will be the noise immunity, and the signal-
noise ratio as well.