T.E.M. A07B500S, Руководство пользователя

Страница: 54 / 131

A07B500S manual
Pag. 54
calculated and consequently the K factor. It can be noticed that K value constantly keeps each
minute below the
value 0.01 .
So assume this value as limit for the over modulation factor.
Increasing the modulation of 1dB the graphs in fig. 5.f are given, corresponding to a transmitter
which deviates little more than ±8KHz; in this case K value is 0.11. Thus it can be noticed that for
small modulation values higher than ±75KHz, K increases considerably.
The rules rightly have tried to fix some limits for the instruments accuracy which need to perform
this kind of measurements, but the system weakness is surely constituted by the receiver, with all its
problems concerning the answer to quick transitory and also the peaks, which is almost ever
distorted by the medium and low frequency filters group delaying with over elongations or
miscompensated attenuation.
So it would be ideal to draw the modulating signal, which is usually available on all the
transmitters, performing all the measurements on it, after having made sure of the exact relation
between the audio level and the frequency deviation. In the A07A50 this is automatic and we think
if a modulation peak analysis measurement made far from the transmitter, has produced doubtful
and questionable results, it must be repeated by the instrument inside the transmitter like in the
A07A50.
The measurement must be started for a whole day observation period so to pick up the programs
having most over modulation problems and consequently to act on the dynamic limiter–compressor
every study must have. With this measurement method help it’s possible to set best the limiter-
compressor no longer by ear but by real data and no more subjective elements.
7.3 Modulation power measurement
Another important parameter determining the interference intensity on the adjacent channel is the
modulation power value. The term is not of common use and the idea that the modulation power
can influence the interferences is not easy to understand.
Reading the CEPT 54-01 rule it’s noticed that the transmitter modulation power in object must not
exceed the samples reference signal one, represented by a sinusoidal signal which deviates 19KHz
of peak.
The 19KHz value has no relation with the stereophonic subcarrier value but it’s the
frequency deviation which the sample signal creates on the transmitter. On the tuned receiver this
signal will be carried to the loudspeaker with a certain voltage directly proportional to the deviation
value; then there will be a certain electric power on the loudspeaker equal to the effectual voltage
square about divided into the loudspeaker impedance; it, at less of the diffuser efficiency, coincides
to the acoustic power. So it’s possible to believe the modulation power as the equivalent of the
acoustic power spread by the loudspeaker, and perceived by our ears.
Thus as for the electric power, the equivalent mathematical rules are valid for the modulation power
also. In the first case the value depends on the voltage square, in the second one on the deviation
square.