FM Transmitter
Sistemas Electrónicos S.A EM 100 DIG
Technical Manual - v1.2 - November 2005 16
connected to pin 3 (+) and pin 2 (-). The cable shield, connected to the ground of the driving
equipment,has to be connected to pin 1.
In case of unbalanced drive, input pin 2 shall be short-circuited with ground and shield on pin 1,
while the signal shall be available on pin 3. Higher impedance selection, in this case, will be 5k:
instead of 10k:.
With balanced driving signals,the connecting cables to the audio source may be well more than
100-m long.
MPX or an externally processed signal, usually an unbalanced signal, can be fed to the female BNC
connector, marked <MPX>, which is internally parallel-wired with the <RIGHT> channel
connector:for this reason it is not possible to connect signals to these two connectors at the same
time. Higher impedance position is 5k: in this case too.
Connect this input with a 50-: (RG58) cable for a short distance; if the distance exceeds several tens
of meters, use 75-: (RG59) or 92-: (RG62) coaxial cables.
The auxiliary-channel connector is also of the grounded BNC female type. Use 50: (RG58)or 75:
(RG59) cables to connect to the driver. The same applies to the monitor "MODULATION" output, if
needed.
2.3.2 Pre-emphasis setting.
Non-precoded low frequency mono and stereo channel signals have to be adequately pre-
emphasized. Standard preemphasis time constant is 50 and 75 µs, the former being usually factory
pre-set for Europe countries.
Check whether this is correct for your country (it is usually correct for any European country and part
of the Pacific areas). It is not correct for USA and Center & South America standards,which require 75
µs.
If above correction is needed,simply set it on the <MODE> frame of the transmitter menu, which
also includes mono/stereo operation and frequency. See appropriate section further on in this
Manual.
2.3.3 Audio Baseband Input Level Range, Setting and
Requirements.
In the following paragraph we will refer to 0dBm as the audio signal which produce 1mW on 600:,
i.e. a 775mVrms / 2200mVpp sinusoidal. Irrespective of the impedance, we will continue to assume 0
dBm as an audio signal whose peak is + (or -) 1100mV.
In the same way, when talking of the modulation, we will assume as 0dB the signal which produces
100% maximum allowed modulation, i.e . 75kHz deviation.
There is no absolute worldwide standard regarding audio peak level as modulation signal for a
transmitter, nor for the mean deviation. Many Broadcasters use 0 or +6dBm as LF peak level for
100%modulation, USA often uses +4 or +10dBm.
Many European countries specify +6dBm for 40kHz deviation (which is assumed to be a "mean"
modulation). This allows for 5.5dB headroom to max. 75kHz deviation, i.e.+11.5dBm for
100%modulation.
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