DB
ELETTRONICA TELECOMUNICAZIONI S.p.A.
PM 1000 – 1 kW FM Broadcasting Transmitter - U
SER
’
S
M
ANUAL
45 /101
6.
MODULATOR STAGE
6.1
GENERAL DESCRIPTION
The new modulator board SINTEL ’96 allows a high quality direct carrier modulation accepting a
composite stereo (MPX) or a mono input signal with up to 3 SCA subcarriers (for example RDS,
ARI, ...).
The mono input signal is filtered by a 7
th
order elliptic filter with a very flat response (20Hz to
15kHz with
±
0.25 dB) and an optimum out of band attenuation (>50 dB for frequencies higher than
19 kHz), then the signal is amplified (or attenuated) and passed to the VCO oscillator for the FM
carrier modulation. The VCO is controlled by a digital PLL circuit with a high stability quartz
reference to obtain a very stable synthesised oscillation: the frequency setting is customisable using
on board dip-switches (the minimum step size is 10 kHz).
The composite stereo signal and the SCA subcarriers are just amplified or attenuated by the correct
modulation depth adjustment before going through the VCO stage: the stereo performances of
SINTEL ’96 are excellent, (stereo separation >60 dB from 20Hz to 15 kHz, SNR > 80 dB).
Before the VCO stage, an adjustable and bypassable deviation limiter prevents the FM carrier from
an excessive modulation depth and adjacent channel interference according to all the international
standards (CCIR, FCC).
Furthermore, an automatic locking circuit enables the output modulated signal of SINTEL ’96 only
when the internal PLL is locked properly, in order to avoid unwanted emissions.
Summary of Contents for PM 1000
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