7
The inverse transposition is carried out in reception and the channels occupy a basic
bandwidth of 4kHz or 8kHz according to whether the system is single or twin channel.
Figure 3shows the reception modulation plan for a twin-channel system. The following
information describes the frequency-conversion process carried out in reception for a
twin-channel system.
The first demodulation is carried out at a frequency generated digitally by the same
process as that indicated in transmission and the second and third demodulations are
achieved at fixed frequencies.
The high-frequency signal that enters the equipment is demodulated by means of a
selected frequency of between 700 kHz and 1160 kHz to obtain, after 652 kHz to 660 kHz
band-pass filtering, the first-demodulation frequency band. The second demodulation is
carried out at a frequency of 672 kHz or 640 kHz according to whether the high-frequency
band is inverted or not. Immediately after, the second-demodulation frequency band is
obtained, with a 16 kHz to 20 kHz band-pass filter for channel 1 and a 12 kHz to 16 kHz
band-pass filter for channel 2. The third demodulation is carried out at a different fixed
frequency in each channel, this being at 20 kHz for channel 1 and at 16 kHz for channel 2,
and, after filtering in the 4 kHz band, the signal is recovered in the base band.
The pilot tone for each channel is obtained from the band of second-demodulation
frequencies, where it is centred at 19850 Hz for channel 1 and at 15850 Hz for channel 2.
After crossing the AGC circuit, this band is demodulated by means of a 24 kHz carrier
frequency for channel 1 and a 20 kHz carrier frequency for channel 2. In both cases, and
by means of a narrow-band filter centred at 4150 Hz, the pilot signal is extracted for later
use.
In the single-channel system there are two different modulation plans in reception,
Figure 4a) and b), depending on the relative positions of the transmission band and the
reception band.
In the case of Figure 4a) the demodulation process is the same as that of channel 1 of a
twin-channel system.
In the case of Figure 4b) the second demodulation is carried out at a frequency of
676 kHz or 636 kHz, instead of 672 kHz or 640 kHz, according to whether the
high-frequency band is inverted or not. From this point on the process is analogous to
that represented in Figure 4a).
The purpose of these two modulation plans in reception for a single-channel system is to
leave the transmission band out of the pass band of the intermediate-frequency filter.
