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Analog Modulations
SMIQ
1125.5555.03
E-8
2.62
2.7.3.1
FM Deviation Limits
The maximal deviation depends on the RF frequency set (cf. Fig. 2-29). It is possible to enter a
deviation that is too high for a certain RF frequency or to vary the RF frequency to a range in which the
deviation can no longer be set. In this case the maximally possible deviation is set and an error
message is displayed.
In the RF range 450 MHz to 750 MHz and 1200 MHz to 1500 MHz a different synthesis range is
selected depending on the deviation set. If the deviation is smaller than 500 kHz or 1000 kHz, the
synthesizer is in the normal mode with optimal spectral purity. If the deviation set is larger the I/Q mode
is automatically selected.
2000
1000
500
0,3 450 750 1200 1500 3300
I/Q mode
maximal
FM-deviation
[kHz]
RF frequency [MHz]
normal mode
Fig. 2-29
Dependency of the FM maximal deviation on the RF frequency set
2.7.3.2 Preemphasis
Preemphasis results in a preemphasis of the modulation signal with time constants 50 µs or 75 µs. The
higher frequencies of the modulation signal are preemphasized.
When preemphasis is switched on, only 1/4 of the maximal deviation is permissible. The highest
permissible modulation frequency is 15 kHz. Exceeding the permissible modulation frequency can lead
to overmodulation."