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3-44
Advanced digital modulation mode
Advanced digital modulation
In the advanced digital mode of operation the instrument can generate TETRA specific modulation
from either an internal or external data source. The RF level can be varied by applying an
envelope control voltage. Modulation errors and fading effects can be simulated. Provision is
also made for using an external mixer to generate a modulated carrier at a higher frequency than
can be provided internally by the instrument.
To select the advanced digital control function press the
[Advanced Digital]
key on the
Modulation Mode Selection Menu
(see Fig. 3-6), followed by [SIG GEN]. This causes the
Sig
Gen
menu shown in Fig. 3-44 to be displayed.
Fig. 3-44 Sig Gen menu in advanced digital modulation mode
Setting the output carrier
In advanced digital modulation mode the carrier output frequency and level are set by pressing the
[Carrier Freq.]
and
[RF Level]
keys respectively and entering the values in the normal way.
Self-calibration
To achieve the high precision of the complex modulation the 2050T series Signal Generators have
an IQ self-calibration feature which automatically adjusts the modulator for optimum performance
minimising vector errors. The instrument generates a warning message at the top of the screen
when an IQ self-calibration is advisable. This will occur 30 minutes after power-on and thereafter
at 3 hour intervals.
To perform a self-calibration press
[IQ Selfcal]
. The calibration process is entirely automatic and
during operation the message
IQ MODULATOR CALIBRATION IN PROGRESS PLEASE WAIT ...
appears. Calibration takes approximately 45 seconds and normally remains valid for 3 hours.
Modulation type and system selection
In advanced digital modulation mode the modulation system is fixed as TETRA. This is
π/
4DQPSK modulation, 18 ksymbols/s rate, root nyquist filter with
α
of 0.35.