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3-16
Digital modulation mode
Digital modulation
In the digital mode of operation the instrument can generate user-defined modulation formats from
either an internal or external data source. Modulation can be applied in FSK, GMSK, PSK and
QAM formats with specified channel filter characteristics. When defined these parameters may be
stored and subsequently recalled. 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 Digital control function press the
[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. 3-8 to
be displayed.
RF
Level
Carrier
Freq.
LOCAL
Data /
Timing
Mod'n
System
Config.
Select
IQ
Selfcal
Mod'n
Mode
Fading
Control
IQ
Errors
Carrier
:
Freq.
RF Level :
CONFIGURATION:
Sideband : LOWER
IF
: 132 MHz
Envelope : DISABLED
Fading
: DISABLED
IQ Errors: DISABLED
Type :
/4 DQPSK
Rate : 18.0000 kHz
Filter : Root Nyquist
Alpha: 0.35
π
DATA:
Internal PRBS (n=15)
dBm
Digital Mod'n Mode
Modulation ENABLED
MHz
Int Std: 10 MHz
380.000 0000
-138.0
ON
C3198
Fig. 3-8 Sig Gen menu in digital modulation mode
Setting the output carrier
In 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.