ANALOG MODULATION MODE
3-54
Modulation ALC
The automatic levelling control (ALC) is used in conjunction with an external source and can be
disabled when not required. To enable the ALC, proceed as follows:
(1) At the
Sig Gen
menu, press
[Select Source]
. The display will show the
Internal
or
External
Source Selection Menu
(Fig. 3-51 or Fig. 3-53).
(2) If necessary press
[Select External]
to obtain the
External Source Selection Menu
(Fig. 3-53).
(3) Select the required external source from the options shown, e.g.
[Ext 1 ALC Coupling]
or
[Ext
2 ALC Coupling]
. The pictogram at the end of each line will change from a horizontal line to
an arbitrary waveform symbol when the source is selected.
(4) Return to the
Sig Gen
menu by pressing [SIG GEN]. The legend
Ext Mod 1 (
or
2) ALC
coupled
appears at the bottom of the display.
(5) Apply a signal to the EXT MOD 1 or EXT MOD 2 input socket and vary the level. If the
input applied to the external modulation socket is outside the ALC range (at least 0.7 to 1.4 V
RMS)
HI
LO
will be indicated and an error message will be displayed at the top of the screen.
If the level is within the required range, the arbitrary waveform symbol will appear alongside
the modulation value.
Pulse modulation
(applies if Option 002 is fitted)
Enabling the pulse modulation disables the RF ALC system which is used to control the output
level from the generator. The signal generator sets the requested RF output level using a digitally
derived control signal whose level is equivalent to that which would be generated by the RF ALC
system.
The calibration of the RF level can be set up to work in two possible ways. The normal method is
that when a new carrier frequency or RF level is entered the RF ALC system is enabled and a CW
signal is generated. The generator then sets up the digitally derived control signal so that it is at
the same level as the RF ALC control signal and the instrument disables the RF ALC and
subsititutes the digitally derived signal. The user of the signal generator will observe that when
the level or frequency of the generator is changed a CW output burst (at the requested output
level) is generated for up to 100 ms.
In some applications, such as live radar testing, the CW burst can cause problems. The alternative
mode of operation is to enable a CW Burst Suppression facility. In this mode when pulse
modulation is first enabled, a calibration of the digitally derived control signal is performed
automatically which generates a tabulation of the RF level against the control signal level. During
calibration the RF output is suppressed by the output attenuator. After calibration changes in level
or frequency do not generate CW bursts. An RF level recalibration can be performed on demand.
When the carrier frequency is changed by more than 15 MHz from the frequency at which the
level was calibrated, or the new requested frequency results in a major change of characteristic
(such as a VCO change), the RF level display is blanked and an UNCAL annunciator is displayed.
Initiating an RF level calibration will restore the RF level display.
Selecting the pulse modulation mode
(1) Press the
[RF Level Utility]
on the
Utilities Selection Menu 2
. The display will show the
RF
Level Utility Menu
shown in Fig. 3-76.
(2) Use the
[CW Burst Control]
key to disable the Burst Suppression mode for normal operation
or enable it for Burst Suppression mode.
(3) Selecting the [SIG GEN] key will return the display to the
Sig Gen
menu.