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281, 282, 284
Users Manual
Introduction
You can use both internal and external modulation sources. External modulation can be
applied to any or all channels. Internal modulation uses the previous channel as the
modulation source; for example channel 2 can be used to modulate channel 3. Clearly,
internal modulation is not available on channel 1 or on a single channel instrument.
The external modulation mode can be either VCA (voltage controlled amplitude) or SCM
(suppressed carrier modulation). The internal modulation mode can be either true AM
(amplitude modulation) or SCM.
Modulation modes share some of the generator’s inter-channel resources with sum
modes; as a result there are some restrictions on using modulation and sum together but
they are generally outside the range of common-sense applications. To better understand
these constraints the following sections (and chapter 12, Sum) should be read with
reference to the block diagrams in appendix F. These show the control signals in a single
channel and the inter-channel connections.
The block diagrams also show the inter-channel trigger connections described in chapter
7, Triggered Burst and Gate; in general, inter-channel triggering is possible
simultaneously with modulation but few combinations are of real use.
External Modulation
Pressing the MODULATION key calls the MODULATION set-up screen:
MODULATION
source: ext
type: VCA
The source soft-key steps the modulation choice between off, external and
CHx where x is the number of the previous channel; this last choice is not available on
single channel instruments or on channel 1 of multi-channel instruments.
With ext selected the modulation can be switched between VCA and SCM with
alternate presses of the type soft-key. Both types of external modulation can be used
with internal or external sum.
External modulation can be applied to any or all channels.
External VCA
Select VCA with the type soft-key on the MODULATION screen. Connect the
modulating signal to the EXT MODULATION socket (nominally 1 kΩ input
impedance). A positive voltage increases the channel output amplitude and a negative
voltage decreases the amplitude. Note that clipping will occur if the combination of
channel amplitude setting and VCA signal attempts to drive the output above 20 V p-p
open-circuit voltage.
External AM is achieved by setting the channel to the required output level and applying
the modulation signal (which can be ac coupled if required) at the appropriate level to
obtain the modulation depth required. If the channel output level is changed the
amplitude of the modulating signal must also be changed to maintain the same
modulation depth.
11-2
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