56
Modulation
Introduction
Both internal and external modulation can be selected. External modulation can be applied to
any or all channels. Internal modulation uses the previous channel as the modulation source,
e.g. channel 2 can be used to modulate channel 3; internal modulation is not available on
channel 1 or on a single channel instrument.
The external modulation mode can be set to VCA (Voltage Controlled Amplitude) or SCM
(Suppressed Carrier Modulation) modes. Internal modulation can be set to 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 these are generally
outside the range of common
−
sense applications. To better understand these constraints the
following sections (and the SUM chapter) should be read with reference to the fold
−
out block
diagrams at the end of the manual which show the control signals of a single channel and the
inter
−
channel connections.
These diagrams also show the inter
−
channel trigger connections described in the Triggered
Burst and Gate chapter; 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
,
ext
ernal and
CHx
where
x
is the number of the previous channel; note that channel 1 does not have a previous channel,
refer to the Inter
−
Channel Block Diagram.
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 1k
Ω
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 20Vpp 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 will have to be changed to maintain the same modulation depth.
The VCA signal is applied to the amplifier chain prior to the output attenuators. The amplifier
itself is controlled over a limited range (~10dB) and the full amplitude range of the channel is
achieved by switching in up to five –10dB attenuation stages. Peak modulation cannot exceed
the maximum of the “range” within which the channel output has been set by choice of amplitude
setting. Whereas with internal AM the generator gives warnings when the combination of
modulation depth and amplitude setting cause waveform clipping (see Internal Modulation
section), it is up to the user to observe the waveforms when using external VCA and to make
adjustments if the waveform is clipping. Note that it is not possible to give a simple guide as to