57
where the “range” breakpoints are because the use of DC Offset, for example, changes these
points.
Within each “range” the maximum output setting of the channel at which clipping is avoided is
reduced from range maximum to half this value as modulation is increased by 0% to 100%;
100% modulation will be achieved at this mid
−
range setting with an external VCA signal of
approximately 1Vpp. Modulation frequency range is DC to 100kHz.
It is also possible to modulate a DC level from the generator with a signal applied to the EXT
MODULATION socket, as follows. Set the generator to external trigger on the
TRIGGER IN
setup screen but do not apply a trigger signal to TRIG IN; select squarewave. The MAIN OUT is
now set at the peak positive voltage defined by the amplitude setting; pressing the ± key with
AMPLITUDE
displayed will set the level to the peak negative voltage. This DC level can now be
modulated by the signal applied to the EXT MODULATION input.
External SCM
Select
SCM
with the
type
soft
−
key on the
MODULATION
screen. Connect the modulating
signal to the EXT MODULATION input (nominally 1k
Ω
input impedance). With no signal the
carrier is fully suppressed; a positive or negative level change at the modulation input increases
the amplitude of the carrier. Note that clipping will occur if the SCM signal attempts to drive the
output above the 20Vpp open
−
circuit voltage.
Peak modulation, i.e. maximum carrier amplitude (20Vpp), is achieved with an external SCM
level of approximately ±1V, i.e. a 2Vpp signal. Modulation frequency range is DC to 100kHz.
When external SCM is selected for a channel the amplitude control of that channel is disabled;
the
AMPLITUDE
setup screen shows the message
fixed by SCM
.
Internal Modulation
Pressing the MODULATION key calls the
MODULATION
setup 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;
CHx
is the source for internal modulation.
Note that channel 1 and single channel instruments do not have a previous channel, i.e. they
have no internal modulation capability; refer to the Inter
−
Channel Block Diagram.
With
CHx
selected the modulation mode can be switched between
AM
and
SCM
with
alternate presses of the
type
soft
−
key.
When
AM
is selected the screen has an additional soft
−
key labelled
depth
; selecting this
key permits the modulation depth to be set directly from the keyboard or by the rotary control.
Warnings are given when either a modulation depth or output amplitude change has caused
clipping; the new setting is accepted but it must either be changed back or the other parameter
must also be changed to avoid the contention.
When
SCM
is selected the screen has an additional soft
−
key labelled
level
; selecting this
key permits the peak carrier output level to be set directly from the keyboard or by the rotary
control. The maximum output level that can be set is 10Vpp.
When internal SCM is selected for a channel both the amplitude control of that channel and the
previous channel (used as the modulation source) are disabled. The
AMPLITUDE
setup
screen of the channel being modulated shows the message
fixed by SCM
. The
AMPLITUDE
screen of the previous channel shows the message
Set by CHx mod.
and its
status screen shows the message
→
x
to indicate that it is being used as a source for channel x.
Internal modulation cannot be used with internal or external Sum.
