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Modulation
Internal Modulation11
11-3
The VCA signal is applied to the amplifier chain prior to the output attenuators. The
amplifier itself is controlled over a limited range (approximately 10 dB); the full
amplitude range of the channel is achieved by switching in up to five 10 dB 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 below), it is up to the user to
observe the waveforms when using externally-driven VCA and to make adjustments to
prevent clipping. Note that it is not possible to give a simple guide as to where the range
clipping limits are because the use of dc offset, for example, changes them.
Within each 10 dB attenuator step the maximum output setting of the channel at which
clipping is av
oided is reduced from the range maximum to half this value as the
modulation is increased from 0 to 100 %. 100 % modulation will be achieved at this
mid-range setting with an external VCA signal of approximately 1 V p-p. The frequency
range for the modulating signal is DC to 100 kHz.
It is also possible to modulate a dc level from
the generator with a signal applied to the
EXT MOD
ULATION socket, as follows:
Set the generator to external trigger on the TRIGGER IN set-up screen but do not
appl
y a trigger signal to the TRIG IN socket; select square in the STANDARD
WAVEFORMS screen. The MAIN OUT socket now provides a constant voltage which is
the peak positive voltage defined by
the current amplitude setting. Pressing the ± key
with AMPLITUDE displayed will set the level to the peak negative voltage. T
his 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
m
odulating signal to the EXT MODULATION input (nominally 1 kΩ input impedance).
With no signal the carrier i
s 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 20 V p-p open-circuit voltage
limit.
Peak modulation, i.e. maximum carrier amplitude (20 V p-p), is achieved with an
external SCM level of appr
oximately ±1 V, i.e. a 2 V p-p signal. The modulation
frequency range is DC to 100 kHz.
When external SCM is selected for a channel the am
plitude control of that channel is
disabled. The AMPLITUDE set-up screen shows the message fixed by SCM.
Internal Modulation
Only the multi-channel instruments (models 282 and 284) can make use of internal
modulation; the single-channel model 281 has no internal modulation capability.
Pressing the MODULATION key calls the MODULATION set-up screen.
MODULATION
source: Ch3
type: SCM
level
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