Other Modulation Modes
External VCA
When used in VCA (Voltage Controlled Amplitude) mode the MAIN OUT amplitude is directly
proportional to the external modulating signal voltage applied to the VCA/SUM IN socket.
Suppressed carrier modulation (SCM) is achievable in this mode.
To use VCA mode set
VCA/SUM input:
to
VCA
on the
EXT VCA/SUM SETUP
screen which
is called by pressing the VCA/SUM IN key:
EXT VCA/SUM SETUP
VCA/SUM input: VCA
Connect the modulating signal to the VCA/SUM IN socket (nominally 6k
Ω
input impedance); a
positive voltage increases the generator output and a negative voltage decreases the output.
Note that clipping will occur if the combination of generator setting and VCA signal attempts to
drive the input above 20Vpp open-circuit voltage.
External AM is achieved by setting the generator 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 generator 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 instrument is
achieved by switching in up to five -10dB attenuation stages; a table of the approximate “ranges”
for each attenuator setting is given at the end of this section. Peak modulation cannot exceed the
maximum of the “range” within which the generator output is set.
Within each “range” the maximum output setting of the generator at which clipping is avoided is
reduced from range maximum to half this value as modulation is increased from 0% to 100%;
100% modulation will be achieved at this mid-range setting with an external VCA signal of
approximately 2·5Vpp. Modulation frequency range is DC to 100kHz.
The generator’s amplitude control circuit has four quadrant operation, allowing the generator
output to be inverted if the external VCA voltage is taken sufficiently negative. Suppressed carrier
modulation (SCM) can be achieved by applying a modulating signal with a negative offset
between 0V and -3V (depending on output level setting) sufficient to reduce the carrier output to
zero.
It is also possible to modulate a DC level from the generator with a signal applied to VCA/SUM IN
as follows. Set the generator to external trigger on the
TRIGGER INPUT 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 VCA/SUM IN.
External SUM In
In SUM mode, set by setting
VCA/SUM input
to
SUM
on the
EXT/VCA/SUM SETUP
screen,
an external signal applied to the VCA/SUM IN socket is summed with the selected waveform.
This can be used to add “noise” to a waveform, for example, or to add two signals for DTMF
(Dual Tone Multiple Frequency) testing.
The relationship between the SUM input and the summed output depends on the generator
amplitude setting because the SUM input is applied to the amplifier chain prior to the output
attenuators. The amplifier is controlled over a limited range (~10dB) and the full amplitude range
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