58
Sum
Introduction
Both internal and external Sum can be selected; summing can be used to add ‘noise’ to a
waveform, for example, or to add two signals for DTMF (Dual Tone Multiple Frequency) testing.
External Sum can be applied to any or all channels. Internal sum uses the previous channel as
the modulation source, e.g. channel 2 can be summed into Channel 3; internal Sum is not
available on channel 1 or on a single channel instrument.
Summing shares some of the generator’s inter
−
channel resources with Modulation modes; as a
result neither internal nor external Sum can be used with internal modulation but external
modulation is possible.
To better understand the constraints, the following sections (and the Modulation 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
summing.
External Sum
In Sum mode an external signal applied to the SUM input is summed with the waveform(s) on the
specified channel(s). The same Sum input signal can be used at different amplitudes with each of
the channels with which it is summed.
Pressing the SUM key calls the
SUM
set
−
up screen.
SUM source: ext
◊
ratio: 0dB
◊
CH2 +2.00 Vpp
Pressing the
source
soft
−
key steps the Sum sources between
off, ext
ernal and
CHx
where
x
is the number of the previous channel, refer to the Inter
−
Channel Block Diagram.
With
ext
selected the screen is as shown above. The level of the SUM can be adjusted
independently for the selected channel by pressing the
ratio
soft
−
key; use the rotary knob or
cursor keys to set the SUM input attenuation for that channel from 0 to –50dB in –10dB steps.
This facility permits the same SUM signal to be used at different levels with each channel.
Clipping will occur if the Sum input level attempts to drive the channel amplitude above the
maximum 20Vpp open
−
circuit voltage. However, the relationship between the SUM input and the
maximum summed output depends not only on the Sum input level but also on the channel
amplitude setting. This is because the Sum input 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.
The summed output cannot exceed the maximum of the “range” within which the channel output
has been set by choice of amplitude setting. Whereas with internal Sum the generator gives
warnings when the combination of Sum input and amplitude would cause waveform clipping (see
Internal Sum section), it is up to the user to observe the waveforms when using external sum and
to make adjustments if the waveform is clipping. Note that it is not possible to give a simple
guide as to where the “range” breakpoints are because the use of DC Offset, for example,
changes these points.
Within each “range” a SUM signal of ~2Vpp will force the channel output from range minimum to
range maximum; if the channel amplitude is set to mid
−
range the SUM signal needed to force the
output to range maximum is about half, i.e. ~1Vpp.
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