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Sum
Internal Sum12
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 approximately 2 V p-p will force the channel output
from the range minimum to the range maximum; if the channel amplitude is set to mid-
range then the SUM signal needed to force the output to range maximum is halved to
approximately 1 V p-p.
To facilitate the setting of appropriate sum and amplitude levels the output amplitude of
the selected channel can also be changed from the SUM set-up screen. Press the CHx
soft-key and adjust the amplitude with direct keyboard entries or by using the rotary
control.
External sum cannot be used with internal modulation.
Internal Sum
Only the multi-channel instruments (models 282 and 284) can make use of internal sum;
the single-channel model 281 has no internal sum capability.
Pressing the SUM key calls the SUM set-up screen:
SUM source: CH1
ratio: 1.00000
CH2 +2.00 Vpp
CH1 +2.00 Vpp
Pressing the source soft-key steps the sum source between off, external and
CHx (where x is the number of the previous channel). CHx is the source of the
internal sum signal.
With CHx selected for internal sum the screen is as shown above. The amplitude of
both the summing channel, CHx
+1, and the internal sum signal, CHx, are shown in
the display, together with the ratio in which they are combined. All three parameters
can be selected with the appropriate soft-key and set directly from the keyboard or by
using the rotary control. Changing any one parameter will also adjust one of the other
two; for example adjusting the amplitude of either channel will cause the displayed ratio
to change.
The value shown in the ratio field is the CHx amplitude divided by the CHx
+1
amplitude. Adjusting the ratio value directly changes the amplitude of the sum input
signal, CHx, not the channel’s output amplitude. When a value is entered into the
ratio field it is initially accepted as entered but may then change slightly to reflect the
actual ratio achieved with the nearest sum input amplitude that could be set for the given
channel output amplitude.
Warnings are given when either a ratio, sum input or output amplitude change is
attempted which would cause the channel's output to be driven into clipping.
In general it is recommended that the amplitude of the sum input is smaller than the
channel amplitude, i.e. the ratio is less than or equal to 1; ratio values can be set from
very small signal levels up to unity. If the sum input is greater than the channel amplitude
there will be combinations when the ratio can be set to a little greater than 1. Note that
the software will always accept an entry, make the calculation and, if the combination is
not possible, return the instrument to its last legitimate setting.
12-3
1.888.610.7664 sales@GlobalTestSupply.com
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