8
2.3.5
ZERO, Offset Subtraction
This key turns offset subtraction on and off. When turned on, the ZERO
annunciator lights and the last signal reading is saved as S
d
and subtracted
from all subsequent signal readings S. This causes subsequent signal calcula-
tions (and the display) to use the value S-S
d
instead of S.
Offset subtraction allows one to remove the effects of ambient DC signals, by
zeroing the display before making a measurement. Pressing the ZERO key a
second time turns off the ZERO annunciator and stops offset subtraction.
2.3.6
AUTO, Automatic Gain Ranging
The AUTO key toggles automatic signal ranging on and off. When on, the
AUTO annunciator lights and the signal range (amplifier gain) is adjusted to
utilize maximum analog-to-digital converter resolution. When AUTO is turned
off, the AUTO annunciator is turned off and the signal range is left in its
current state.
Signal range can be manually controlled by the
, arrow keys. Pressing an
, arrow key when AUTO is on (and STATS, MENU and
λ
are off), turns
AUTO off and executes the manual range change. See RANGE, Section 2.3.11.
Signal range changes will often not coincide with observable changes to the
display value as the display is scaled by the detector responsivity and so must
adjust independently. Signal range gains will occur in 1 decade steps when auto
ranging and 1 or
1
/
2
decade steps (depending on MODE) when manual ranging.
When auto ranging in CONT PULSE mode, the arrival rate of pulses must be
above 1 Hz. In P-P CONT mode, the arrival rate of peaks must be above 50 Hz.
AUTO gain ranging is not allowed in SNGL measurement modes. When exiting
a SNGL measurement MODE, AUTO will turn back on if it was on when the
SNGL measurement mode was entered. (See MODE, Section 2.3.13)
2.3.7
CH A (B), Display Channel A (or B)
CH A causes channel A to be the display channel. CH B causes channel B to be
the display channel. The display channel annunciators will indicate which
channel is currently the display channel. When a display channel is selected,
the front panel will change to reflect its settings and measurement data.
Operations from the keypad or menu that affect a specific channel will only
affect the currently displayed channel and not the other.
2.3.8
STO REF, Store Reference Value
STO REF causes the last measurement, D, to be stored as D
ref
for subsequent
use in relative measurement calculations. When units are relative, REL, and
the stored reference is selected, the displayed value is the ratio D/D
ref
. When
units are logarithmic relative, dB, and the stored reference is selected the
displayed value is the function 10 log (D/D
ref
). D
ref
is always a power reading
stored in the units of Watts. Press STO REF to cause a new D
ref
to overwrite the
existing D
ref
value.
NOTE
When not using remote interface operation and when displaying relative dB
or REL measurements with the stored reference selected, the message
display area will show the STO REF value used in the calculation.
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