:VOLT
<value> <unit>
/
The
El440
accepts three different 'base units' in the
VOLT
-
:VOLT:
UN
IT
~~mmand-
V
(
Volts peak
-
peak
)
VRMS
(
Volts root mean square
)
dBm
(
dB ref. to
1
mW on
50R
-
0
dBm-b
1
mW on
5OR)
There are two ways to tell the instrument which unit to use:
set the default unit via :VOLT:UNIT command. (e.g.
':VOLT:UNIT dBm;')
append the unit string to the parameter in the :VOLT command.
(e.g. ':VOLT
1.3
dBm;')
A unit appended to the :VOLT command has higher priority than
the default unit, however the default unit is NOT changed by the
:VOLT command.
To keep track of all this, the instrument deals internally with two
units. One is the default unit (entered via
':VOLT:UNIT7), the other
is the so-called effective unit (entered implicitly via ':VOLT7). The
effective unit is not queryable by the user but has a much higher
impact on the instrument operation than the default unit.
The default unit is used in only two situations
:
When a VOLT command has no unit then effective
=
default.
All query responses are given in the default unit.
The effective unit is used when
:
the volt value is checked against bounds. (observe that the
transformation rules from dBm or VRM to V are different for
different output functions. The hardware limits are V
-
limits!)
rn
an attempt is made to switch the output amplifier on, this is not
possible when the real unit is dBm! (the dBm value is only correct
for a
500
load, and the output amplifier cannot drive a
500
load.)
the output function is changed. The instrument always keeps the
output voltage in the effective unit: that means that the volt
peak-peak value for example, measured on the output connector,
changes when the
FUNCtion is changed from SIN to SQU and the
effective unit is not V.
This implementation
-
although straightforward
-
might lead to little
confusion when not understood. It is quite usual for the following
odd-looking thing to happen
:
Error 'NO dBm allowed when output amplifier is active' on command
':AMPL ON' and response for ':VOLT:UNIT?' is 'V'.
REASON
:
effective unit is dBm.
Understanding the
HP E1440A
4-17
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