User manual

IEEE-488 Reference
2001-900-01 Rev. K / August 2010 4-225
*RST V
:SYSTem:PRESet V
Query :AC? Query AC voltage units
Short-form format: :unit:volt:ac?
Response message: V, DB or DBM
Description This command is used to select the units for ACV measurements. With volt (V) units selected, normal
AC voltage measurements will be made for the ACV function.
NOTE
In order to select dB or dBm units, the advanced filter must be disabled. See paragraph
4.19.11 for details on controlling the advanced filter.
With DB units selected, AC dB voltage measurements are calculated according to the following
equation:
where; V
IN
is the AC input signal.
V
REF
is the specified reference level in volts.
The dB reference level (V
REF
) is specified using the :DB:REFerence <n> command. Note that any AC
type can be used for dB or dBm measurements except Positive and Negative Peak. These functions may
return negative values. The log of a negative number is undefined.
The DBM units selection is used to make decibel measurements referenced to 1mW and can be expressed
as follows:
where; V
IN
is the AC input signal.
Z
REF
is the specified dBm reference impedance.
The dBm reference impedance (Z
REF
) is specified using the :DBM:IMPedance <n> command. Note that
any AC type (except Positive and Negative Peak) can be used for dbm measurements.
Programming example 10 OUTPUT 716; “:unit:volt:ac db; ac?”
20 ENTER 716; A$
30 PRINT A$
40 END
dB 20 log
V
IN
V
REF
-------------
=
dBm = 10 log
P
1mW
-------------
P
V2
IN
Z
REF
------------=