Keithley 2001 Operator's Manual page 383

Query
Description
Programming example
2001-900-01 Rev. K / August 2010
*RST
:SYSTem:PRESet
:AC? Query AC voltage units
Short-form format: :unit:volt:ac?
Response message: V, DB or DBM
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.
In order to select dB or dBm units, the advanced filter must be disabled. See
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 is the AC input signal.
IN
V is the specified reference level in volts.
REF
The dB reference level (V
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 is the AC input signal.
IN
Z is the specified dBm reference impedance.
REF
The dBm reference impedance (Z
any AC type (except Positive and Negative Peak) can be used for dbm measurements.
10 OUTPUT 716; ":unit:volt:ac db; ac?"
20 ENTER 716; A$
30 PRINT A$
40 END
V
V
NOTE
V
IN
=
------------ -
dB 20 log
V
REF
) is specified using the :DB:REFerence <n> command. Note that any AC
REF
P
------------ -
dBm = 10 log
1mW
V 2
IN
=
P ----------- -
Z
REF
) is specified using the :DBM:IMPedance <n> command. Note that
REF
IEEE-488 Reference
paragraph
4-225