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7 Additional Parameters and Functions
A number of additional capabilities can be accessed using the measurement parameter keys,
as described below. Further functions also can be selected from the Functions menu key – see
section 10.1
7.1 Combined AC + DC Measurements
Vdc+Vac or Idc+Iac measurements are made by pressing both keys together. The reading
displayed is the RMS sum (dc
2
+ ac
2
) and ac+dc is shown on the display.
Both ac and dc measurements are made on the same range: the instrument will auto-range to a
range which gives an in-range reading for both the ac and dc component of the parameter.
Clearresult to be shown as an in-range reading.
Note that this is a special case of Dual Measurement mode (see section 8.3
) and the
measurement update rate is slowed accordingly.
7.2 Decibel Measurement (dB)
dB (deciBel) can be selected only when Vac is already in the main display. Pressing [Shift]
followed by [Vac] shows the dB value of the Vac measurement (referred to the chosen
impedance setting) and displays the {dB} symbol. If no secondary function is selected the
normal reading in volts will be shown in the secondary display.
dBs are shown in a fixed format with 0.1dB resolution, whatever range the Vac measurement is
being made on. The value displayed is in dBm and is calculated from the formula:
dB = 10 log
10
(1000 x V
2
/R), where R is the selected reference impedance.
The default reference impedance is 600: but a different value can be selected within the
Utilities menu. Press [Utilities], use the Navigator keys to select {rEF} and confirm with [OK].
The existing reference value will flash. Alternative values can be selected using the Navigator
keys and confirmed with [OK].
Available values are 50, 75, 93, 110, 124, 125, 135, 150, 250, 300, 500, 600, 900, 1000, 1200,
and 8000:. Pressing [Shift] followed by [Auto] (Clear is the shifted function of Auto) returns
the value to 600:. Press [Utilities] again to exit the Utilities menu.
dB is cancelled by pressing [Vac], or by selecting any other measurement parameter or
function.
7.3 Removing Test Lead Resistance (:
Null)
Residual test lead resistance can be nulled out using the Ohms Null facility as follows:
Connect the test leads together and press [Shift] followed by [
2/4] (: Null is the shifted
function of :). The 100: range is set automatically and the {NULL} symbol flashes; the meter
stores the reading that it detects after 5 seconds provided that it is less than 1.000: (1000
counts). A beep sounds when the null is completed and the display should show zero Ohms
and no {NULL} symbol.
If the reading cannot be nulled, because the offset is too large, the reading will not change and
the buzzer will not sound.
The Ohms null is stored as a floating point value which is used on all ranges; it is not lost when
the function is changed or when the instrument is turned off. Ohms null can be cancelled by
selecting Ohms Null and not generating a sub 1.000: reading within the next 5 seconds, or by
restoring the instrument to factory defaults – see section 12.8
.
Normal Null can be used together with Ohms Null.
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