BASIC DMM OPERATION
2.6.9 dB Measurements
The dB measurement mode makes it possible to compress
a large range of measurements into a much smaller scope.
AC dB measurements can be made with the instrument
in the ACV or AC4 function. The relationship between dB
and voltage and current, can be expressed by the follow
ing equations:
In ACV, the instrument will read OdB when 1V is applied
to the input. With ACA dB selected, the instrument will
read OdB when ImA is applied to the input.
Reference levels other than 1V and l.mA cannot be directly
programmed, but they can be established with the zero
feature. This procedure simply consists of applying a signal
to the instrument and pressing the ZERO buttonThat
suppressed level is the dB reference (OdB point).
The following procedure explains how to use the zero
feature to establish a reference:
1. Apply a voltage or current signal, that is to~@used as
the dB reference, to the input of the Model 199.
2. Press the ZERO button. The ZERO indicator will turn
on and the display will zero. The reference is now
whatever the applied signal is.
3. Disconnect the signal from the instrument.
AC dB Measurements-Perform the following steps to
make dB measurements:
1. Select the AC volts or AC amps function. (Press,VOlX
or AMPS, then AC).
2. Select the front or rear panel input terminals with the
INPUT switch.
3. Check and/or change the dB reference as previously
explained.
4. Connect the signal to be measured to the .tiput of the
Model 199.
5-tenable the dB measurement mode by pressing SHIFT
dB.
6. Take the dB reading from the display.
WARNING
With dB enabled, a hazardous voltage baseline
level (34OV or more), not displayed, may be pre-
sent on the input terminals. If not sure what is
applied to the input, assume that a hazardous
voltage is present.
dBm Measurements-dBm is defined as decibels above or
below a lmW reference. dB measurements can be made
in terms of impedance rather than voltage or current.
Because the instrument cannot directly establish im-
pedance references, a voltage reference must be calculated
and established for a particular impedance reference. Use
the following equation to calculate the voltage reference
needed for a particular impedance reference:
Example: Calculate the voltage reference needed to make
dBm measurements referenced to 6000.
For OdBm, V,., = ~O.oOlW l 6OOQ
= -iiF
= 77456V
Once the necessary voltage reference is known, it~~can be
established in the Model 199 with the dB program. Subse-
quent dBm readings will be referenced to the correspond-
ing impedance reference. Table 2-4 lists the voltage
references needed for some commonly used impedance
references.
dBW Measurements-dBW is defined as decibels above or
below a 1W reference. dBW measurements are made in the
same manner as dBm measurements; that is, calculate the
voltage reference for a particular impedance and set the
instrument to it with the dB program. The only difference
between dBm and dBW is the reference point; lmW vs 1W.
The following equation can be used to calculate the voltage
reference:
For OdBW, V,., = K
24.3
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