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4.4
Cautions to be observed during Actual Measurement
(1)
dB/dBm selection—AMPTD switch
The switch selects whether the oscillator
output amplitude is represented in dB or
dBm (see 4.3 14)} .
Set the switch to the desired position
prior to the use and be careful not to
Fig.
4-4
touch the switch during-measurement.
The dBm unit is naturally used in a 600-ohm terminated line since
the unit is clearly defined in terms of power dissipated in a
600-ohm load.
The dB unit is only conditionally definite because it is a relative
voltage ratio as 1 V rms being 0 dB;
a 6-dB difference depends on
whether the output has a matched load or not.
Remember that the
output of the VP-7721A has been calibrated with a 600-ohm load.
This is noted by INTO 600Q marked beneath the switch on the front
panel as shown in Fig.
4-4.
Measuring the voltage at the OSC OUTPUT connector directly with
a high-impedance voltmeter
(e.g. the level meter section of the
VP-7721A)
reveals that the output is 6 dB higher than the displayed
value.
To supply the exact level appearing in the digital display,
the oscillator output should be terminated with a 600-ohm load.
dB§j[]dBm
IVOdB 1 mW
INTO Boon
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CP
(2)
Oscillator output
OSC OUTPUT
600ft
Fig.
4-5
Fig.
4-6
4-21
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