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Operating Information
2–26
AM700 Audio Measurement Set Service Manual
When the generator is driving an output load of 600 W, and the internal series
resistor is also set for 600 W, the generator dBm reading will be correct. This
also true when driving a 150 W load with the internal 150 W series resistor
selected. Since half the voltage is dropped across the internal resistor and half
across the external resistor, the dBu (voltage across the load) output will be
6 dBu down from the output value in dBm for these matching conditions.
Internal series
resistor
User load
resistance
E
out
Figure 2–21: Generator output voltage divider for dBm calculations
Input dBm. This setting defines the external resistance on the input of the AM700
and is used for calculating the input signal amplitude in terms of dBm. Since the
AM700 is measuring voltage, the internal termination load of the AM700, which
is in parallel with the external load, has no effect on the calculation as the
applied voltage is dropped across both in parallel (see Figure 2–22).
NOTE. This is calculated dBm, so if the external load is not as stated in the Input
dBm box, the measured value stated in dBm will not be correct.
AM700 internal
load resistance
E
in
User’s external
parallel resistor
To acquisition
Figure 2–22: Input circuit for dBm calculations
Analog to Digital Conversion Factor. This selection permits measurement results
obtained from digital audio sources to be represented as analog values as if they
were produced by a D/A converter. The scale factor of a hypothetical D/A
converter is specified in this entry.