Fluke CombiScope PM3370B User Manual page 70

USING THE COMBISCOPE INSTRUMENTS
Absolute FFT amplitudes are calculated from the true signal using the information
on the actual attenuator setting in the range from 5 V/div. to 2 mV/div. This results
in an offset value to be added to the relative FFT amplitude for each attenuator
setting. In any attenuator setting, the reference level for the absolute FFT value is
calculated from a peak-to- peak amplitude of a sine wave on a screen of 6.34
divisions. This amplitude equals an RMS value of:
6,34 2 ⁄ 2
This level is used as the reference level (top of screen) for the FFT amplitude
display. For any attenuator setting, the reference level can be calculated as
follows:
2,24 * <number of millivolts per divisions>
Examples: At 20mV/div. :
At 100mV/div.:
For a 50Ω system, a signal amplitude of 224 mVrms corresponds to the following
signal power:
P =
This can also be expressed as a signal level of 0dBm at 50Ω impedance.
The same voltage measured in a 600Ω system corresponds to the following
power level:
P =
This can be calculated as a signal level of:
10 * log
Vrms offset calculation:
A signal of 1 mW at 50Ω impedance is taken as voltage reference at 100 mV/div.
From this signal the RMS voltage is calculated as follows:
Urms
For a whole screen of 10 divisions, Urms = 2.236068. Depending on the
attenuator setting, the Vrms offset voltage is calculated as follows:
Vrms offset = attenuation
Example for attenuator setting 0.5 V/div.:
Vrms offset
≈
2,24
2.24
2.24
2
( ) ⁄ ≈
0,224 50 0,001 W 1 mW
2
( ) ⁄ ≈ 0,0000836 W 83,6 µW
0,224 600
10
( ⁄
83.6E-6 1 mW
( )
= P *R =
= 0,5 *2,236068
20 ≈ 44.8 mVrms
*
100 ≈ 224 mVrms
*
≈
≈
10
) (
= 10 * log 83.6E-3
( )
1E-3 *50 = 0,2236068
Urms
*
= 1,118034
3 - 51
) ≈
– 10.7 dBm