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USING THE COMBISCOPE INSTRUMENTS 3 - 51
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:
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:
Examples: At 20mV/div. : 2.24
*
20 ≈ 44.8 mVrms
At 100mV/div.: 2.24
*
100 ≈ 224 mVrms
For a 50Ω system, a signal amplitude of 224 mVrms corresponds to the following
signal power:
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:
This can be calculated as a signal level of:
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:
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
*
Urms
Example for attenuator setting 0.5 V/div.:
6,34 2⁄ 2 2,24≈
2,24 * <number of millivolts per divisions>
P 0,224()
2
50⁄ 0,001 W 1 mW≈≈=
P0,224()
2
600⁄ 0,0000836 W 83,6 µW≈≈=
10 * log
10
83.6E-6 1 mW⁄()10 * log
10
83.6E-3()10.7 dBm–≈=
Urms P *R() 1E-3 *50()0,2236068== =
Vrms offset 0,5 *2,236068 1,118034==