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:

6,34 2 2 2,24

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. : 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:

P= (0,224)2 50 0,001 W 1 mW

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= (0,224)2 600 0,0000836 W 83,6 μW

This can be calculated as a signal level of:

10 *10log(83.6E-6 1 mW) = 10 *10log(83.6E-3)– 10.7 dBm

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 = (P *R ) = (1E-3 *50) = 0,2236068

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.:

Vrms offset = 0,5 *2,236068 = 1,118034

Page 70
Image 70
Fluke PM-3380B This can be calculated as a signal level, Vrms offset calculation, Example for attenuator setting 0.5 V/div