The LeCroy Spectrum Analyzer option, shown in Figure 1, displays the frequency spectrum of an acquired signal using an interface like that of an RF spectrum analyzer. This includes readouts in dBm based on the voltage into the scope 50 Ω input. In some applications (e.g. when using a probe that has a high impedance input) users would like to read spectral amplitudes in dBV (decibels relative to 1 V). A simple rescaling allows this to be accomplished and by using the Save Table feature the spectrum peak table can be converted using an Excel spreadsheet.
The spectrum analyzer reads in dBm assuming a 50 Ohm load. You can recalibrate this to read in dBVrms using the following derivation:
|dBm= 10*Log10 ((V2/R)/ .001) = 10*Log10 (V2) +10*Log10 (1000/R)
dBm = 20*Log10(V) + 10*Log10(20)
|But 20*Log10(V) =dBV, so
|dBm= dBV+13 or
So simply subtracting 13 dB from the table values for each harmonic will give you the correct values in dBVrms.
Let’s do a measurement to check. In Figure 2 we have analyzed a 400 Hz sine wave. The sine has a measure rms voltage level of 1.0367 V as seen in the parameter P1 readout.
The Spectrum Analyzer peak table shows a component a 400 Hz with an amplitude of 13.3 dBm. Applying our conversion the level 0.3dBVrms. Doing the math to convert back to volts we expect the rms voltage level of the 400 Hz component to be 1.035 V. This is within 0.1%.
LeCroy scopes have the ability to export the spectrum analyzer peak table in comma delimited (.csv) file format. This can be imported into a spreadsheet with the rescaling done there. Figure 3 shows the SaveTable tab on the Save/Recall dialog box.
Once you have saved the table you can import it into a spreadsheet and apply the rescale operation. Figure 4 shows the peak table and spectrum display for a 400 Hz square wave with an Excel spreadsheet overlaid showing the rescale operation added to the table.
Using the Save Waveform selection under the file pull down menu you can save the whole spectrum trace and rescale that as well.
For very small voltages it might be more convenient to convert the voltages to dBm V (decibel referenced to 1 mV). Simply add 60 dB from the dBV value to get the value in dBm V. Adding another 60 dB will give the value in dBμV.