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Converting Spectrum Analyzer Option
Readouts from dBm to dBV
LAB WM-454
February 27, 2012
Summary
The LeCroy Spectrum
Analyzer option reads spectral
values in dBm (decibels
relative to 1 mW). In some
applications, users would like
to read spectral values in dBV
(decibels relative to 1 V). This
application brief outlines a
simple rescaling to accomplish
this.
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.
Figure 1: The LeCroy spectrum analyzer option uses an RF spectrum analyzer format to
display the frequency spectrum of an acquired waveform including amplitude readouts in
decibels relative to 1 mW (dBm)
LeCroy Corporation
Converting Spectrum Analyzer Option Readouts from dBm to dBV
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The spectrum analyzer reads in dBm assuming a 50 Ohm load. You can recalibrate this to read in dBVrms using
the following derivation:
2
2
dBm= 10*Log 10 ((V /R)/ .001) = 10*log10 (V ) +10*Log10 (1000/R)
for R=50:
dBm = 20*Log10(V) + 10*Log10(20)
But 20*Log10(V) =dBV, so
dBm= dBV+13 or
dBV= dBm-13
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.
Figure 2: Analysis of a 400 Hz sine
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 Corporation
Converting Spectrum Analyzer Option Readouts from dBm to dBV
page | 2 of 3
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.
Figure 3: The Save Table setup to save the Spectrum Analyzer peak table in comma delimited (.csv) format
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.
Figure 4: The exported spectrum analyzer peak table imported into a spreadsheet with the rescale operation added to
convert the table of peak values into dBVrms
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.
LeCroy Corporation
Converting Spectrum Analyzer Option Readouts from dBm to dBV
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