Spectrum Analyzer : Spectrum Analyzer Measurements : Total Harmonic Distortion (THD)
 
Total Harmonic Distortion (THD)
To access THD measurement go to main menu and press MEASURE > MEASUREMENT >THD.
The THD result is calculated from the relative values of the 2nd, 3rd, 4th, 5th, 6th and 7th harmonics to the fundamental (carrier) frequency. To calculate the THD value, the user must set the frequency range of the instrument to include the fundamental frequency and up to the 7th harmonic. The instrument determines the fundamental frequency position from the manually set Fundamental Freq or by defining the fundamental as highest magnitude signal.
If the signal under test has a low THD value the higher order harmonics may not appear above the analyzer noise floor. In this case the noise floor value is used in the calculation, but under normal measurement conditions (the fundamental will be at least 30 dB above the noise floor) the impact on the measured noise floor value on the THD is negligible.
The noise floor can be reduced by manually selecting a lower RBW or reducing the input attenuation (be careful not to overdrive the RF input circuits). Refer to AM/FM Metrics to understand how THD percentage and THD in dB are calculated.
THD Measurement
1. The status panel provides a quick access to common spectrum analyzer settings. See STATUS PANEL (THD).
2. Fundamental signal.
3. Harmonic signals.
4. All THD measurement parameters are set via the SETUP menu. See SETUP Menu (THD).
STATUS PANEL (THD)
Below is the status panel with the corresponding minimized status panel icons for THD measurement. From main menu, select MEASURE > MEASUREMENT > THD.
 
Status Panel with Minimized Status Panel Icons (THD)
 
REF LEVEL
Sets the reference level of the top graticule line in the selected units. If the reference level offset is not zero, OFFSET REF LEVEL is displayed at this location. Refer to Setting Amplitude Parameters.
SECOND REF LEVEL
The reference level is the top graticule line to the right on the measurement display. Its functionality is exactly same as reference level, but it is only available when trace math is enabled.
SCALE/DIV
Sets the graticule scale/division for log-based units. This setting does not apply to linear units.
SECOND SCALE/DIV
It applies to second reference level and is available only when trace math is enabled.
REF LEVEL OFFSET
Sets the reference level offset in dB units. This setting can compensate for the presence of external input attenuation or gain.
FREQ OFFSET
Accounts for frequency conversions outside of the analyzer. Refer to Setting Frequency Parameters.
ATTEN LEVEL
When auto attenuation is off, sets input attenuation.
PRE AMP
Toggles the low-noise front-end preamplifier on or off. Refer to Setting Amplitude Parameters.
TRACES
Displays the current status of up to six traces in a quick-view summary.
The summary information includes the trace number, type, mode, and detector type. The active trace is shown with a highlighted background including the mode and detector type restated under the table. The reference trace settings are applied to all traces. Selecting a trace in the summary panel activates the pressed trace and opens the TRACE menu, allowing you to select and set up an individual trace as desired. Refer to Setting Trace Parameters.
TRACE STATS (dBm)
Displays the spectrum analyzer received Maximum, Average, and Minimum power of each frequency range sweep.
SWEEP
Toggles the current sweep setting between continuously or sweep once. Refer to Setting Sweep Parameters.
FREQ REFERENCE
Indicates the current frequency reference source of Internal High Accuracy (used after GPS has lost sync, but while the internal clock still has good GPS reference), Internal Standard Accuracy, External, or
GNSS (GPS) Hi Accuracy (requires GPS). The instrument automatically selects the frequency reference in the following order of priority: external, GPS, then the internal time base.
SETUP Menu (THD)
The THD SETUP menu is available in MEASURE > MEASUREMENT > THD.
SETUP Menu (THD)
 
AUTO FUND FREQ
Sets fundamental frequency automatically, when turned on. Turn it off to manually enter the value of FUNDAMENTAL FREQ.
FUNDAMENTAL FREQ
Sets the fundamental frequency manually.