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Distortion measurement with a Spectrum Analyzer (Part 3)

In this article we will introduce how to use S3302 handheld spectrum analyzer to make an Accurate Harmonic Measurement

It takes more steps to measure in this method, but since measurement is done with smaller span and resolution bandwidth for each signal, the signal-to-noise ratio is increased and more accurate measurement result will be obtained. It will be explained below how to measure the harmonic of 1 GHz signals.


1)       Set the output signal of the signal generator:

Set the frequency of the signal generator as 1GHz and power as -10dBm. Connect the output of the signal generator to the input port of the spectrum analyzer, as shown in Fig. below. Enable the ON state of the radio frequency.

1-spectrum analyzer pulse measurement.png

2)       Set the starting frequency and stop frequency of the spectrum analyzer.

l  Press [Reset] key.

l  Press [frequency], [Start Freq], 800[MHz], [Stop Freq] and 2.5[GHz].


3)       Set the video bandwidth to smooth noise so as to improve the resolution.

l  Press [Bandwidth], [Video Bandwidth Auto Man] to enable auto off.

l  Use [↓] key to reduce video bandwidth.


4)       Reduce the span by the signal tracking function.

l  Press [Peak] to activate the signal peak of marker search.

l  Press [frequency] and [Signal Track Off On].

l  Press [frequency], [Span] and 100[kHz].


5)       Disable signal tracking.

l  Press [Frequency] and [Signal Track Off On].


6)       Move the signal peak to the top division to obtain the highest amplitude measurement accuracy.

l  Press [Peak] and [Peak Search], and read the peak power.

l  Press [Amplitude] and [Ref Level], and set it as the peak power. Results are shown in Fig.1.

Fig. 1 Input Signal under 100kHz Span.png

Fig. 1 Input Signal under 100kHz Span


7)       Set the step of the center frequency as the signal frequency of the fundamental wave.

l  Press [Frequency] and [Frequency Step Auto ON/OFF] and enter 1 GHz.


8)       Measure the second harmonic.

l  Press [Maker], [Marker→], [Marker→Center] and step key [↑]. Change the center frequency of the spectrum analyzer to second harmonic by stepping operation. Press [Peak] and [Peak Search], and read the peak power.

l  Press [Amplitude] and [Ref Level], and set it as the peak power. Adjust the harmonic peak to the reference level. The second harmonic amplitude is shown in Fig.2.


Fig. 2 Second Harmonic Amplitude.png

Fig. 2 Second Harmonic Amplitude

 

9)       Calculate the harmonic distortion.

Change the distortion percentage of the second harmonic to fundamental wave. The unit can be changed again to volt in order to read the voltage ratio of two signals.


10)    Measure other harmonics.

Repeat Step (i) to (j) to other harmonics to be measured. Calculate the distortion percentage of each harmonic.


Total signal harmonic distortion percentage is also subject to frequent test. In order to test such parameter, amplitude of each harmonic should be tested with linear unit (such as volt) rather than relative unit dBc. Press [Amplitude], [Units] and [Volt] to set the amplitude unit as volt. The measured signal amplitude can be applied in the following equation to calculate the total harmonic distortion:

3-fomular.png

Where:

n  A1 - refers to fundamental wave amplitude (V)

n  A2 - refers to second harmonic amplitude (V)

n  A3 - refers to third harmonic amplitude (V)

n  An refers to nth harmonic amplitude (V)

If the signal amplitude is carefully measured as shown in the above example, the obtained harmonic distortion percentage is accurate.

  

Now Saluki has following 5 series of spectrum analyzers to fit all your need on spectrum measurements. Please check following links for details:

S3302 series handheld spectrum analyzer (9kHz - 20GHz / 44GHz)

S3531 series spectrum analyzer (9kHz - 1.8GHz / 3GHz)

S3532 series spectrum analyzer (9kHz - 3.6GHz / 7.5GHz)

S3331 series handheld spectrum analyzer (9kHz - 3.6GHz / 7.5GHz)

S3503 series spectrum analyzer (3Hz - Max 50GHz)