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Distortion measurement with a Spectrum Analyzer(Part 1)

Mutual interference between equipment in common in crowd operating environment of the communications system. For example, second-order and third-order intermodulation distortion is common in narrowband systems. When there are two signals (F1 and F2) in one system, they and second harmonic distortion signals generated by them (2F1 and 2F2) mix and become third-order intermodulation products 2F2-F1 and 2F1-F2 very close to original signals, thus resulting in high-order intermodulation distortion. Such distortion products are mostly generated by devices such as amplifiers and mixers in the system. Most transmission units and signal generators have harmonics and their components need to be measured.

We will show how to use S3302 handheld spectrum analyzer to do distortion measurement. 

Identification of distortion generated by spectrum analyzer

In the case of large signal input, the spectrum analyzer may be subject to distortion, which will affect the distortion measurement results of true signals. You can set the attenuator to determine which signal is a distortion signal generated by the spectrum analyzer. This example shows whether the spectrum analyzer is subject to harmonic distortion based on the input signal.

1)       Set the output signal of the signal generator:

Set the frequency of the signal generator as 200MHz and power as 0dBm. Connect the output of the signal generator to the input port of the spectrum analyzer, as shown in Fig. 4-1. Enable the ON state of the radio frequency.

2)       Set the center frequency and span of the spectrum analyzer.

l  Press [Reset].

l  Press [Frequency], [Center Frequency] and 400[MHz].

l  Press [frequency], [Span] and 500[MHz].

You can see on the trace of the spectrum analyzer that the harmonic distortion of the signal is subject to 200MHz deviation from the original 200MHz signal, as shown in Figure 1.

3)       Set the center frequency of the spectrum analyzer at the first harmonic distortion location.

l  Press [Peak] and [Next Peak]

l  Press [Peak] and [Marker→Center].

4)       Set the span as 50MHz and reset the center frequency.

l  Press [frequency], [Span] and 50[MHz].

l  Press [Marker→] and [Marker→Center].

5)       Set the attenuator as 0dB.

l  Press [Atten Auto Man] and 10[dB].

l  Press [Peak] and [Peak Track Off On].

l  Press [Maker] and [Delta].

1-observation of harmonic distortion.png

Fig. 1 Observation of Harmonic Distortion

6)       Increase the attenuator setting to 10dB:

l  Press [Atten Auto Man] and 10[dB].

Observe the differential marker reading, as shown in Figure 2. The reading is the distortion difference of the attenuator at 0dB and 10dB. If the attenuator is changed and the differential marker reading is 1dB or more, it indicates that the spectrum analyzer is subject to certain distortion. If the differential marker reading is not obvious, the attenuation may be increased.

2_Attenuator Setting.png

Fig. 2 Attenuator Setting - 10dB

The amplitude reading of the differential marker is provided by two sources: 1. If the RF attenuation is increase, S/N will decrease, and this reading will be positive. 2. If the harmonic distortion of the spectrum analyzer decreases, this reading will be negative. The larger the reading, the bigger the error of measurement. In such case, the attenuator setting may be changed to reduce absolute amplitude of such delta marker reading.

Now Saluki has following 5 series of spectrum analyzers to fit all your need on spectrum measurements.

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)