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How to measure Noise Signals with a Spectrum Analyzer
Release time :2017.12.15 Reading quantity:21 source:Salukirf, original

Definition of noise signal

In communications systems, signal-to-noise ratio (SNR) is usually used to express noise amplitude. When the nose level in the system increases, SNR decreases and it will be harder to demodulate modulated signals. SNR measurement is also used to indicate measurement of ratio of carrier against noise in communications systems.


Measurement of the S/N and noise by the marker function of S3302 series spectrum analyzer is described below. In the example, SNR is measured with the signal (carrier) having only single frequency point. If the modulation signal is tested, this test process should be modified to correct the level of the modulation signal.

Measure SNR

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 figure below. Enable the ON state of the radio frequency.

1-spectrum analyzer pulse measurement.png


2)       Set the center frequency, span, reference level and attenuator.

l  Press [Reset].

l  Press [Frequency], [Center Frequency], 1[GHz].

l  Press [Frequency], [Span] and 5[MHz].

l  Press [Amplitude], [Ref Level] and -10[dBm].

l  Press [Amplitude], [Atten Auto Man] and 40[dB].


3)       Set the marker at the signal peak, and the differential marker at the noise location with the offset of 200kHz.

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

l  Press [Marker], [Delta] and 200[kHz].


4)       Enable the noise marker function and observe the S/N:

Press [Marker] and [Noise Marker Off On]. As shown in Figure 1, the S/N reading is in dBc/Hz, as the noise value refers to the noise bandwidth normalized to 1Hz.

This value decreases by 10×log(BW). If you wish to obtain noise values under different channel bandwidth, the measurement result needs to be corrected based on current bandwidth. For example, if the reading of the spectrum analyzer is -85dBc/Hz and the channel bandwidth is bandwidth, S/N is:

S/N=85dBc/Hz - 10×log(30kHz) =40.2dBc/(30kHz)

If the differential marker is less than one fourth of the edge distance between the signal peak and response, errors may occur in noise measurement.

1-spectrum-analyzer-application.jpg

Figure 1 S/N Measurement

Noise measurement by noise marker function

In this example, the noise of 1Hz bandwidth is measured by the noise marker function, using the 1GHz external signal.


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, and enable the ON state of the radio frequency.


2)       Set the center frequency, span, reference level and attenuator.

l  Press [Reset].

l  Press [Frequency], [Center Frequency], 999.98[MHz]

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

l  Press [Amplitude], [Ref Level] and -10[dBm].

l  Press [Amplitude], [Atten Auto Man] and 40[dB].


3)       Activate the noise marker.

l  Press [Maker] and [Noise Marker Off On].

Note: The "Sample" mode of the detector will be enabled automatically. To obtain the noise power under different bandwidth, you can correct the current bandwidth based on 10×log(BW). For example, if the noise power within 1 kHz bandwidth is to be obtained, 10×log (1000) or 30 dB has to be added to the reading.


4)       Reduce the measurement error by increasing the sweep time:

Press [Sweep], [Sweep Time Auto Man] and 3[s].

In the "Average" mode of the detector, you can increase the sweep time so that the trace data are averaged in a longer interval, so as to reduce the measurement error.


        5)    Press [Maker] and rotate the knob on the front panel until the noise marker reading is 1GHz.

The noise marker value is calculated based on 5% of points on the whole sweep trace, with the marker location as the center. The noise marker will not be at the signal peak since such position has no enough trace points for calculation. Therefore, when the resolution bandwidth is narrow, the noise level will average trace points below the signal peak. As shown in Fig.2:

                     2-spectrum-analyzer-noise-measurement.jpg3-spectrum-analyzer-noise-marker.jpg

Fig. 2 Noise Measurement by Noise Marker Function

6)       Set the spectrum analyzer into the zero span mode, with the marker location as the center.

l  Press [Peak] and [Marker→Center].

l  Press [frequency], [Span] and [Zero Span].

l  Read the [Maker].

In this case, the amplitude reading of the noise marker is correct, as the averages of all points are at the same frequency, which is not affected by the shape of the resolution bandwidth filter. The noise marker is calculated based on the average of the interested frequency points. The power of discrete frequency points should be measured in the zero span mode, with the spectrum analyzer tuned to the interested frequency point.

 

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)