Basic knowledge of real time system

What's real time system?

MSA500 series signal analyzer offers both the real time system based on Fast Fourier Transform(FFT) and the conventional sweep system.

Sweep system

Graph:Sweep system

One RBW filter moves in the sweep range specified and then spectrum is displayed.

The speed at which the RBW filter moves is set by the sweep time.

If a spectrum doesn't exist the moment RBW filter comes to a position because the spectra change, that spectrum won't be observed like The red dotted line of the above figure.

Therefore, the sweep system basically treats a signal that spectrum does not change with This is called "steady signal". The signal which changes with time is called "unsteady signal". Typical example is the modulation signal.

Real time system

Graph:Real time system

Many filters (1024 filters in MSA500) are arranged in parallel.
Therefore, the signal in a certain period is simultaneously transformed into spectrum.

This is a reason called real time.

Since the filters equal to the frequency resolution (Called FFT bin) are arranged in parallel, the signal in a certain period is processed at the same time. The spectra won't be missed at all even if those change with time. Any "unsteady signal" such as modulation signal can be treated.

About fast Fourier transform

In the real time system, time domain signal is converted into frequency domain signal using Fourier transform.

Time domain signal f(t) ⇒ Frequency domain singnal F(ω)

Conversion equation

$$ F(ω)\int_{-∞}^∞ f(t)ε^{-jωt}dt $$

$ω=2πf$ ($f$:frequency)

The fast Fourier transform FFT realizes faster calculation by devising calculation algorithm of the above equation.

Window function

The integration to ∞ from -∞ is necessary as shown in the above equation, but the signal captured during a certain period is actually processed.

Image graph:Fourier transform

The caluclation of Fourier transform is performed as the signal captured during T seconds is repeated.
Then, discontinuity points come out as shown in the above figure.

The spurious are generated by discontinuity point as shown in the figure below.

Called Side lobe.

Graph:Side lobe

Of course, these side lobes are obstructive.
Window function is used to suppress these side lobes.

In order to eliminate the discontinuity, it is necessary that both first part and final part in the captured waveform are zero as shown in the figure below.

Graph:Window function

There are various kinds of window functions according to the purpose of use.

MSA500 has adopted 4-term Blackman-Harris window.

Shape of spectrum

In sweep system, the shape of a spectrum waveform is defined as shown in the following figure.

Shape of spectrum

In real time system, the RBW setting is not performed.

As shown in the figure of 4-term Blackman-Harris window function on the previous page, the shape of spectrum is the same as shown in the figure below in every span.

The bin expresses frequency resolution and every span consists from 602 bins (a part of 1024 bins by FFT operation).

  • ∴3dB width =(2/602)x(span)
  • And, shape factor=1:4(2bin:8bin)

As shown in the upper photograph, the shape is same on the screen in every span.

Span Δf(1bin) 3dB width(2bin)
20MHz 33.22kHz 66.2kHz
10MHz 16.61kHz 33.2kHz
5MHz 8.31kHz 16.6kHz
2MHz 3.32kHz 6.6kHz
1MHz 1.66kHz 3.3kHz
500kHz 831Hz 1.66kHz
200kHz 332Hz 662Hz
100kHz 166Hz 332Hz
50kHz 83.1Hz 166Hz
20kHz 33.2Hz 66Hz

Strong and weak points of real time system and sweep system

Real time system

Sweep system

Maximum span 20MHz

Although the maximum span in sweep system is as wide as 3.3GHz@MSA538/538TG/538E or 8.5GHz@MSA558/558E, the maximum span in real time system is 20MHz. This is decided by 3RD IF frequency and the sampling rate of A/D converter.

However, in wireless communications system, especially modulation analysis, since allowable bandwidth is 20MHz or less in almost all systems, it will be acceptable.

Maximum span 20MHz

What's I and Q?

Refer to the whole block diagram. The figure below shows from 3RD IF to IQ conversion in it.

image:The figure below shows from 3RD IF to IQ conversion in it.


  1. When the multiplication is executed in signal processing, the image won't be generated then.
    • Multiplication with real number $(f_A x f_B)$ Result:$fA+fB$と$fA-fB$(image)
    • Multiplication with complex number $(f_A x f_B)$ Result:$f_A+f_B$
  2. Time domain analysis can be performed by simple calculation as described.
    «Power vs. time», «frequency vs. time», «Phase vs. time», «IQ vs. time», «Q vs. I»
  3. If the input signal is a modulation wave, EVM or constellation can be calculated from I and Q data.

Bits of knowledge

Other favorite analyses

1. Time domain analysis

In real time mode, since 3RD IF which is analog signal is digitized by A/D converter and then is separated into I and Q, various time domain analyses are available.

Sampling frequencyfs=34MHz x (specified span/20MHz)

image:Time domain analysis
  1. Power vs. time
    • Power=(Vi2+Vq2)/50

      The ASK signal, which is appeared in burst and whose amplitude is digitally modulated, can be observed.

    • ASK modulation wave of ETC

      ASK modulation wave of ETC
  2. frequency vs. time
    • frequency=(φnn-1)/360Ts

      The signal wave modulated by frequency can be observed.

      • φn:Current phase
      • φn-1:Previous phase
      • Ts:Sampling rate(1/fs)
    • FM modulation wave

      FM modulation wave
  3. Phase vs. time
    • Phase=tan-1(Vq/Vi)

      It can be observed how the phase of the QPSK modulation wave changes over time.

    • Phase waveform of QPSK

      Phase waveform of QPSK
  4. I, Q vs. time
    • V axis:Vi and Vq, H axis:Time

      The time domain waveforms of I and Q of phase modulation such as QPSK can be observed directly. Two waveforms of Vi and Vq are displayed.

    • I and Q waveform of QPSK modulation

      I and Q waveform of QPSK modulation
  5. Q vs. I
    • V axis:Vq, H axis:Vi

      The raw constellation waveform can be observed. It does not include initial phase compensation and frequency difference compensation of digital phase modulation.

    • BPSK constellation

       BPSK constellation

2. Spectrogram analysis


3. OverWrite analysis


  • Observation of unnecessary spectrum which appears rarely

    The unnecessary spectrum (spurious) which disturbs a communication system may appear rarely. When span is wide, some spectra may be missed, but the probability of capturing spurious signal increases by setting the long accumulation time.

  • Observation of unnecessary spectrum which appears rarely

4. Modulation analysis

The modulation analysis can be accomplished by using the data separated to I and Q.

  • 16K frame IQ memory:MSA500 series has the large IQ memory of 16K frames (64M bytes).
  • USB interface:The data can be transferred as fast as 19ms/frame from IQ memory to PC through USB interface.
  • PC:The transcerred IQ data are stored in PC.
  • Modulation analysis software:Modulation analyses such as EVM measurement and constellation display
    *It is necessary to design PC software at the user side.

Concept of trigger

In the sweep system, there is no concept of trigger basically. Because it handles the steady signal. In the real time system, however, FFT processing is performed to the signal captured on time domain. That is, the signal can be captured with trigger ev erywhere.

Optimum for measurement of such unsteady signal as modulation wave which occurs in burst.

Image:Concept of trigger1

The capturing range of a waveform is determined by Trigger, Pre-trigger and Span.

Image:Concept of trigger2

1. Trigger

  1. Channnel power trigger
  2. Channnel power triggerSpan is equally divided into five channels (CH1 to CH5).
    When the instantaneous value of whole power in the specified channel crosses the trigger preset value, the trigger signal is generated. The slope of "rising" or "falling" can be also set. It is conv enient when acquiring the burst signal.

  3. Power tigger
  4. Power tiggerWhen the instantaneous value of whole power in the screen crosses the trigger preset value, the trigger signal is generated. The slope of "rising" or "falling" can be also set.

  5. IF level trigger
  6. IF level triggerWhen the level of IF signal (modulated with 17MHz) crosses the trigger preset value, the trigger signal is generated. The slope "rising" or “falling” is not available.

  7. External trigger
  8. The trigger signal is generated by the signal input to EXT TRIG connector. The input voltage range is from 1 to 10 Vp-p, and the frequency range is from DC to 5MHz. The slope of "rising" or "falling" can be a lso set.


By setting Pre-trigger, the signal before a trigger point is analyzable. When Pre-trigger is set to 0%, the signal after trigger point is captured. When being set to 50%, each 50% of signal after and before trigger point is captured. When being set to 100%, the signal before trigger point is captured. Five positions can be set 0% to 100% in 25% step.



The frame time depends on span and is decided by it.

Span Sampling rate Frame time
20MHz 34MHz 30.12μs
10MHz 17MHz 60.24μs
5MHz 8.5MHz 120.5μs
2MHz 3.4MHz 301.2μs
1MHz 1.7MHz 602.4μs
500kHz 850kHz 1.205ms
200kHz 340kHz 3.012ms
100kHz 170kHz 6.024ms
50kHz 85kHz 12.05ms
20kHz 34kHz 30.12ms

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