EEVblog #845 – Oscilloscope FFT Comparison

Dave compares the FFT modes on 7 different oscilloscopes:
Rohde & Schwarz HMO1202 Series
Tektronix MDO3000
Keysight 3000X Touch
Lecroy WaveJet 354 Touch
GW Instek GDS-1104B
Rigol DS1054Z
Rigol DS2000
Which is the best?
Which one sucks the most?
Forum HERE

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  1. I like the PicoScope 4227 (under US $1500). This is a 12bit 100MHz scope with great flexibility. I have uploaded an example of a plot of a 120Vac, 60Hz waveform with a 15A load having a power factor of 0.83. Both voltage and current are displayed in time and frequency domains. The waveform was captured with 10M samples and the FFT is calculated using 1.048 million bins. The resulting display was changed to a logarithmic scale, making it easier to display all of the harmonics. While working with this waveform, I was using a 22 inch monitor – a great advantage. Try this with one of the reviewed scopes.

  2. For the spectrum analysis, comparing with Spectrum Analyzer and Digital oscilloscope, the hardware structure are quite different: the Spectrum Analyzer is a narrow band instrument, could provide <100dB dynamic range easily; the digital scope is a wide band instrument, it's helpful and useful to do the spectrum analysis with FFT also, some points just for reference:

    1. Dynamic range of Digital oscilloscopes with FFT function

    The dynamic range of Digital oscilloscopes with FFT function is limited by ADC and nonlinear responses from the front-end amplifiers , the ADC could also produce harmonic distortions they will be shown up as artifacts in the FFT spectrum, you need to distinguish the spectral information is from the signal or from oscilloscope.

    Digital oscilloscopes usually use 8 bits ADC inside, all of the ADC has effective bits , it is less than 8 bits, maybe around 6 to 7 bits, it's a measure of the signal-to-noise ratio (SNR). The SNR can be calculated from effective bits using the equation:
    SNR = Effective Bits * 6.02 + 1.8 dB
    For an ideal 8-bit ADC, it has an SNR of 50 dB.

    The Spectrum Analyzer is a narrow band instrument, could provide <100dB, even -150dB dynamic range easily.

    2.Frequency resolution of Digital oscilloscopes with FFT function

    If want to distinguis two closed signal, we need higher frequency resolution bandwidth,in order to achieve higher RBW, we need more data points to do the FFT caculation, more data calculation cost longer time, this is the relationship.
    For example, if the sampling rate of the scope is 1 G Sa/s,

    1) if the data points is 100Kpts, the Min. RBW will be: 1 G Sa/s / 100Kpts = 10 kHz, you could see the two signals separated more than 10KHz;

    2) if the data points is 1Mpts, the Min. RBW will be: 1 G Sa/s / 1Mpts = 1 kHz, you could see the two signals separated more than 1KHz;

    3) if the data points is 10Mpts, the Min. RBW will be: 1 G Sa/s / 10Mpts = 100 Hz, you could see the two signals separated more than 100Hz;

    For the economic level Spectrum Analyzer such as DSA815 or DSA832 , it's easy to provide 100Hz or even 10Hz RBW.

    3. High resolution FFT real tests with RIGOL DS1054Z

    I use a PC software and RIGOL DS1054Z to do the high resolution FFT calculation as below, I could use the whole memory 12Mpts of DS1054Z to do the FFT, so the RBW could be 83.3Hz, much higher resolution, it's very easy to distinguish the signals separated by ~100Hz, of course, it's very easy to see the FM signal in this video which has the sidebands seprated by 5KHz.

    If need highed RBW, the PC based FFT Software + Deep memory Scope is a good solution.

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