HF2 FFT Spectrum Analyzer / Zoom FFT

The HF2 FFT Spectrum Analyzer is a standard feature of all HF2 Instruments, fully integrated inside the ziControl graphical user interface. It builds on a technique also called Zoom FFT, which is the numerical calculation of the FFT spectrum on samples demodulated by the lock-in amplifier. This postprocessing of demodulated samples can be applied to the output of all demodulators inside the HF2 Instruments.

Key Features

  • Very-high frequency resolution FFT spectrum analyzer on demodulated samples
  • Increases resolution of FFT analysis by orders of magnitude compared to FFT performed on raw data
  • Wide frequency span selection range
  • Performance specification comparable or better to standalone spectrum analyzers costing thousands - Technical Note
  • Included into standard ziControl software delivery

The performance of the HF2 FFT Spectrum Analyzer is amazing, as it delivers an extremely high resolution output compared to an FFT performed on raw data before demodulation. As the resolution of an FFT spectrum is determined only by the total length of time spanned by the samples (and not by the sample rate), it is a numerically challenging calculation if performed on a data stream at high-frequency (it would be 210 MSamples/s for the HF2LI). Instead, if performed on demodulated samples, the length of time can be comfortably increased and the resulting resolution is impressive.

The method applied by the HF2 FFT Spectrum Analyzer is called Zoom FFT as it represents a detailed spectrum on a relatively narrow band signal (after demodulation). In communication technology this method is called multi-rate filtering.

Objectives

  • Precise analysis of any periodically modulated data source
  • Accurate measurements of a system's thermal noise
  • Simple characterization of the dynamics in a demodulated signal

Specifications

FFT span window range 0.22 Hz to 460 kHz
FFT frequency resolution (minimal) 7 µHz to 14 Hz
FFT spectrum plot lines up to 32'768 lines per plot
amplitude accuracy ± 0.2 dB
overall phase noise over all carriers -125 dBc/Hz @ 1 kHz
spurious free dynamic range -80 dB typical
display units VRMS, VRMS^2, Vpk^2, VRMS/VHz, Vpk/VHz, Vrms^2/Hz, Vpk^2/Hz

FFT Span Window vs. Frequency Resolution

The selection of the span window automatically determines the maximum achievable frequency resolution.

FFT Span Window Minimal Frequency Resolution
460 kHz 14 Hz
100 kHz 3 Hz
10 kHz 300 mHz
1 kHz 30 mHz
100 Hz 3 mHz
10 Hz 300 µHz
1 Hz 30 µHz
0.22 Hz 7 µHz

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