ANALOG SPECTRUM ANALYSIS

TOPICS Bank-of-filters spectrum analyzer Sequential spectrum analyzer –Variable tuning filter spectrum analyzer –Superheterodyne spectrum analyzer

BANK-OF-FILTERS SPECTRUM ANALYZER

Measurement principle A ff 1 f 2 Parallel filters f c RBW

Main design choices Fixed passband filters Minimum overlap between the frequency responses of two adjacent filters Detectors for measuring the amplitude of the “sinusoidal” output of the filters

Basic architecture Trigger pulses Y X s(t)s(t) Detector MUXMUX Sweep generator Display Filter 1 Filter 2 Filter n

Frequency resolution It is strictly dependent on RBW (resolution bandwidth) value Minimum frequency separation between two spectral components having the same amplitude to be correctly detected and discriminated.

Advantages Straightforwardness (only in case of few filters) Reduced measurement time Real-time analysis Suitable to any kind of signal

Drawbacks The better (the smaller) the frequency resolution, the more complex the architecture. Frequency interval = 0 Hz – 100 kHz Resolution = Δf =100 Hz Example # filters = M == 1000 !

SEQUENTIAL SPECTRUM ANALYZER (Variable tuning filter spectrum analyzer)

Measurement principle A ff 1 f 2 A single filter moving along the frequency axis

Single passband filter with adjustable central frequency Complete coverage of the frequency interval of interest Detector for measuring the envelope of the filter output signal Main design choices

s(t) Envelope detector Basic architecture Variable tuning filter Ramp generator Display

Why the envelope detector? Thanks to the filter movement, the envelope of its output signal precisely describes the evolution of its frequency response.

Advantages Single filter Straightforward architecture Drawbacks Only stationary signals It is difficult to assure a constant frequency resolution

SEQUENTIAL SPECTRUM ANALYZER (Superheterodyne spectrum analyzer)

Measurement principle A f f IF Single fixed filter (IF Filter, f IF ) Moving spectrum

Single passband filter with constant central frequency Mixer implementing the superheterodyne technique Detector for measuring the envelope of the filter output signal Main design choices

Superheterodyne technique (ideal case) IN LO IF f sig f lo Mixer f sig f sig - f lo f sig + f lo IF Filter, f IF f lo f lo - f sig f lo + f sig IF Filter, f IF

Tuning equation Ambiguity f lo being equal and f sig 2 > f sig 1 if f sig 2 image frequency of f sig 1 Superheterodyne technique (ideal case)