SUPERHETERODYNE SPECTRUM ANALYZER

TOPICS Superheterodyne spectrum analyzer – Basic architecture – Frequency resolution – Sweep time – Video section

BASIC ARCHITECTURE

Superheterodyne Technique Mixer: real case Output frequencies = m and n integer, zero included f sig, f lo are present in output For reliable measurements : Input signal must not contain f IF !

f IF Operating choices along the frequency axis Frequency range of the oscillator f in min f in max f lo min f lo max f IF Analysis range IF Filter f sig x f lo x f IF

Operating choices along the frequency axis Frequency range of the oscillator f in min f in max f lo min f lo max f IF Analysis range IF Filter f sig x f lo x Image frequency range f img min f img max f img x f IF Input filter

Basic architecture s(t)s(t) Low-pass filter Voltage controlled oscillator (VCO) Reference oscillator (LO) Ramp generator Mixer IF filter f IF Envelope detector Video filter Logarithmic amplifier Input attenuator Log IF amplifier IF Attenuator

Example of measured spectrum

FREQUENCY RESOLUTION

Frequency resolution f IF s(t)s(t) RBW - 3 dB Display RBW

Frequency resolution -3 dB 10 kHz 10 kHz RBW

-3 dB - 60 dB BW - 60 dB BW - 3 dB BW Selectivity = Selectivity

Resolution vs Selectivity RBW = 10 kHz RBW = 1 kHz Selectivity 15: dB BW = 15 kHz -3 dB -60 dB 10 kHz

SWEEP TIME

Sweep Time (ST) Time taken to scan the frequency range of interest (span) Time interval covered by the ramp provided by the generator v sw, sweep speed

Sweep Time , IF filter response time t, time during which a spectral component remains within the IF filter bandwidth RBW t

Sweep Time To carry out reliable measurements ST is in inverse relation to the square of RBW !

Sweep Time If the relation is not satisfied…. Too rapid sweep

VIDEO SECTION

Envelope detector s(t)s(t)

Video filter s(t)s(t) before after