1. SUPERHETERODYNE SPECTRUM ANALYZER

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

3. BASIC ARCHITECTURE

4. 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 !

5. 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

6. 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

7. 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

8. Example of measured spectrum

9. FREQUENCY RESOLUTION

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

11. Frequency resolution -3 dB 10 kHz 10 kHz RBW

12. -3 dB - 60 dB BW - 60 dB BW - 3 dB BW Selectivity = Selectivity

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

14. SWEEP TIME

15. 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

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

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

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

19. VIDEO SECTION

20. Envelope detector s(t)s(t)

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