Swept-tuned spectrum analyzer. Gianfranco Miele, Ph.D

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1 Swept-tuned spectrum analyzer Gianranco Miele, Ph.D

2 Envelope detector Spectrum analyzers typically convert the IF signal to video with an envelope detector. In its simplest orm, an envelope detector consists o a hal wave rectiier circuit V in R C V out Due to envelope detection, the phase inormation o the input signal gets lost, so that only the magnitude can be indicated in the display.

3 Envelope detector The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal alls. The diode in series rectiies the incoming signal, allowing current low only when the positive input terminal is at a higher potential than the negative input terminal.

4 Envelope detector

5 Envelope detector V in R C V out Full-wave rectiier

6 Envelope detector

7 Video ilter To reduce the eect o noise on the displayed signal amplitude and smooth the trace a video ilter is used. It is a low-pass ilter that comes ater the envelope detector and determines the bandwidth o the video signal (video bandwidth VBW). Similar or the resolution bandwidth, the video bandwidth also limits the maximum permissible sweep speed. In act a ilter with a non-zero bandwidth has a settling time with k v 1. τ = k v VBW

8 RBW, VBW and sweep time In order to have a reliable response the signal must be in the video ilter bandwidth or a time Being the sweep speed t τ = k v VBW v = span t sweep The time that the signal is in the IF ilter bandwidth is t = RBW v = RBW span t sweep

9 RBW, VBW and sweep time As a consequence, t sweep k VBW RBW span This means that a change in video bandwidth has a dramatic eect on sweep time. Consider that the degree o reduction (degree o averaging or smoothing) is a unction o the ratio o the video to resolution bandwidths. At ratios o 0.01 or less, the smoothing is very good. Consequently in order to obtain a very smoothed trace, we must wait more time.

10 Video ilter From C. Rauscher "Fundamentals o Spectrum Analysis" Rohde&Schwarz VBW=1 MHz VBW=300 Hz

11 In a precious lecture we learnt that in order to reject image requencies a high IF value and a low-pass ilter are used. Adopting this solution is not possible to realize a IF ilter with a narrow bandwidth. A possible solution could be the adoption o a second downconversion stage.

12 RF input section IF section x(t) RF Att. LPF x IF1 IF ampliier IF 1 ilter IF2 IF 2 ilter LO1 LO2 Re. Osc. 1 st LO 2 nd LO Saw-tooth wave generator Display Video ilter Envelope detector Logarithmic ampliier Video section

13 x IF1 LO1 x =100 MHz IF1 =3.4 GHz LO1 =3.5 GHz

14 IF1 LO1 IF2 =20 MHz IF2

15 Evaluating the tuning equations at the second stage it is possible to identiy the requency o the second local oscillator to be applied in order to downconvert IF1 at IF2. IF2 = LO2 ± IF1 = LO2 + IF1 LO2 IF1 IF1 LO2 LO2 =3.42 GHz LO2 =3.38 GHz

16 IF1 LO1 IF2 LO2

17 IF1 LO1 IF2 LO2

18 x1 x2 IF1 LO1 IF1 =3.4 GHz LO1 =3.5 GHz x1 =100 MHz x1 =140 MHz

19 3.36 GHz IF2 =20 MHz IF1 LO1 IF2

20 = 2 IF2 IF1 LO1 Image requency at the 2 nd stage IF2 LO2

21 We need more stages IF1 LO1 IF2 LO2

22 RF input section IF section 2 nd LO x(t) RF Att. LPF x LO1 IF1 IF ampliier IF 1 ilter LO2 IF2 IF 2 ilter Re. Osc. 1 st LO 3 rd LO LO3 IF3 IF 3 ilter Saw-tooth wave generator Display Video ilter Envelope detector Logarithmic ampliier Video section

23 x x2 IF1 LO1 x =100 MHz IF1 =3.4 GHz LO1 =3.5 GHz x1 =140 MHz

24 IF1 LO1 IF2 IF2 =300 MHz

25 Evaluating the tuning equations at the second stage it is possible to identiy the requency o the second local oscillator to be applied in order to downconvert IF1 at IF2. IF2 = LO2 ± IF1 = LO2 + IF1 LO2 IF1 IF1 LO2 LO2 =3.7 GHz LO2 =3.1 GHz

26 IF1 LO1 IF2 LO2 Because o 2 IF2 =600 MHz, the image requency o x1 is equal to 700 MHz. It can be rejected by IF 1 ilter, because ilters that are centered at 3.4 GHz with a bandwith narrower than 600 MHz are physically implementable

27 IF1 LO1 IF2 LO2

28 IF1 LO1 IF2 IF3 =20 MHz LO2 IF3

29 Evaluating the tuning equations at the second stage it is possible to identiy the requency o the second local oscillator to be applied in order to downconvert IF2 at IF3. IF3 = LO3 ± IF2 = LO3 + IF2 LO3 IF2 IF2 LO3 LO3 =320 MHz LO3 =280 MHz

30 IF1 LO1 IF2 LO2 IF3 LO3

31 IF1 LO1 IF2 LO2 IF2 LO3

32 Image requency at the 3 rd stage is rejected by IF 2 ilter IF1 LO1 IF2 LO2 IF3 LO3

33 Full-tuning equation The tuning equations applied at each stage are IF1 = LO1 x IF2 = IF1 LO2 IF3 = IF2 LO3 It is possible to state that IF3 = LO1 LO2 + LO3 + x This equation is called ull tuning equation

34 Zero span I we ix the requency o the LO so that our analyzer is tuned to a ixed requency, so the displayed span is 0 Hz (zero span).

35 Zero span

36 Zero span

37 Zero span

Swept-tuned spectrum analyzer. Gianfranco Miele, Ph.D

Swept-tuned spectrum analyzer. Gianfranco Miele, Ph.D Swept-tuned spectrum analyzer Gianfranco Miele, Ph.D www.eng.docente.unicas.it/gianfranco_miele g.miele@unicas.it Video section Up until the mid-1970s, spectrum analyzers were purely analog. The displayed