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TE4201-Communication Electronics 1 9. SSB Demodulation and Receivers SSB demodulation SSB demodulationSSB demodulationSSB demodulation BFO drift effect.

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Presentation on theme: "TE4201-Communication Electronics 1 9. SSB Demodulation and Receivers SSB demodulation SSB demodulationSSB demodulationSSB demodulation BFO drift effect."— Presentation transcript:

1 TE4201-Communication Electronics 1 9. SSB Demodulation and Receivers SSB demodulation SSB demodulationSSB demodulationSSB demodulation BFO drift effect BFO drift effectBFO drift effectBFO drift effect SSB receivers SSB receiversSSB receiversSSB receivers

2 TE4201-Communication Electronics 2 SSB demodulation SSB wave - envelope, frequency and amplitude Medium signal amplitude High signal amplitude Low signal amplitude Information signal DSB AM wave f C -f i, f C, f C +f i Suppressed Carrier DSB AM wave f C -f i, f C +f i Suppressed Carrier SSB AM wave f C +f i (USB)

3 TE4201-Communication Electronics 3 Demodulation of SSB wave – Beat Frequency Oscillator Mixer (Demodulator) Low-pass filter Mixer output Carrier frequency generator Beat Frequency Oscillator SSB input f c + f i Oscillator f c fcfc fcfc f c + f i + f c f c + f i – f c = f i f c + f i Low-pass filter passes all LF (intelligent frequencies) Low-pass filter rejects all RF (high frequencies) Intelligence output fifi

4 TE4201-Communication Electronics 4 BFO drift effect In SSB demodulation, Carrier freq. generator has to be accurate or no error so that the original information could detected. The beat freq. produced by inaccurate carrier generator will produce false information. In SSB demodulation, Carrier freq. generator has to be accurate or no error so that the original information could detected. The beat freq. produced by inaccurate carrier generator will produce false information. Suppose original information is 1kHz. signal, and that USB is used, then (f C +1k) USB will beat with f C to get (f C +1k) – f C = 1k (original information) Suppose original information is 1kHz. signal, and that USB is used, then (f C +1k) USB will beat with f C to get (f C +1k) – f C = 1k (original information) Suppose f C generator drift to f C -15Hz then for the above case (f C +1k) USB will beat with a new drifted carrier (f C -15Hz) to get (f C +1k) - (f C -15Hz) = 1k+15Hz=1015Hz (wrong information) Suppose f C generator drift to f C -15Hz then for the above case (f C +1k) USB will beat with a new drifted carrier (f C -15Hz) to get (f C +1k) - (f C -15Hz) = 1k+15Hz=1015Hz (wrong information) Suppose f C generator drift to (f C +15Hz) then for the above case (f C +1k) USB will beat with drifted new drifted carrier (f C +15Hz) to get (f C +1k) - (f C +15Hz) = 1k-15Hz=985Hz (wrong information) Suppose f C generator drift to (f C +15Hz) then for the above case (f C +1k) USB will beat with drifted new drifted carrier (f C +15Hz) to get (f C +1k) - (f C +15Hz) = 1k-15Hz=985Hz (wrong information) Drift effect in SSB demodulation thus produce an original sound to become like Donald Duck sound (funny sound) Drift effect in SSB demodulation thus produce an original sound to become like Donald Duck sound (funny sound)

5 TE4201-Communication Electronics 5 Original SSB music consists of the sum of: 256Hz (fundamental)+512Hz(second harmonic)+1024Hz(fourth harmonic) Carrier generator of SSB demodulator drift  5Hz Find components of the new music result by BFO drift effect. Assume USB. (f C +256Hz) USB will beat with drifted new drifted carrier (f C +5Hz) to get (f C +256Hz) - (f C +5Hz) = 256-5Hz=251Hz (fundamental) (f C +512Hz) USB will beat with drifted new drifted carrier (f C +5Hz) to get (f C +512Hz) - (f C +5Hz) = 512-5Hz=507Hz(second harmonic) (f C +1024Hz) USB will beat with drifted new drifted carrier (f C +5Hz) to get (f C +1024Hz) - (f C +5Hz) = 1024-5Hz=1019Hz (fourth harmonic) When drifted to +5Hz, component of new music will be 251Hz (fundamental)+507Hz(second harmonic)+1019Hz(fourth harmonic) Similarly when drifted to –5Hz, component of new music will be 261Hz (fundamental)+517Hz(second harmonic)+1029Hz(fourth harmonic) Example 4-3 text

6 TE4201-Communication Electronics 6 SSB receivers Mixer 1 2001k- 2003k Local oscillator RF amplifier 2001k- 2003k 452k- 454k IF amplifier 455k 452k- 454k Mixer 2 Reinserted carrier BFO 455k 1k- 3k Audio amplifier 1k- 3k 2455k f O =f C +f i f O =2000k+455k USB with f C =2000k fifififi

7 TE4201-Communication Electronics 7 Example Draw the block diagram of a SSB receiver which has outputs (information signals) 1kHz and 3kHz. The suppressed carrier at the transmitter is 1MHz and LSB is used. Determine the exact frequencies at all stages if IF used is 465kHz. Mixer 1 997k- 999k Local oscillator 1465k RF amplifier 997k- 999k 466k- 468k IF amplifier 465k 466k- 468k Mixer 2 Reinserted carrier BFO 465k 1k- 3k Audio amplifier 1k- 3k LSB with f C =1000k f O =f C +f i f O =1000k+465k fifififi HW : Draw the block diagram of a SSB receiver having an IF freq of 455kHz. Input to the receiver is a band of LSB freq. from 1997kHz to 1999kHz. Indicate all the frequencies at the receiver blocks if BFO>IF but Local oscillator has f 0 < LSB.


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