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university:courses:alm1k:circuits1:alm-cir-envelope-detector [24 Feb 2018 17:22] – [Envelope Detector] Doug Merceruniversity:courses:alm1k:circuits1:alm-cir-envelope-detector [16 Mar 2018 14:21] – add biased detector Doug Mercer
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 1 - 10 KΩ resistor (brown black orange)\\ 1 - 10 KΩ resistor (brown black orange)\\
 2 - 0.1uF capacitors (104)\\ 2 - 0.1uF capacitors (104)\\
-1 - 1N914 diode\\+1 - 1N914 diode
  
 ===== Envelope Detector ===== ===== Envelope Detector =====
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig1.png?500 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig1.png?500 |}}
  
-<WRAP centeralign> Figure 1. Envelope Detector Circuit </WRAP>+<WRAP centeralign>Figure 1, Envelope Detector Circuit </WRAP>
  
 The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series rectifies the incoming signal, allowing current flow only when the positive input terminal is at a higher potential than the negative input terminal.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig2.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig2.png?650 |}}
  
-<WRAP centeralign> Figure 2. Generated modulating and carrier signals</WRAP>+<WRAP centeralign>Figure 2, Generated modulating and carrier signals</WRAP>
  
 To generate the modulated signal we will use the Math function from the ALICE signal generator CH A. With the program paused, select the Math option under the Shape drop down menu. Enter the following equation which multiplies the captured modulation waveform from channel A with the captured carrier waveform from channel B. Because the signals are centered on 2.5 V that DC portion of the waveforms much be subtracted out. The 2.5 volt offset is then added back to after the multiplication to center the modulated signal in the 0 to 5 V range of the ALM1000. To generate the modulated signal we will use the Math function from the ALICE signal generator CH A. With the program paused, select the Math option under the Shape drop down menu. Enter the following equation which multiplies the captured modulation waveform from channel A with the captured carrier waveform from channel B. Because the signals are centered on 2.5 V that DC portion of the waveforms much be subtracted out. The 2.5 volt offset is then added back to after the multiplication to center the modulated signal in the 0 to 5 V range of the ALM1000.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig3.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig3.png?650 |}}
  
-<WRAP centeralign> Figure 3. Positive Half of the generated AM signal </WRAP>+<WRAP centeralign>Figure 3, Positive Half of the generated AM signal </WRAP>
  
 Use the vertical range and position controls to shift the traces so that they don not overlap each other. This make it easier to see the two waveforms. Without the capacitor connected, the circuit works like a positive half-wave rectifier that keeps the part of the signal that is above 2.5 V. Use the vertical range and position controls to shift the traces so that they don not overlap each other. This make it easier to see the two waveforms. Without the capacitor connected, the circuit works like a positive half-wave rectifier that keeps the part of the signal that is above 2.5 V.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig4.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig4.png?650 |}}
  
-<WRAP centeralign> Figure 4. Filtered demodulated signal </WRAP>+<WRAP centeralign>Figure 4, Filtered demodulated signal </WRAP>
  
 The resulting demodulated signal is the envelope of the positive half wave obtained previously. It is actually the 100 Hz message signal with some 10 KHz ripple on it. The resulting demodulated signal is the envelope of the positive half wave obtained previously. It is actually the 100 Hz message signal with some 10 KHz ripple on it.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig5.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig5.png?650 |}}
  
-<WRAP centeralign> Figure 5. Carrier and modulating signal spectrum</WRAP>+<WRAP centeralign>Figure 5, Carrier and modulating signal spectrum</WRAP>
  
 As we can see we have the single peak at 100 Hz of the modulating signal on CH-A and the single peak at 10 KHz of the carrier signal on CH-B. As we can see we have the single peak at 100 Hz of the modulating signal on CH-A and the single peak at 10 KHz of the carrier signal on CH-B.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig6.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig6.png?650 |}}
  
-<WRAP centeralign> Figure 6. Modulated Carrier signal spectrum</WRAP>+<WRAP centeralign>Figure 6, Modulated Carrier signal spectrum</WRAP>
  
 With the filter capacitor, if we look at the spectrum of the demodulated signal after the envelop detector we see the large peak at 100 Hz and a greatly attenuated peak at the 10 KHz carrier frequency as shown in figure 7. With the filter capacitor, if we look at the spectrum of the demodulated signal after the envelop detector we see the large peak at 100 Hz and a greatly attenuated peak at the 10 KHz carrier frequency as shown in figure 7.
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 {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig7.png?650 |}} {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig7.png?650 |}}
  
-<WRAP centeralign> Figure 7. Filtered Demodulated signal after envelope detector spectrum</WRAP>+<WRAP centeralign>Figure 7, Filtered Demodulated signal after envelope detector spectrum</WRAP> 
 + 
 +===== Biased Envelope Detector ===== 
 + 
 +The simple diode based envelope detector of figure 1 does not well or at all if the amplitude i.e. Swing is less than the forward turn voltage of the diode. It will suffer significant distortion 1n the negative half of the modulation signal for high modulation indexes (near 100%) when the diode is not fully turned on. A way around this limitation is to introduce a small DC bias to the diode. This small bias current moves to quiescent operating point of the circuit to right at the turn on point of the diode. 
 + 
 +==== Materials: ===== 
 + 
 +ADALM1000 Active Learning Module\\ 
 +Solder-less breadboard, and jumper wire kit\\ 
 +1 - 1.5 KΩ resistor (brown green red)\\ 
 +1 - 10 KΩ resistor (brown black orange)\\ 
 +1 - 20 KΩ resistor (red black orange)\\ 
 +1 - 0.22uF capacitor (224)\\ 
 +1 - 1.0uF capacitor\\ 
 +1 - 2N3904 NPN transistor\\ 
 +1 - 1N914 diode 
 + 
 +On your solder-less breadboard construct the biased envelope detector circuit as shown in figure 8. 
 + 
 +{{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig8.png?550 |}} 
 + 
 +<WRAP centeralign>Figure 8, biased envelope detector</WRAP> 
 + 
 +Use the same modulated signal you used in the simple diode envelope detector example. 
 + 
 +**For Further Reading:** 
 + 
 +**Return to ALM Lab Activity Table of Contents** 
 + 
university/courses/alm1k/circuits1/alm-cir-envelope-detector.txt · Last modified: 03 Jan 2021 22:12 by Robin Getz

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