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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 [17 Mar 2018 14:28] – more biased detector stuff Doug Mercer
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 ===== Objective: ===== ===== Objective: =====
  
-In this lab activity we will use the [[http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/adalm1000.html|ADALM1000]] to introduce amplitude modulation (AM) and envelope detection demodulation. A signal's envelope is equivalent to its outline, and an envelope detector connects the amplidude peaks of the signal. Envelope detection has numerous applications in the fields of signal processing and communications, one of which is amplitude modulation (AM) detection or demodulation.+In this lab activity we will use the [[http://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/adalm1000.html|ADALM1000]] to introduce amplitude modulation (AM) and envelope detection demodulation. A signal's envelope is equivalent to its outline, and an envelope detector connects the amplitude peaks of the signal. [[university:courses:electronics:text:chapter-7#envelope_detector|Envelope detection]] has numerous applications in the fields of signal processing and communications, one of which is amplitude modulation (AM) detection or demodulation.
  
 Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio frequency carrier wave. In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels.  Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio frequency carrier wave. In amplitude modulation, the amplitude (signal strength) of the carrier wave is varied in proportion to the waveform being transmitted. That waveform may, for instance, correspond to the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels. 
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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, C<sub>1</sub> (224)\\ 
 +1 - 1.0uF capacitor, C<sub>2</sub>\\ 
 +1 - 2N3904 NPN transistor\\ 
 +1 - 1N914 diode 
 + 
 +On your solder-less breadboard construct the biased envelope detector circuit as shown in figure 8. The amplitude modulated signal is AC coupled into the Base of NPN transistor Q<sub>1</sub> which is configured as an emitter follower. Voltage divider R<sub>1</sub> and R<sub>2</sub> along with diode D<sub>1</sub> act to set the DC bias point of the AC coupled input ([[university:courses:electronics:text:chapter-7#diode_clamp|DC restoration]]). Absent any modulated input the DC quiescent operating point seen at the emitter of Q<sub>1</sub> will be the voltage at the junction of R<sub>1</sub> and R<sub>2</sub> minus the diode drop of D<sub>1</sub> and the V<sub>BE</sub> of Q<sub>1</sub>. The base current of Q<sub>1</sub> flows in diode D<sub>1</sub> forward biasing it. During the positive half cycles of the modulated input D<sub>1</sub> turns off and the input signal peaks charge filter capacitor C<sub>2</sub>. During the negative half cycles of the input signal transistor Q<sub>1</sub> turns off and D<sub>1</sub> turns on harder supplying the input current. 
 + 
 +{{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig8.png?550 |}} 
 + 
 +<WRAP centeralign>Figure 8, biased envelope detector</WRAP> 
 + 
 +To test this circuit first use the same modulated signal you used in the simple diode envelope detector example. Compare the new design to the simple diode envelope detector. Using the same steps as earlier generate AM signals with smaller amplitudes / higher modulation indexes and compare the outputs of these two detector designs. 
 + 
 +**For Further Reading:** 
 + 
 +  * [[http://www.analog.com/en/technical-articles/integrated-diode-based-rf-detectors.html|Understanding, Operating, and Interfacing to Integrated Diode-Based RF Detectors]] 
 +  * [[http://www.analog.com/media/en/technical-documentation/application-notes/AN-423.pdf|Amplitude Modulation of the AD9850 Direct Digital Synthesizer]] 
 +  * [[http://www.analog.com/en/analog-dialogue/raqs/raq-issue-92.html|Multipliers and Modulators]] 
 +  * [[http://www.analog.com/media/en/technical-documentation/data-sheets/ADL5511.pdf|Envelope and TruPwr RMS Detector 
 +]] 
 + 
 +**Return to ALM Lab Activity [[university:courses:alm1k:alm-labs-list|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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