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| university:courses:alm1k:circuits1:alm-cir-envelope-detector [16 Mar 2018 14:21] – add biased detector Doug Mercer | university:courses:alm1k:circuits1:alm-cir-envelope-detector [20 Mar 2018 16:42] – [Biased Envelope Detector] Antoniu Miclaus |
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| ===== Objective: ===== | ===== Objective: ===== |
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| 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. |
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| 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. |
| ===== Biased Envelope Detector ===== | ===== Biased Envelope Detector ===== |
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| 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. | 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 on 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. |
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| ==== Materials: ===== | ==== Materials: ===== |
| 1 - 10 KΩ resistor (brown black orange)\\ | 1 - 10 KΩ resistor (brown black orange)\\ |
| 1 - 20 KΩ resistor (red black orange)\\ | 1 - 20 KΩ resistor (red black orange)\\ |
| 1 - 0.22uF capacitor (224)\\ | 1 - 0.22uF capacitor, C<sub>1</sub> (224)\\ |
| 1 - 1.0uF capacitor\\ | 1 - 1.0uF capacitor, C<sub>2</sub>\\ |
| 1 - 2N3904 NPN transistor\\ | 1 - 2N3904 NPN transistor\\ |
| 1 - 1N914 diode | 1 - 1N914 diode |
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| On your solder-less breadboard construct the biased envelope detector circuit as shown in figure 8. | 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. |
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| {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig8.png?550 |}} | {{ :university:courses:alm1k:circuits1:alm-cir-envel-det-fig8.png?550 |}} |
| <WRAP centeralign>Figure 8, biased envelope detector</WRAP> | <WRAP centeralign>Figure 8, biased envelope detector</WRAP> |
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| Use the same modulated signal you used in the simple diode envelope detector example. | 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. |
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| **For Further Reading:** | **For Further Reading:** |
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| **Return to ALM Lab Activity Table of Contents** | * [[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 |
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| | **Return to ALM Lab Activity [[university:courses:alm1k:alm-labs-list|Table of Contents]]** |
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