This version is outdated by a newer approved version.
This version (02 Mar 2018 12:28) was approved by Antoniu Miclaus.The Previously approved version (28 Feb 2018 14:04) is available.
This version (02 Mar 2018 12:28) was approved by Antoniu Miclaus.The Previously approved version (28 Feb 2018 14:04) is available.This is an old revision of the document!
Table of Contents
Activity: Envelope Detector
Objective
In this lab activity we will use ADALM2000 and Scopy to introduce envelope detection and amplitude modulation. The signal's envelope is equivalent to its outline, and an envelope detector connects all the peaks in this signal. Envelope detection has numerous applications in the fields of signal processing and communications, one of which is amplitude modulation (AM) detection.
Amplitude modulation (AM) is a modulation technique used in electronic communication, most commonly for transmitting information via a radio 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.
A typical amplitude modulated signal has the following equation:
where:
- message signal
- carrier signal
- k - modulation index (typically varies between 0 and 1)
- ωm - message frequency
- A - carrier amplitude
- ωc - carrier frequency
An envelope detector is an electronic circuit that takes a high-frequency signal as input and provides an output which is the envelope of the original signal. (ωc » ωm)
It consists of two main elements:
- Diode / rectifier - serves to enhance one half of the received signal over the other.
- Low pass filter - required to remove the high frequency elements that remain within the signal after detection / demodulation. The filter usually consists of a very simple RC network but in some cases It can be provided simply by relying on the limited frequency response of the circuitry following the rectifier.
Materials
ADALM2000 Active Learning Module
Solder-less breadboard, and jumper wire kit
2 - 1 KΩ resistor
2 - 1uF capacitor
2 - 1N914 diode
Envelope Detector
Background
Consider the circuit presented in Figure 1.
Figure 1. Basic Envelope Detector Circuit
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.
Hardware Setup:
Build the following breadboard circuit for the envelope detector circuit.
Figure 2. Envelope Detector breadboard circuit
Procedure
Use the first waveform generator as source to provide the AM signal, with the following parameters:
- k = 0.5
- ωc = 10KHz
- ωm = 100Hz
- A = 3
To generate the AM signal use the math function from the Scopy signal generator. Set the main frequency to 100Hz and apply the following function: (1+0.5*cos(t))*3*cos(100*t). The generated waveform is presented in Figure 3.
Figure 3. Generated AM signal
Configure the scope so that output signal is displayed on channel 1.
Disconnect the capacitor from the circuit and observe the output signal. A plot example is presented in Figure 4.
Figure 4. Positive Half of the generated AM signal
Without the capacitor connected, the circuit works like a positive half-wave rectifier that keeps the part of the signal that is above 0V.
Now connect the capacitor back to the circuit. A plot example is presented in Figure 5.
Figure 5. Positive half wave Envelope
The obtained signal is the envelope of the positive half wave obtained previously. It is actually the 100 Hz message signal with some 10 KHz variations on it.
Frequency Domain Spectrum
We can also view these signals in the frequency domain using the Spectrum Analyzer tool. First we can look at the 10 KHz carrier and 100 Hz message signals together. Enable channel 1 and set the sweep range from 10 Hz to 15kHz. Run a single sweep. Enable markers 1 and 2 from the Markers tab and the Marker Table. Move each marker using “Prev Peak”, “Next Peak” to set them on the carrier and the message signal. A plot example is presented in Figure 6.
Figure 6. Message and carrier signals
Set the Sweep range from 9 kHz to 11kHz. In Figure 7, the main peak is at the 10 KHz carrier frequency and there are the modulation side-bands +/- 100 Hz on either side of the carrier. (i.e. 9900 Hz and 10100 Hz).
Figure 7. Demodulated Carrier signal spectrum
Set the Sweep range from 20Hz to 180Hz. In Figure 8, the main peak is at the 100Hz message frequency.
Figure 8. Deodulated message signal spectrum
Extended Envelope Detector
Background
Consider the circuit presented in Figure 9.
Figure 9. Positive and negative Envelope Detector Circuit
A similar circuit is added to the circuit in Figure 1 , the only difference being that the diode is reversed, allowing the negative voltages to pass through the RC circuit.
Hardware Setup:
Build the following breadboard circuit for the extended envelope detector circuit.
Figure 10. Extended Envelope Detector breadboard circuit
Procedure:
Use the first waveform generator as source to provide the AM signal, with the following parameters:
- k = 0.5
- ωc = 10KHz
- ωm = 100Hz
- A = 3
To generate the AM signal use the math function from the Scopy signal generator. Set the main frequency to 100Hz and apply the following function: (1+0.5*cos(t))*3*cos(100*t). The generated waveform is presented in Figure 11. (with 5 displayed periods)
Figure 11. Generated AM signal
Configure the scope so that output signal is displayed on channel 1.
Disconnect the capacitors C1 and C2 from the circuit and observe the output signal. A plot example is presented in Figure 12.
Figure 12. Positive Half and Negative Half of the generated AM signal
Without the capacitors connected, the circuit works like a positive half-wave rectifier and negative half-wave rectifier, separating the positive half from the negative one.
Now connect the capacitor back to the circuit. A plot example is presented in Figure 13.
Figure 13. Positive half wave Envelope and Negative half wave Envelope
The obtained signal is the envelope of the positive half wave and negative half wave obtained previously.
Questions
1. What happens if the capacitor/resistor values are changed? Which are the drawbacks in this case?
2. For the circuit in Figure 1, if a resistor is added in series with the diode, between D1 and R1, how is the output affected? Explain the differences.
Further Reading
university/courses/electronics/electronics-lab-envelope-detector.1519990085.txt.gz · Last modified: 02 Mar 2018 12:28 by Antoniu Miclaus