The second part of the experiment introduces an asynchronous envelope detector and examines its operation.
Theory:
Amplitude modulation is when the amplitude of a high frequency wave is varied and a wave of frequency 1MHz is obtained carrying the information in its amplitude. It is used primarily for transmitting information via a radiocarrier wave. An amplitude modulator is used to simulate the amplitude modulated radio frequency carrier wave.
The key purpose of a detector is to remove the modulation from the carrier in order to obtain the audio frequency representation of the signal. Envelope detectors operate by extracting information from the envelope or shape of the wave which is carried by the carrier wave at a high frequency. These contain a number of disadvantages such as they are not linear and their distortion levels are high.
An envelope detector for AM signals
Figure 1
Synchronous demodulation is superior to envelope detection but is less often used due to the complexity of the circuitry. This operates by producing a local oscillator signal on exactly the same frequency as the carrier and mixing it with the incoming signal. This converts the signal directly down to audio frequencies and the sidebands appear as the audio signals in the audio frequency band.
Synchronous demodulation
Figure 2
The modulation index, m, can be calculated by:
Where V1 and V2 can be taken as the maximum and minimum peak-to-peak voltages of the AM wave as seen in Figure 1.
Figure 3 – An AM waveform Amplitude
Experimental Method:
1.1 Operation of the Modulator:
Set up the equipment as seen in Figure 4 below connecting the oscillator output to the carrier in on the modulator.
Vary the oscillator frequency slowly until a maximum output signal is obtained at the AM Out. This is referred to as the resonant frequency of the coupling transformer.
Obtain 2V peak- to- peak at the modulator output by adjusting the oscillator output signal amplitude.
Examine the modulator output signal after connect a 1kHz sine wave to the Modulator’s In input. Examine how the modulator output waveform changes as the amplitude of the modulating wave is varied.
Figure 4 – AM Modulator
1.2 Frequency Response of the Modulator:
Ensure that the percentage of modulation remains a constant at 30% as the modulation frequency is altered between 100Hz and 20KHz.
At each frequency measure and record the modulating signal amplitude without changing the carrier frequency or amplitude for the duration of these measurements.
1.3 Linearity of the Modulator:
Modulate the carrier with a 1KHz sine wave. Obtain percentages of modulation, by varying the modulating signal amplitude, between 20% and 100%. For each measurement of the percentage obtained, measure and record the modulating signal amplitude.
Then repeat for modulating frequencies of 100Hz and 10KHz.
1.4 Overmodulation:
Increase the modulating signal amplitude until an overmodulated AM signal has been obtained. This is what the AM waveform looks like when the percentage of modulation is over 100%.
2.1 Asynchronous AM Detector (Envelope Detector) – Frequency Response:
Assemble the circuit as seen in Figure 5. By feeding the AM modulator with a 1KHz modulating signal and with a carrier signal from the oscillator, a 30% modulated AM wave can be obtained.
Measure and record the detector output signal amplitude at a variation of frequencies between 100Hz and 10kHz.
Compile a graph of the frequency response of the envelope detector (the detector output signal amplitude) as a function of frequency.
Figure 5
2.2 Linearity of the Envelope Detector:
After modulating the carrier with a 1KHz sine wave, vary the modulating signal amplitude in order to obtain percentages of modulation between 20% and 100%. Measure and record the detector output signal amplitude for the resulting percentages.
Repeat with the modulating frequencies of 100Hz and 10kHz.
Compile a graph of the linearity envelope detector as a function of frequency.
2.3 Detection of an Overmodulated AM Wave:
Modulate the carrier with a 1kHz sine wave. Increase the modulating signal amplitude until an overmodulated AM signal is obtained. Draw the detector output signal waveform.
2.4 Synchronous AM Detector:
Set up the circuit as presented in Figure 4 and obtain a 30% modulated signal from the modulator. Observe the waveform at the collector of Q2. Draw the two waveforms and the Audio Out waveform.
Results:
1.1 Operation of the Modulator:
The resonant frequency obtained was 455KHz.
1.2 Frequency Response of the Modulator:
Bandwidth = 2.26
1.3 Linearity of the Modulator:
2.1 Asynchronous AM Detector (Envelope Detector) – Frequency Response:
2.2 Linearity of the Envelope detector –:
Discussion:
A synchronous modulator appears to be the better of the two despite the fact that the circuit is more complex. The Envelope detector is not completely linear and as can be seen from the graphs and there are traces of distortion
Conclusion:
The Envelope detector is not as sophisticated as the Synchronous detector which is a better modulator despite being more complex in design.
References:
Appendix A: Amplitude and Frequency Modulation, Page 10
- 14th February
Appendix A: Amplitude and Frequency Modulation, Page 10
