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LABORAORTY GENERATION OF AM AND DSB-SC

Module by: B Kanmani. E-mail the author

Summary: The ‘switching-modulator’ or the ‘square-law’ modulator can be used for generating amplitude modulation (AM/DSB), while the ‘ring-modulator’ or the balanced modulator can be used to generate double-side-band-suppressed-carrier (DSB-SC) in the laboratory. It has been accepted that different circuits are required for generating these two forms of AM, which differ only in the presence or absence of the independent carrier. In this module, the switching-modulator is modified by introducing an additional active device. With this modification, the modulator becomes capable of generating AM with varying depths of modulation, including the DSB-SC. Thus a single circuit can be used to generate both the DSB and the DSB-SC. The simplicity of the proposed method makes it ideally suited for laboratory implementation.

LABORAORTY generatION OF AM and DSB-SC

1. Introduction

The ‘switching-modulator’ or the ‘square-law’ modulator can be used for generating amplitude modulation (AM/DSB), while the ‘ring-modulator’ or the balanced modulator can be used to generate double-side-band-suppressed-carrier (DSB-SC) in the laboratory. It has been accepted that different circuits are required for generating these two forms of AM, which differ only in the presence or absence of the independent carrier. In this module, the switching-modulator is modified by introducing an additional active device. With this modification, the modulator becomes capable of generating AM with varying depths of modulation, including the DSB-SC. Thus a single circuit can be used to generate both the DSB and the DSB-SC. The simplicity of the proposed method makes it ideally suited for laboratory implementation.

2.1 The switching modulator

The blocks in the ‘switching modulator’ used for low-power generation of the AM waveform are: (i) an adder, (ii) an active device and (iii) a band pass filter (BPF) tuned to the carrier frequency, as shown in figure 1.

Figure 1
Figure 1 (graphics1.png)

Figure 1: The switching modulator used for the generation of amplitude modulated waveform

For proper functioning of this switching modulator, |m(t)|<<Ac, which implies the modulation index of the generated AM waveform is very low. In addition, the assumption of an ideal switch for the active device may not be realistic. Hence, AM generated using the switching modulator being of low modulation depth, cannot be used to demonstrate ‘over-modulation’.

2.2 The square-law modulator

Another method of generating the amplitude modulation is using the ‘square-law’ modulator. This modulator uses the same blocks as the ‘switching modulator’, but the active device is operated by in the non-linear region, such that its output can be represented as a power series to include the square term. The proper functioning of this modulator depends on operating the active device in the ‘square-law’ region. This is a valid assumption for any active device, for a very small operating range. Moreover, maintaining the operating range within permitted levels is practically difficult, since the amplitude swing of the message signal is unknown. Hence, the square-law modulator generates AM, for limited operating range in the carrier and signal strengths. This is the main drawback of the square-law modulator.

2.3 Existing methods for low power AM generation

To summarize the existing methods of low power AM generation: the square-law modulator has low operating amplitude range for the incoming message and carrier, as the active device needs to be operated only in the square-law region, while the switching modulator produces AM only for low modulation index. Moreover, both these methods cannot generate the DSB-SC waveform, which is the AM with the carrier suppressed.

2.4 Existing methods for DSB-SC generation

The DSB-SC can be generated using either the balanced modulator or the ‘ring-modulator’. The balanced modulator uses two identical AM generators along with an adder. The two amplitude modulators have a common carrier with one of them modulating the input message , and the other modulating the inverted message . Generation of AM is not simple, and to have two AM generators with identical operating conditions is extremely difficult. Hence, laboratory implementation of the DSB-SC is usually using the ‘ring-modulator’, shown in figure 2.

Figure 2
Figure 2 (graphics2.png)

Figure 2: The ring modulator used for the generation of the double-side-band-suppressed-carrier (DSB-SC)

This standard form of DSB-SC generation is the most preferred method of laboratory implementation. However, it cannot be used for the generation of the AM waveform.

2.5 A comment

The DSB-SC and the DSB forms of AM are closely related as; the DSB-SC with the addition of the carrier becomes the DSB, while the DSB with the carrier removed results in the DSB-SC form of modulation. Yet, existing methods of DSB cannot be used for the generation of the DSB-SC. Similarly the ring modulator cannot be used for the generation of the DSB. These two forms of modulation are generated using different methods. Our attempt in this work is to propose a single circuit capable of generating both the DSB-SC and the DSB forms of AM.

3 The modified switching modulator

The block diagram of the ‘modified switching modulator’ given in figure 2, has all the blocks of the switching modulator (figure 1), but with an additional active device. In this case, the active device has to be of three terminals to enable it being used as a ‘controlled switch’. Another significant change is that of the ‘adder’ being shifted after the active device. These changes in the ‘switching-modulator’ enable the carrier to independently control the switching action of the active device, and thus eliminate the restriction existing in the usual ‘switching-modulator’ (equation (2)). In addition, the same circuit can generate the DSB-SC waveform. Thus the task of modulators given in figures 1 and 2 is accomplished by the single modulator of figure 3.

Figure 3
Figure 3 (graphics3.png)

Figure 3: The modified ‘switching modulator’

It is possible to obtain AM or the DSB-SC waveform from the ‘modified switching-modulator’ of figure 3, by just varying, the amplitude of the square wave carrier . It may be noted that the carrier performs two tasks: (i) control the switching action of the active devices and (ii) control the depth of modulation of the generated AM waveform. Thus, the proposed modification in the switching modulator, enables the generation of both the AM and the DSB-SC from a single circuit. Also, it may be noted that the method is devoid of any assumptions or stringent difficult to maintain operating conditions, as in existing low power generation of the AM. We now implement the ‘modified switching modulator’ and record the observed output in the next Section.

4. experimental results

The circuit implemented for testing the proposed method is given in figure 4, which uses transistors CL-100 and CK-100 for controlled switches, two transformers for the adder, followed by a passive BPF. The square-wave carrier and the sinusoidal message are given from a function generator (6MHz Aplab FG6M).The waveforms are observed on the mixed signal oscilloscope (100MHz Agilent 54622D, capable of recording the output in ‘.tif’ format).

Figure 4
Figure 4 (graphics4.png)

Figure 4: The implementation of the modified ‘switching modulator’ to generate the AM and the DSB-SC waveform

The modified switching modulator is tested using a single tone message of 706 Hz, with a square-wave carrier of frequency 7.78 KHz. The depth of modulation of the generated waveform can be varied either by varying the amplitude of the carrier or by varying the amplitude of the signal. Figure 5 has the results of the modulated waveforms obtained using the ‘modified switching modulator’. It can be seen that the same circuit is able to generate AM for varying depths of modulation, including the over-modulation and the DSB-SC. The quality of the modulated waveforms is comparable to that obtained using industry standard communication modules (like the LabVolt for example).

Figure 5
Figure 5 (graphics5.png)

Figure 5: The message signal of 706 Hz along with the modulated waveforms obtained using the modified switching modulator, with a carrier of frequency 7.78 KHz : (a) to (e) AM for varying depths of modulation, and (f) the DSB-SC waveform

Acknowledgements

The author acknowledges Smt. Jayashubha, the laboratory Instructor, for patiently testing and recording the results of all the circuits.

NOTE:

A modified form of this work was presented during the IEEE Signal processing society: 13th DSP Workshop & 5th SPE Workshop, Marco Island, 4th -7th January, Florida, 2009

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