The two methods of SSB generation are (i) frequency discrimination method and (ii) the phase discrimination method. The frequency discrimination method of SSB generation given in figure 1, is based on suppressing one of the sidebands from the double-side-band suppressed carrier (DSB-SC) modulated waveform. For a perfect SSB to be generated using this method, the band pass filter (BPF), should have sharp cut-off, which is a difficult constraint for practical implementation, especially when the message signal has significant components near the ‘zero’ frequency.
Figure 1: The frequency discrimination method of generating the SSB waveform
The second method of SSB generation, the ‘phase discrimination method’, is based on the time domain representation of the SSB waveform, and is given in figure 2.
Figure 2: The phase discrimination method of generating the SSB waveform
Hence laboratory implementation of the ‘phase discriminator’ method of SSB generation requires two ‘DSB-SC’ generators, in addition to two phase shifters and an adder.
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. Generation of AM is not simple, and to have two AM generators with identical operating conditions is extremely difficult. Hence, the preferred implementation of the DSB-SC is usually using the ‘ring-modulator’, shown in figure 3.
Figure 3: The ring modulator used for the generation of the double-side-band-suppressed-carrier (DSB-SC)
Now, if one uses the ‘phase discrimination’ method for generating the SSB waveform, which requires two ‘ring modulators’, each of which requires four diodes and two transformers, the circuit becomes too huge for implementation, besides having significant ‘noise’ due to presence of transformers. Hence, the laboratory implementation of the SSB waveform, may involve use of industry standard communication modules like the oscillator, multiplier, differentiator, adder, etc. However, a discrete component realization of any circuit is always preferred, as it helps comprehend the concepts better. Recently, the transformer-less method of DSB-SC generation has been developed, which has the advantages of having less electronic noise and reduced physical size. In this module the attempt is to suitably modify the phase discriminator of SSB generation (figure 2), by using the transformer-less method of DSB-SC generation, so that the modification makes it suitable for laboratory implementation.
The modified phase discriminator
The modified phase discriminator given in figure 4, differs from the usual SSB generator of figure 2, by using a square wave for the carrier instead of a sinusoidal carrier. This change in the type of carrier results in significant advantages over the usual SSB generator using the phase discriminator.
Figure 4: The block diagram used for the generation of the SSB waveform
The first advantage of using a square wave carrier is that digital ICs can be used for generating the two phase co-related carriers. Since analog circuits designed for producing necessary phase shift are highly frequency sensitive, the use of digital ICs provides a wide operating frequency range (dependent on the bandwidth of the IC). Hence the operating bandwidth of the modulator is enhanced.
The second advantage of using a square wave carrier is the replacement of the DSB-SC modulator by a ‘multiplier’. While the DSB-SC modulator, is also a multiplier, producing the product of two analog signals, the ‘multiplier’ in this case is the product of two signals with one of them being a binary waveform. When one of the two signals to be multiplied is binary, the product can be obtained using a combination of an analog inverter and a two channel analog switch, as shown in figure 5. The binary input acts as the select input to the ‘two-channel’ analog switch, and hence,
Figure 5: The ‘multiplier’ used to obtain the product of an analog signal and a binary waveform
The third advantage of using a square wave carrier is the ‘modified phase discriminator’ becoming transformer-less, since the need for two DSB-SC modulators is eliminated. Transformers being bulky, circuits devoid of them are preferred. Elimination of transformers, in turn reduces the electronic noise in the generated waveform. Thus, the modified phase discriminator generates the SSB waveform. In the next Section, experimental implementation of the proposed method for single tone message input is attempted.
Experimental Results
The practical implementation of each of the blocks of the proposed modified phase discriminator is not unique, as a number of methods of implementation are possible. We have realized the multiplier using the two ICs: LF-398 (sample and hold) and LM-741 (operational amplifier), as shown in figure 6. It may be noticed that this ‘multiplier’ does not use any transformers.
Figure 6: The ‘multiplier’ in the modified ‘phase discriminator’ for generation of the SSB
The other blocks in the modulator are: Hilbert transformer is a passive phase shifter for the message signal, the BPF is a passive LC tuned filter, and the adder is using the operational amplifier LM 741. The square-wave carrier and the sinusoidal message are given from the dual function generator (Lab-Volt Model 9402-00).The waveforms are observed on the mixed signal oscilloscope (100MHz Agilent 54622D, capable of recording the output in ‘.tif’ format). The observed output at various stages of the SSB modulator for single tone message signal is given.
Figure 7: The signals at the output of the multipliers
Figure 7 has the outputs of the multipliers when two phase synchronized carriers with quarter-cycle phase difference as one of their inputs. The final output
***SORRY, THIS MEDIA TYPE IS NOT SUPPORTED.***
of the SSB generator is the sum/difference of the in-phase and quadrature phase components, which can be observed by not applying one of the inputs, w1(t) or w2(t), to the adder. Each of these components is a DSB-SC waveform, and is given in figures 8 and 9.
Figure 8: The analog input and the corresponding DSB-SC output of BPF obtained by setting w2(t) to zero
Figure 9: The analog input and the corresponding DSB-SC output of BPF obtained by setting w1(t) to zero.
The USB obtained using difference of w1(t) and w2(t), is shown in figure 10, along with one of the square carriers. The LSB waveform obtained as the sum of w1(t) and w2(t), is shown in figure 11. The quality of the generated SSB is evident. The addition of two DSB-SC waveforms obtained using carriers with quarter period phase shift to produce the SSB waveform is also demonstrated. Thus the ability of the modified transformer-less phase discriminator to generate the SSB waveform is proved. The simplicity of the developed method makes it suitable for laboratory implementation. The addition of the in-phase component and the quadrature phase component, resulting in the USB / LSB is an exciting observation.
Figure 10: One of the carriers along with the USB generated.
Figure 11: One of the carriers along with the LSB generated.
Conclusion
The existing methods for generation of SSB using discrete components, is not suitable for laboratory implementation. The proposed SSB modulator, is comparatively simple and suitable for laboratory implementation. The method is tested for single tone modulation. The method being transformer-less, is compact in size and has reduced electronic noise.
Acknowledgment
The author acknowledges Smt. Jayashubha, the laboratory Instructor, for patiently testing and recording the results of the circuits.
NOTE:
A more detailed presentation of this work is contained in: WCSET 2009: World Congress on Science, Engineering and Technology, Rome, Italy, April 28-30, 2009
