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A complete SSB station installation using Technical Materiel eguipment.

PART

Linear Amplifiers and Receivers

ALAN ANDREWS

Mełhods used ło suppress one sideband, along wiłh a discussion of linear amplifiers and receivers for SSB.

IN PART 1 of this series the advan-tages of singlc-sidcband transmis-sion were discussod. Also includcd wore descriptions of the basie single-sideband system, as weli as methods of suppressing the carrier signal. In this article we will cover the methods used for suppressing the unwanted sideband and provide information about trans-mitter amplifiers and single-sideband receivers.

Suppressing a Sideband

Two basie methods of sideband sup-pression are in current use. The first employs a balanced modulator, the out-put of which consists of the two side-bands. One of these sidebands is then removed by a very selective filter. In the other method, two balanced modu-lators are used. To one of them are fed both the audio and the r.f. signals. The input of the other includes the same audio and r.f., except that both are shifted 90° in phase. This process elim-inates the carrier and one sideband

without the use of a separate filter.

A błock diagram of the filter method is shown in Fig. 6. The balanced modulator functions the same as described in Part 1. and the output of the modulator consists of the two sidebands produced as a result of modulation, the carrier having been suppressed. The filter eliminates either sideband, so that only the other sideband appears in the filter output.

This filter must be extremely selec-tive and it is difiicult to obtain the re-quired selectivity except at Iow fre-quencies. So most often, when using this arrangement, the r.f. oscillator is operated at a much lower frequency than the carrier. The remaining sideband is then also much lower than that required for transmission. A SSB signal cannot be multiplied in frequency by the usual methods. without creating extreme distortion, so some other method must be used.

As shown in Fig. 6 another oscillator is used, this one producing the actual

carrier frequency although it is not used for transmission. The single-sideband heterodynes with the carrier in the mixer stage to produce two addi-tional frequencies. One of these is equal to the carrier plus the sideband.

Drakę Model 1>A sideband receWer.

Fig. 6. Błock diagram oi the iilter method used to suppress one oi the two sidebands.

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the other the carrier minus the sideband. There is a large frequency dif-ference existing between these two signals, so the mixcr stage output can be tuned to either of these newly created sidebands. Now we can proceed to the following step.

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