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The sideband whtch remains is then amplified by one or morę stages of linear amplification to inerease the power level to that required for radi-ation. When the transmitted frequency is much higher than that produced in the initial stages of the transmitter, several successive stages of r.f. gener-ation and mixing can be used to step up the frequencv.

The second method used for sideband suppression. illustrated in Fig. 7, eliminates the need for mixing stages as used in Fig. 6. It is known as the “phasing method'* and suppresses both the carrier and one sideband simultane-ously.

With this arrangement the audio sig-nal is fed to an “audio phase shifter" which gives two outputs. which are 90° out-of-phase with each other. The snme process is performed on the r.f. signal with the phase shift being in the same direction as the audio. If the audio shift were 90° leading. the r.f. phase shift must be the same.

Into balanced modulator No. 1 are fed both the audio and the r.f. in their original phases. Balanced modulator No. 2 is supplied the phase-shifted audio and r.f. s^;gnals. By the process already described. each balanced modulator removes the r.f. carrier and pno-duces two sideband frequcncics in its output. These signals can be analyzed by using F, for the carrier frequency and Fa for the audio, as shown in Fig. 7. The phase-shifted components are labeled F, + 90° and F« + 90°.

In the output of Circuit No. 1 the two sidebands are Fr + F« and F.- — Fa. From No. 2 one sideband is F, + 90° + F_a + 90°, which is cqual to Fc + Fa + 180°. The other output is Fc + 90° - F_A - 90°. or F,. - F_A. These four outputs are labeled in Fig. 7 in the order listed here. Outputs (1) and (3) are identical except that they are 180° out-of-phase. therofore they can-ccl. Outputs (2) and (4i are identical. and thus reinforce each other. This is the F, — Fa signal and it is then amplified before being transmitted.

In the process the carrier and the upper sideband were eliminated. lcav-ing only the lower sideband to be radi-ated. By reversing either the audio or the r.f. the lower sidebands can be cancelled, leaving only the upper. One way of doing this is to change the phases of the audio and r.f. in different directions. for example if the r.f. were shifted to lead by 90° and the audio to lag by 90°.

The major problem encountered in this method is that of maintaining the phase shift at exactly 90° for all audio modulating frcquencics. Phase shifting can be performed with RC networks but the values must be exact and all the components needed are not standard commercial values. However, some manufacturers are now providing these networks in packaged units. One type which is a\ailable has all its compo-nents mounted in a metal tubę en-velope w-hich plugs into a standard octal Socket. This unit maintains the phase shift to within less than two de^rees for a rangę of about 300 to 3000 cps.

The availability of these units has eliminated the primary disadvantages of the phasing system—difficult design and alignment. With these disadvan-tages removed, the phasing method can be considered better than the filter method because of its lower initial and operating costs.

An example of an audio phase-shift network is shown in Fig. 8. The audio signal Fa is applied to the grid of V,, Fa + 90° is applied to V* Notice espe-cially the non-standard values of components needed for proper phase shift over a wide enough band of frequen-cies.

The r.f. phase shifting is not as criti-cal, nor as difficult to obtain. There are two reasons for this, the r.f. is a much higher frequency. also, the r.f. is al-ways the same frequency and does not vary over a band as does the audio.

Two r.f. phase-shifting networks are shown in Figs. 9A and 9B. Operation is similar. the only difference is that in Fig. 9B an output resistor replaces the coil used in Fig. 9A. In each Circuit the No. 1 output leads the No. 2 output by 90°.

Linear Amplifiers

Single-sideband and suppressed-car-rier transmitters require the use of linear amplifiers after the modulation stages. Schematically, linear amplifiers are not very different from any other type used in transmitters, but in performance they should have two primary characteristics. These are Iow distor-tion and maximum power gain, which are mutually exclusive factors in most amplifiers.

Linear amplifiers are those in which the output signal voltage is propor-tional to the input signal voltage. Audio amplifiers can be classed as linear. so can the r.f. and i.f. amplifier stages used in radio receivers.

In order to utilize its fuli effective-ness. carrier and sideband suppression

(1) CANCCLS13)

(2) A00S TO(4)

Fig. 7. Błock diaqram of sideband suppression by means of the phasing method.

should take place at very Iow signal lcvcl. otherwise no appreciable savings in power would result. It would be senseless to wastc power in generating a high-Ievel carrier and then immedi-ately suppress it. This means that single-sideband systems must amplify the signal many times before it reachcs the fuli power required for radiation.

In any amplitude-modulation system, class C amplification cannot be used for any stage after the one in which modulation occurs. This limits the us-

Heathkit "Mohawk" amateur-band receirer.    to CithCl Class A Ol B. or in SOmC

cases AB. if serious distortion is to be The RME—Electro-Voice sideband selector. avoided.

Class A amplifiers can bc used in the transmitter, and they are quite linear, but efiiciency is Iow (usually less than 30^ i. This may not be too much of a disadvantage at Iow power, but de-cidedly inereases costs for a higher powered transmitter. Class B push-pull gives much better efficiency (about 60 to 70^r/t ) and, if designed properly, can givc the des i red linearity.

To design a stage to amplify in a very linear manner often results in Iow' gain, which means that morę stages are required to give a certain amount of gain. As class C cannot be used, the maximum possible efficiency cannot be obtained, all the morę reason why few stages are desirable In many cases negative feedback is used to im-

77

October, 1959