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Single-Sideband Transmission

(Conłmued from page 78)

origina! carrier by too many cycles. the rcproduced audio sounds unnatural and. in cxtremo cascs. can become un-intclligible. Pitch of the reproduced audio varies directly with the fre-quency difference between the side-band and the re-inserted carrier. As this frequency difference inereases. pitch of the audio is also inereased. This difference should be kept to less than about 50 cycles-pcr-sccond for best results. If we divide the trans-mitter and receiver drift equally it means that each can have a maximum frequency deviation of ± 25 cps. At Communications frequencies this is ex-tremely close tolerance.

If broadcast, a smali degree of carrier amplitudę can help greatly in tun-ing the rcceiver morę accurately. In some senices. especially in the v.h.f. band and higher. it is essential that some carrier be transmitted. Current practice, at least in some cases. is to transmit only about 10^ of fuli carrier power. This still represents con-siderable savings and climinatcs one of the primary disadvantages of single-sideband transmission. that of critical tuning. Some ('quipment uses auto-matic-fref|uency-control in a further effort toward stabilization.

For a group of SSB stations operat-ing as a network. the transceiver-type system is advantagcous. Transmission and reception are both on the same carrier freąuency, giving duplex ad-vantages with a simplex arrangement. This minimizes tuning problems be-cause if an operator hears a message, he knows automatically that he is on the correct frcquency for transmission.

SSB eąuipment is available from a large number of manufacturers for al-most any frcquency rangę in which it can be used. For lower power systems the initial eąuipment is morę costly than for conventional AM systems, pri-marily because of the added stages which are necessary. However. as the power rating of the transmitter is inereased, SSB eąuipment becomes pro-|x>rtionally cheaper than other AM gear. The saving in cost for the larger eąuipment is due primarily to the re-duction of power reąuirements for the modulator. driver. and finał stage of the transmitter.

For use with existing AM transmit-ters. SSB generators are ayailablc and at least one manufacturer has a model avai!ahle in kit form. These separate genei-ators create SSB output from conventional AM signal input. Receiver costs do not di Afer greatly when com-pared to other AM receivers.

The two disadvantages of SSB. sta-bility reąuirements and critical tuning. are morę than offset by the advantages alrcady listed. Improved components, both in transmitter and receiver. have madę SSB a definite possibility and a definite probability for extensive use in all Communications services.    —SJ-

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