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Translation equipment pilots
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Reference pilots are discrete frequencies added to each translated band of traffic channels for monitoring and automatic gain control regulation at the far end of the transmission path. Some of the more common reference pilots are given in Table 20.3.
Additional pilots can be added for measurement or monitoring of wide band systems and are usually positioned in the gaps between supergroups. These are called intersupergroup pilots.
20.6.1.1 Use of reference pilots for automatic gain control
The normal practice is to provide gain or loss correction to the traffic path at the received end of transmission using the pilot to control the correction. (See Figure 20.8). This correction is applied most commonly at group and supergroup level.
Correction is applied to maintain the design traffic level across the circuit link which may contain many stages of translation and several tandem connected line systems. Seasonal temperature changes to the cable link is foremost in the contributing factors. The correction is applied equally across the frequency band and with an Automatic Gain Control ratio of approximately 10:1.
Thermistors are normally used as gain control elements to avoid any unlinearity distortion to the traffic path. In addition the slow thermal time constant is suited to the slow diurnal gain change of the cable system and the envelope gain of the AGC relatively easy to control. ± 4dB regulation range is normally required from AGC equipments.
Дата публикования: 2015-03-29; Прочитано: 161 | Нарушение авторского права страницы | Мы поможем в написании вашей работы!
