A Recording Transmission Measuring System for Telephone Circuit Testing

HE making of transmission measurements on telephone circuits is essentially a delicate operation. However, with the aid of vacuum tubes and, more lately, copper-oxide rectifiers, devices have been developed for measuring the various important transmission characteristics of telephone circuits, including transmission losses and gains for single frequencies, speech volume and noise, all of these measurements being made with meters as are measurements of the performance of electric power systems. There has now been developed an experimental model of a system not only for indicating but also for recording the results of transmission measurements on telephone circuits. It was developed particularly for the purpose of automatically plotting curves of transmission loss versus frequency, this characteristic of telephone circuits being a very important index of the ability of the circuit to transmit speech clearly. It is, however, also suitable for making various records of performance as a function of time, including transmission loss, speech volume and noise. The essential elements of the automatic recording system are shown in Fig. 1 as they are used in making a transmission-frequency run on a telephone circuit. At one end of the circuit is an adjustable frequency oscillator which generates testing power, a sending panel for supplying this power to the circuit and adjusting it to the proper value and a synchronous motor for varying the oscillator frequency. At the other end is a receiving panel which amplifies the weak received testing power and converts it to direct current which causes the pointer of the recording meter to move.