RECORDING TRANSMISSION MEASURING SYSTEM 



23 



mainder being parts already in use in the field in the making of ordinary 

 transmission measurements. 



The general operation is as follows: Constant testing power is 

 supplied to one end of the circuit by the adjustable frequency oscillator, 

 the frequency generated being varied continuously from one end of 

 the range to the other by slowly turning the frequency control dial 

 with the synchronous motor. While this takes place the recording 

 meter at the other end of the circuit makes a record of the received 

 power on a strip of paper, which is moved steadily by a synchronous 

 motor, the resulting curve being a graph of the variation of the trans- 

 mission efficiency of the circuit with respect to frequency. The purpose 

 of the tuned circuit shown in Fig. 1 is to cause a mark to be made on 

 the paper in the recording meter when a particular frequency is re- 

 ceived. This mark serves as a reference point for applying a frequency 

 scale to the record after the curve has been made. 



Fig. 1 — Schematic arrangement of recording system. 



If it is desired to obtain a record of transmission efficiency with 

 respect to time, the same arrangement is used without the motor at 

 the sending end, the oscillator frequency being fixed. The recording 

 meter will then draw a line showing how the received power, and 

 therefore the loss introduced by the circuit, changes with respect to 

 time. If it is desired to record noise on the circuit instead of trans- 

 mission loss the oscillator is disconnected from the circuit and the 

 amplification at the receiving end increased until the very small noise 

 currents are sufficient to cause readings on the meter. If the receiving 

 apparatus is connected across a working telephone circuit it will serve 

 as a recording speech volume indicator. 



The oscillator, amplifier and other parts of the system have great 

 stability and when left in continuous operation will maintain adjust- 

 ments over long periods so that they may be connected to and used 

 in the same manner as an ordinary voltmeter. 



Figure 2 shows an experimental setup of the oscillator used at the 

 sending end of a circuit and the recorder and associated parts at the 

 receiving end. The motor-driven oscillator is at the left and the 



