690 



BELL SYSTEM TECHNICAL JOURNAL 



Visual Method 



In the visual method of observation, a steady potential proportional 

 to the inphase component of the transfer factor is impressed across 

 one pair of plates of a cathode ray oscillograph and another steady 

 potential proportional to the quadrature component is impressed 

 across the other pair of plates, the constant of proportionality being 

 the same for the two components. In this way the transfer factor 

 at any frequency appears as a single point, the vector from the origin 

 to the displaced beam constituting the transfer factor. The locus 

 of all these points, i.e., vector tips, over the frequency range constitutes 

 the transfer factor polar diagram. 



To provide rectified potentials proportional to inphase and to quad- 

 rature components respectively, use is made of the properties of the so- 

 called vacuum tube wattmeter.^" As used in practice, this device con- 

 sists of two triodes in push-pull connection (Fig. 6), the series arm of 

 the grid circuit being connected to the unknown potential, and the 



Fig. 6 — Circuit of a vacuum tube wattmeter used to provide a rectified potential 

 proportional to the product of the two impressed grid potentials (both of the same 

 frequency) multiplied by the cosine of the phase angle between them. 



shunt arm of the grid circuit being connected to a source of the same 

 frequency but of standard phase. Under these conditions the rectified 

 output in the plate circuit flowing in series with the two plates is pro- 

 portional to the product of the two impressed voltages multipled by 

 the cosine of the angle between them. 



As shown in Fig. 7, two separate wattmeters are employed, one for 

 each phase, their series input terminals being connected together across 

 the output of the circuit under test. To the common branch of one of 

 these wattmeters is supplied the same potential as is fed to the input 

 of the circuit under test. The rectified output of this wattmeter there- 

 fore is proportional to the product of the input and output voltages 

 multiplied by the cosine of the transfer factor phase angle. This po- 



10 U. S. Patent 1,586,533; Turner and McNamara, Proc. I. R. E., vol. 18, p. 1743; 

 October (1930). 



