230 BELL SYSTEM TECHNICAL JOURNAL 



To obtain maximum signal-to-noise ratio, a tube must be chosen to have 

 the lowest product of noise multiplied by the ratio of amplifier to conversion 

 gain, the latter requirement being in conflict to overload requirements. 

 When inputs below the low-level Hmit are to be utilized a preamplifier 

 ahead of the modulator tube is required. This ampUfier also contains a 

 noiseband centered on F2 — Fi . If the ampUfier is selective and rejects 

 this noise band or if an F2 — Fi rejection filter is inserted ahead of the 

 modulator tube the resultant new low-level Umit is determined by the signal- 

 to-noise ratio of the preamplifier at the signal frequency Fi only. 



Dynamic range is defined as the useful range of a modulator limited by 

 the high-level limit on one end and by the low-level limit on the other. 

 The dynamic range of a number of pentodes was determined. It was found 

 experimentally that differences in dynamic range between pentodes of 

 different power ratings, such as 6AK5, 6AC7, 6AG7, 6L6, 829B, are small. 

 A dynamic range of 30-36 db can be reaUzed with a 6AK5 for a .01 db 

 linearity requirement. The 6AK5 was the most suitable tube of those 

 investigated considering all other requirements of the circuit such as band 

 width, available signal levels, etc. 



Buffer amplifiers are required ahead of the modulator tube to prevent 

 crosstalk between measuring and reference modulator through common 

 paths. These buffer ampHfiers are of conventional video ampUfier design, 

 with phase and gain characteristics closely controlled to the order of 0.01 

 db and 0.1 degree. 



The Phase and Transmission Detector 



In the null type of phase measurement an initial circuit zero is made. 

 When the circuit is rebalanced with the apparatus under test inserted, the 

 phase detector must be able to verify that the same phase relationship has 

 been reestablished as existed when the initial circuit zero balance was made. 

 Bridge circuits yield high sensitivity and a high degree of independence of 

 input voltage amplitudes. In Fig. 6 a four-arm resistance phase bridge 

 is shown, which has two inputs E\ and Eo , and two outputs Es and Ed 

 corresponding to the vectorial sum and difference of the input voltages Ei 

 and E2 . 



As derived in the appendix, for the equal arm bridge, the amplitudes of 

 the voltages Es and Ed are equal for phase angles of ^ = 7r/2 + nir, where 

 n is any integer, regardless of the amplitudes of Ei and £2 • Thus equality 

 of I £s I and [ £d | is convenient to define the circuit phase zero. Equality 

 of \ Es\ and \ Ed\ by itself does not distinguish between 90° and 270° 

 phase shifts. This ambiguity can be resolved with a detection circuit 

 which responds to both the amount and the sign of the difference | £s | — 

 I Ed I and by making provision for the introduction of a smaU increase A<p 



