538 



BELL SYSTEM TECHNICAL JOURN^iL 



vary the i.f. impedance by small amounts to achieve a better i.f. match, 

 since the proper spacing for best gain is not a critically exact dimension. 

 That is to say, there is a fairly large range of spacings which give good 

 performance as far as conversion gain is concerned so that, as long as the 

 critical distance which gives poor gain is avoided, the i.f. impedance can 

 be adjusted by varying the spacing of the input filter.* 



It is imi)ortant that the i.f. impedance of the modulator be adjusted to 

 match the impedance of the 'i.f. amplifier which drives it, since any mis- 

 match would cause a degradation of the system performance. In the 

 design of the matching transformer the inductance of the leads, the 

 capacity of the tube and by-pass condenser and the resistance of the elec- 



0.25 0.50 0.75 1.00 1.25 1.50 

 FILTER SPACING IN INCHES 



Fig. 9. — Data showing the effect of the sjjacing of a rejection filter for the wanted side- 

 band in the input circuit. 



tron stream were measured at the base of the tube. A broad-band trans- 

 former was designed and the inductances were thrown into an equivalent 

 T network, thereby utilizing the lead inductance inside the tube as a 

 part of the transformer, absorbing it in the L2-M branch as indicated in 

 Fig. 10. In several experimental tubes the lead inductance was .OAnH. 

 The impedance match obtained with such a transformer gave less than 

 two db SWR over a band from 55 to 75 mc with the loop at the cusp on 

 the reflection coefiicient chart characteristic of slightly over-coupled tuned 

 transformers as shown in Fig. 11. 



The broadband matching of the output circuit of the modulator re- 

 quired a different technique. Xot only is this filter called upon to pro- 

 vide a broad-band imi)edance match, but also it should provide dis- 



* The spacing of the input filter also affects the plate impedance in a complicated way. 



