Abstract :
In transistor IF design, the use of double-tuned transformers with non-critical coupling factors is of importance, especially with the introduction of F M multiplex. Undercoupled circuits are desirable in this relatively wide band application to reduce envelope delay distortion. Many previous design procedures have been restricted to having both equal primary and secondary quality factors and critical coupling. Those without these restrictions have proved too cumbersome. Another problem associated with previous design methods which do consider non-critical coupling factors is the calculation of the bandpass and admittance characteristics of the double-tuned filters. The method presented here reduces this factor to the inspection of plots of computer derived, double-tuned transformer characteristics. The most commonly used bandpass characteristics are tabulated. In addition, equations are presented to enable calculation of any desired transformer characteristic not presented in plot or tabular form. A main consideration in IF design is the stability criterion. An equation is derived relating the total damping required for stable operation to a defined stability factor. A very simple relation is shown to exist between the required damping of the transistor input and output circuits. These relationships are used to obtain an expression for stable power gain. It is shown that from this expression stable power gain is a direct function of coupling factor. Conventional calculations complete the unneutralized design theory. A procedure is derived for the neutralized case which through the use of correction factors makes the neutralized design procedure similar to the unneutralized design, both in rapidity and technique. A four stage, 10.7 Mc, undercoupled amplifier is designed using both neutralized and unneutralized stages. Although the degree of undercoupling used causes the loss of approximately 3db gain, compared to a critical coupling case, the neutralized stages sti- l exhibit a gain of 28.7db. The overall bandwidth is 250 kc. Measured results indicate correlation with the designed data within 2% for bandwidth and within 0.1 db for power gain.
