Undercoupling in Tuned Coupled Circuits to Realize Optimum Gain and Selectivity

Setting the coupling of intermediate-frequency transformers below the critical value gives an increase in skirt selectivity without much loss of gain. If the coupling is set too far below the critical value more gain is lost than selectivity is gained. It is desirable to be able to set the units to get the best gain and selectivity. The mathematical analysis gives an indication of what can be done to get this condition without considerable trial-and-error work.