Cross-modulation coupling is caused by the nonlinearity of the converter. The carrier of the principal signal is amplitude modulated by the interference signal and is received together with the principal signal. To eliminate this undesirable effect, the ability of the input circuit to suppress the interference signal must be increased. One way to achieve this would be to use a directional rotary antenna, although a magnetic antenna would be best for this purpose. By using the capacitive coupling of the antenna to the input circuit, the Q value hence, selectivity of the antenna circuit can be made larger than by using inductive coupling. A loose coupling between the antenna and the tuning circuit will decrease the sensitivity somewhat, but will increase the overall selectivity. The selection of the working point for the remote cut-off tubes 6U1, 6K4, and 6K3P is important for the elimination of envelope distortion. The output of the local oscillator should be adjusted for optimum performance. Author
Edited trans. of Wu Hsien Tien (Chinese People's Republic) n10 p22-3 1965.