Switchable Bandselector

EE JULY/AUGUST 1987 In many older types of SW receiver, intermodulation in the mixer was generally avoided by including a tuneable, often automatically tracking, pre- selector. In a computercon- trolled preselector, the use of varactor diodes for tuning the inductors often leads to con- siderable intermodulation dis- tortion. A different approach is therefore used in this design. 16...32MHz The circuit diagram shows the • use of PIN diodes Type BA244 for selecting one of 5 band- filters followed by a low-pass section. Selection of a filter is effected by having the com- puter drive the associated input high. An impedance trans- former is provided at the input to enable connection of 50 Q as well as 500 Q aerials. For most c). o purposes, the 500 Q input is preferable, since it allows short 15V ECM O BA244 02 IN 4148 aerials to be correctly ter- minated. Input transformer Tri is wound on a ferrite core Type FT37-75 from Micrometals. The total number of turns is 19, with a tap at ...