Patent GB-1018730-A (GB)
Title: Multiband turnable circuit
IPC: H03J5/24
Filed: 1962-11-19
Abstract: 1,018,730. Radio received switching arrangements; transistor receiving circuits. RADIO CORPORATION OF AMERICA; Nov. 19, 1963 [Dec. 4, 1962], No. 45682/63. Headings H3Q and H3T. A transistorized tuner for a television receiver uses on the UHF band transmission lines having their outer and inner conductors joined electrically at their adjacent ends, being tuned by means of ganged variable capacitors in series with the inner conductors. Each inner conductor comprises first and second electrically isolated portions, the variable capacitor being located in the first portion and switching means provided which in one position connect the two portions together and in other positions disconnect them and connect inductances in circuit for tuning over VHF bands. An embodiment comprises a metal box 12 divided by screening partitions 22, 24, 26 into aerial tuning, RF band-pass, and local oscillator compartments, each section including an inner strip conductor 34 66, 104 and 132 respectively, forming with its compartment a concentric line. Tuning is by means of ganged capacitors 32, 64, 102, 134 in series in the respective inner strip conductors, each of which is split into two portions 34a, 34b &c. at a point of relatively low impedance, the two portions being joined together on the UHF band by the sliding contacts 42, 68, 106, 136 of the waveband switch. In the input compartment 14 the UHF aerial is coupled into the line by means of a balanced inductive coupling loop 56; the inner line conductor 34 is earthed via a series capacitor 36 and this forms the coupling for the VHF aerial which is connected to terminal 60. The switch contact 42 is so shaped that on two VHF bands inductors 50, 52 are successively connected in series with the upper portion 34a of the line inner. The RF amplifier comprises transistor 28 which is in common emitter connection, the base being connected to a tapping point on the line inner 34 via capacitor 62. The coupling between the collector of transistor 28 and the mixer stage comprises a band-pass filter formed by compartments 16, 18. In compartment 16 the collector of transistor 28 is connected to a tapping point on line inner 66 and switch contact 68 successively brings inductors 74, 76 in series with the variable capacitor 64 on VHF bands. The arrangement is neutralized, at UHF by means of a coupling loop 94 which feeds back energy to the transistor base via neutralizing capacitor 96, and on the first only of the VHF bands by means of a feedback network comprising capacitors 80, 100 and inductor 98. In the output compartment 18 switch contact 106 successively brings inductors 116, 118 in shunt with variable capacitor 102; the coupling between input and output compartments comprises mutual inductance, a gap being provided in the partition wall 24 so that coupling occurs between line inners 66, 104 and also between inductors 74, 76 and 116, 118 respectively. The output is fed to the emitter of mixer-oscillator transistor 30 in compartment 20; on UHF lead 160 provides an inductive coupling to the line inner 104, whereas on the VHF bands coupling is provided by capacitors 120, 122. The mixer-oscillator 30 is in common base connection and its collector is connected via an IF output circuit comprising capacitor 154, IF transformer 152 and RF chokes 156, 158, to a tap on the line inner 132. On UHF oscillation occurs due to the interelectrode capacities of the transistor; on VHF bands switch contact 138 successively connects inductors 146, 148 in shunt with variable capacitor 134 and additional back-coupling is provided by capacitor 150. The Specification also describes 'a modification (Fig. 3, not shown) in which a separate mixer and oscillator are employed; it is further stated that the wavechange switch may be mechanically operated by the ganged tuning capacitor.