GB285654A - Improvements in or relating to wireless or like high-frequency transmission installations - Google Patents

Abstract

285,654. Jackson, L. Mellersh-, (Lorenz Akt.-Ges., C.). March 1, 1927. Filter circuits; harmonics, producing and absorbing; modulating systems.-A filtering arrangement for eliminating undesired frequencies from a circuit, particularly applicable to the elimination of higher harmonics and side waves in a radio transmission system, comprises a balanced transformer, the two primary windings of which are connected in series respectively with a selective acceptor or rejecter circuit, and with a non-selective impedance.The filter may be used for suppressing the pure carrier wave in a modulated high-frequency transmission system; and for suppressing a powerful fundamental wave when' determining and measuring the frequency and strength of harmonics. Fig. 1 shows a circuit for eliminating a single disturbing oscillation from the output of a frequency-multiplying choke 1. The output is passed in parallel through the two primary windings 5, 6 of a transformer, one branch including an acceptor circuit 16 tuned to the undesired frequency, and the other branch an equivalent resistance 17. For the undesired frequency, these branches pass equal currents which balance out in the secondary coils 7, 8. Other frequencies are passed into a tuned secondary circuit II coupled to the aerial coil 12. The primary and secondary coils 5 - - 8 are symmetrical in space as well as electrically. When it is desired to transmit selectively one wave, and to suppress all others, a rejector circuit 18, Fig. 2, tuned to the desired wave replaces the acceptor circuit 16. The circuit 18 should comprise large capacity and small inductance; and the coils 5, 6 should have high inductance so that they substantially determine the current phase in the branch circuits. Currents below the desired frequency can be more accurately balanced out by switching in an inductance coil 19, and current above the desired frequency by switching in a capacity 20. By using two balancing arrangements (of the kind shown in Fig. 2) in cascade, both higher and lower undesired frequencies may be accurately balanced out. Filters of the kind shown in Fig. 1 may also be connected in cascade, and may be followed by filters of the Fig. 2 type. Fig. 4 shows a modified filter employing separate instead of branched circuits. One of the circuits interposed between the source 1 and the aerial 12 includes a rejector circuit 21, or an acceptor circuit. For the suppression of the pure carrier wave in radio telephony, the modulated wave is applied to the two branched circuits of Fig. 1, and the acceptor circuit 16 - tuned to the carrier wave which is thereby eliminated from the circuit II. Additional filter circuits may also be used for the suppression of one of the side-bands. Higher harmonics or side waves may be measured by coupling a measuring- circuit to the circuit II of Fig. 1, the powerful fundamental wave being excluded by the acceptor circuit 16, in combination with the balanced transformer 5 - - 8. For this purpose the circuit II is untuned.