GB620656A - Modulating system for radio beacons - Google Patents

Abstract

620,656. Radio navigation. STANDARD TELEPHONES & CABLES, Ltd. Aug. 18, 1944, No. 15863. Convention date, Aug. 26, 1943. [Class 40 (vii)] In a system for providing a carrier wave modulated with two different signals, for use, for example, in defining a course line for aircraft approach and landing, a modulator system is fed with a carrier frequency and two signal frequencies, and provides one output comprising the carrier frequency and the two pairs of side-bands corresponding to the two signal frequencies, and another output comprising only the said two pairs of'side-bands with one pair reversed in phase with respect to the corresponding pair in the first output. The two outputs are fed to conjugate input points of a bridge network, one of the sides of which includes a phase reverser. There are thus derived at two independent output points the carrier and one of the pairs of side-bands, with different pairs at the respective points. Referring to Fig. 1, 10 and 11 are sources of modulating frequency, e.g. 90 and 150 c/s, both of which are fed via lines 15 and 16 to an unbalanced modulator 13 and a balanced modulator 14, the said modulators being fed with the same carrier from a source 12. The line 16 is transposed at 18 so that the 150 c/s frequency fed to the balanced modulator 14 is reversed in phase with respect to the 150 c/a frequency fed to the unbalanced modulator 13. In the output of the unbalanced modulator 13 there appears the carrier and the side-band components corresponding to both the 90 and 150 c/s frequencies, whilst in the output of the balanced modulator 14 there appears only the said side-band components with the side-band components corresponding to the 150 c/s frequency in phase opposition to the side-band components corresponding to the latter frequency at the output of the modulator 13. The outputs from the two modulators are applied respectively to conjugate input terminals 21, 22 of a balanced bridge network 20, that from the modulator 14 passing through a phase shifter 17. One of the arms of the bridge 20 is transposed at 24, whereby there appears at one output terminal 23 of the bridge the carrier and the total side-band energy corresponding to the 90 c/s frequency, the sideband energy corresponding to 150 c/s having cancelled out, whilst at the output terminal 24 there is obtained the carrier and the total sideband energy corresponding to the 150 c/s frequency, the other side-band energy having cancelled out. The two differently modulated signals may be applied via lines 25 and 26, for example, to an equi-signal course beacon of the type disclosed in Specification 563,007 or, as in Fig. 2 (not shown), to an equi-signal glide-path beacon of the type disclosed in Specification 590,491. Monitoring means, comprising a loop 27, a filter 28 tuned to the unwanted sidebands, and a meter 29, may be provided to test for any unbalance, which may then be corrected by adjustment of phase-shifter 17.