GB695616A - Improvements in or relating to frequency changing circuits for receivers of short electromagnetic waves - Google Patents

Abstract

695,616. Radio receiving apparatus. STANDARD TELEPHONES & CABLES, Ltd. Jan. 1, 1952, No. 103/52. Class 40 (v). In a wave-guide frequency-changing circuit in which an incoming signal and a local oscillation are fed to a rectifier, the wave-guide section containing the rectifier has its input impedance matched with that of the guide, and means are provided for suppressing the passage out of said section of image frequency waves generated at the rectifier. Coaxial lines may replace the wave-guide at somewhat lower operating frequencies, and the rectifier may be replaced by a thermionic mixer tube. As shown in Fig. 2, the wave-guide section 1, of rectangular crosssection, is closed at one end 2 and has a flange 3 for connection to the main guide through which are fed the incoming wave and the local oscillation. The rectifier 4 of cartridge type has its lower end held in a socket on a vertical post 6 formed on a horizontal bar 7 extending across the guide parallel with the wide sides. The rectifier 4 is enclosed in a vertical cylinder 8 extending upwards from the wide upper face of the guide and situated a quarter-wavelength from the closed end 2. Matching at the incoming wave frequency is effected by a screw 11 adjustable as regards depth of penetration. The position of the screw 11 along and across the guide is made adjustable by mounting it in a rotatable disc 12 having a flange 15 fitting between upper and lower plates 14, 13 secured to the top surface of the guide, as described in Specification 681,081, [Group XXXVI]. The Specification explains that by the action of the mixer rectifier 4, not only is the desired intermediate frequency produced, but also varies harmonics and combinations of the two input frequencies. Among these, is the image frequency which when mixed with the local oscillator frequency also produces the desired I.F. If the image frequency is allowed to pass outwards along the guide and to be reflected by the receiving aerial, it will cause distortion of the received signal, or in the case of a multiplex system will give rise to cross-talk. The image frequency is prevented from emerging from the guide section 1 by providing a tuning screw 10 behaving as a series resonant circuit in shunt with the wave-guide. The image frequency is thus reflected back to the rectifier, and by adjusting the distance D between the rectifier 4 and the screw 10, the reflected image frequency wave may be arranged to arrive at the rectifier in suitable phase to strengthen the I.F. output. The screw 10 may be placed close to and parallel with one of the short sides of the rectangular guide; or it may be made adjustable in position by a rotatable disc similar to the disc 12.