Patent GB-1448254-A (GB)
Title: Gyromagnetic filter and diode mixer circuit
IPC: H03D7/16
Filed: 1973-11-01
Abstract: 1448254 Radio receivers PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 1 Nov 1973 [4 Nov 1972] 50776/73 Heading H3Q [Also in Division H1] An electromagnetic filter, which may be incorporated in a diode mixer circuit, comprises first and second transmission lines coupled by a gyromagnetic body at a desired frequency, the first transmission line comprising two live conductors and a ground conductor arranged so that even-mode and odd-mode waves at that frequency can exist on the line in the vicinity of the gyromagnetic body, the filter further comprising means for terminating the first line in the vicinity of the gyromagnetic body whereby at the termination the even-mode waves are substantially short-circuited but a high impedance is presented between the live conductors for the odd-mode waves. As shown in Fig. 1, a signal which may represent a television signal in the frequency range 50 to 900 MHz is applied through conductors L 1 , L 2  to a diode bridge R 1 , the output of which is fed via conductors L 3 , L 4  forming with a ground plate (A) (Figs. 2, 3, not shown) a transmission line to a yttrium-iron garnet (YIG) crystal Y 1 , which is polarized by a magnetic field perpendicular to the plane of the conductors, and has a magnetic resonance at 2000 MHz. At points 1, 2 just beyond the crystal Y 1 , the conductors L 1 , L 2  are connected to a line R 5 , having an electric length of half a wavelength at 2000 MHz. Points 1, 2 are also connected by conductors L 6 , L 7  to a local oscillator of which the frequency may be varied from 2050 to 2900 MHz. The oscillator output consists of "odd" waves, i.e. the currents flowing in conductors L 6 , L 3 , and L 7 , L 4  are in phase-opposition and balanced with respect to the ground plate (A), and is applied to the diode bridge R 1 . The mixing of signal and oscillator frequencies gives the usual mixture of frequencies, including an i.f. signal at 2000 MHz which appears as an "even" wave, i.e. the currents in conductors L 3 , L 4  are in the same phase with respect to the ground plate (A). The conductors are formed as flat strips on a dielectric sheet, with the ground plate (A) on the other side of the sheet, and the crystal Y 1  embedded in the dielectric. It is shown that the "even" i.f. signal is much more strongly coupled to the crystal Y 1  than the "odd" oscillator output. The line R 5  acts like a short-circuit at the i.f. of 2000 MHz, so that a current anti-mode occurs at points 1, 2, giving maximum coupling. The crystal Y 1  gives an output to another circuit through an aperture (K) in the ground plate (A). The second circuit may be similar to that described, mounted on the other side of plate (A), and rotated 90 degrees with respect to the first. It has a crystal Y 2 , similar to and situated opposite Y 1 ; an oscillator G2 operating at a fixed frequency 1965 MHz as odd waves travelling along conductors L 11 , L 8  and L 12 , L 9  and mix in diode bridge R 2  with the 2000 MHz i.f. from the first circuit. A second i.f. of 35 MHz appears at the output terminals U 1 , U 2 . The two crystals give a band-pass filter; otherwise a single crystal may be used. The filter may be used in the diode mixer circuit of Specification 1,287,535.