Patent GB-956729-A (GB)
Title: Microwave mixers
IPC: H03D9/06
Filed: 1962-02-13
Abstract: 956,729. Frequency changing arrangements; amplitude modulation systems. VARIAN ASSOCIATES. Feb. 13, 1962 [Feb. 16, 1961], No. 5514/62. Headings H3B, H3F and H4L. The invention relates to a frequency-changing arrangement in which a vertically polarized signal is applied via a waveguide 2, Fig. 1, and horizontally polarized local oscillations via a waveguide 3, to a common waveguide 1 supporting dominant modes of both waves in orthogonal relationship. Mixing is performed by means of crystal diodes 4, 5 connected in series in a vertical direction across the waveguide 1, the beat at intermediate frequency being taken out over an output post 6 extending horizontally across the waveguide 1. The local oscillations are perturbed by the post 6 in such a manner that the electric field thereof couples into the crystal diodes in opposite directions, whereas the signal component is not perturbed and couples in the same direction. In such an arrangement the crystal diodes lie in the same direction. In such an arrangement the crystal diodes lie in the polarization direction of the signal wave and present a capacitive loading thereto which lowers the cut-off frequency of higher order modes, degrading the coupling of signal energy to the diodes at the high end of the band of frequencies which may be propagated by the signal waveguide 2. In accordance with the invention, this is overcome by making the width a<SP>1</SP> of the mixer waveguide 1 substantially less than that a of the signal waveguide 2. The crystal diodes 4, 5 are positioned midway of the width a<SP>1</SP> of the mixer a<SP>1</SP> of the mixer waveguide, the local oscillator waveguide 3 being positioned off-centre so as to increase the perturbing effect of the post 6 on the local oscillations. The signal waveguide 2 is also placed off-centre to conserve space. In a construction, the mixer waveguide 1, Figs. 2, 3, is preferably oblong in the direction of the crystal diodes 4, 5 and is recessed in a rectangular metallic block 1<SP>1</SP> which is provided with flange plates 10, 11 for mating to the signal and oscillator waveguides respectively. The crystal diodes 4, 5 are seated in removable caps 12 threaded through the top and bottom walls and are held in contacting relation at their inner terminals by a connector 13 to which is connected the output post 6. Post 6 extends through an insulating low-pass choke 6<SP>1</SP> in the waveguide wall to form the inner conductor of a coaxial I.F. connector 6<SP>11</SP>. Each cap 12 is provided with a filter network by-passing A.C. to the block 1<SP>1</SP> so that the direct current in the crystals can be monitored at the lugs 14. A thin horizontally-extending rectangular iris 21 in the signal waveguide flange 10, centred with respect to the crystal diodes 4, 5 improves the signal V.S.W.R., while a thin vertically extended bifurcated iris 22 in the local oscillator flange 11, centred with respect to the local oscillator waveguide 11<SP>1</SP> also aids in improving the nose figure. The roles of the various terminals can be reversed, e.g. the signal and local oscillator inputs could be reversed, or the arrangement operated as a side-band modulator by using the post 6 as input for the modulating signal and the opposite waveguide connections for carrier input and modulated output, respectively. It may also be operated as a parametric amplifier by using voltage-controlled diode capacitors instead of rectifiers. Further, a waveguide mode of desired polarization may be established by means of a coaxial probe rather than a single waveguide. Specification 956,728 is referred to.