Patent GB-655826-A (GB)
Title: Improvements in or relating to carrier wave modulation systems
IPC: H01J25/52
Filed: 1948-11-19
Abstract: 655,826. Frequency modulation; radio signalling. PHILCO CORPORATION. Nov. 19, 1948, No. 30125. Convention date, Nov. 20, 1947. [Class 40 (v)] [Also in Group XL (b)] An input F.M. wave of mean frequency f is converted into a F.M. wave of another mean frequency F by mixing the two waves to produce a spectrum F, F+f, F+2f, &c., of which all except F bear the initial frequency modulation. Means are provided for removing the frequency modulation from a selected one of these waves, whereby a frequency modulation is imparted to another of the waves (e.g. the frequency F). Fig. 3 shows a suitable circuit, in which the F.M. input f is amplified in a valve 42, and is applied to the cathode of a magnetron oscillator 21 which generates a wave of higher frequency F and also operates as a mixer or modulator, the output of which to the wave guide 23 includes the spectrum previously referred to. The wave guide 23 communicates through an adjustable slot 30, Fig. 4, with a cavity resonator 29 which is tuned to one of the side frequencies (e.g. F+f), and has the effect of suppressing the frequency modulation borne by the chosen frequency. As a result, frequency modulation appears on the previously undeviating frequency F which is fed to the divided guides 25, 27 leading respectively to a radiating element 26 and to an equal dummy load 28. The guides 25, 27 are provided with tuning stubs 37, 38 at a distance equal to an integral number of half-wavelengths from their junction 24, the stub 37 being of the same effective length. The effective length of the stub 38 is adjusted to an odd number of quarter-wavelengths, the wavelengths being those of waves of frequency F. The effect is to transmit the modulated frequency F to the radiator 26, and to exclude therefrom the side frequencies (F+f, F+2f, &c.). The equivalent filter circuit with lumped impedances is described in the Specification (Fig. 5, not shown). The magnetron 21 feeds the wave-guide 23 through a coaxial line 22 and the guide is fitted on either side of the resonator 29 with means 35, 36 for adjusting the effective lengths of the guide sections. The wider sides of the guides are longitudinally slotted, and the narrower walls are adjusted in their separator by screws 35d threaded into trough-shaped brackets 35c. The adjustment is such that the effective length of guide between the magnetron 21 and the resonator 29 is an integral number of halfwavelengths. A tuning screw 41 is provided at the junction 24 of the branched guides 25, 27. The mixing of the input F.M. wave f with the shorter wave F may be so effected that Class C or other amplitude modulation results, or so as to produce a frequency or phasemodulated output. In a modified arrangement shown in Fig. 7, the modulation-suppressing resonator 29, Fig. 1, is omitted, and suppression is effected by supplying a wave of corresponding frequency from a source 72 through an amplifier 73 to the magnetron 70. The F.M. wave is supplied through an amplifier 71, and the output taken to the load through a line 77. If the frequency modulation of the chosen side frequency is not completely removed, the frequency modulation of the output from the magnetron 60, Fig. 6, may be less than that of the input F.M. wave. In that case, the output may be reduced in frequency by a local source 62 and mixer 61, and the resultant passed to a frequency discriminator 63 to yield the orginal signal. The input wave is applied to a second discriminator 64 and the two resulting signals are compared at 65 to yield a difference voltage which applies supplementary frequency modulation to the magnetron 60. The input F.M. wave may be pre-distorted by a network inserted in front of the amplifier 42, Fig. 3, in order to compensate for a tendency for the output signal from the magnetron to include variations which are a function of the rate of change of the frequency of the input signal (Fig. 8, not shown). The system may be employed at radio relay stations receiving a frequencymodulated carrier wave and transmitting a similarly - modulated carrier of different frequency.