Patent GB-545586-A (GB)
Title: Frequency-modulated carrier-translating systems
IPC: H03D3/12
Filed: 1941-02-03
Abstract: 545,586. Wireless signalling. HAZELTINE CORPORATION. Feb. 3, 1941, No. 1401. Convention date, March 1, 1940. [Class 40 (v)] In a system for transmitting or receiving frequency-modulated waves, two or more carrier currents each having frequency-modulation components are derived (e.g. by beat methods) from the frequency-modulated wave and are then combined in such a way as to develop a modulated output. In a transmitter, this modulated output may be a frequency-modulated wave in which the frequency deviation is increased as compared with the original wave. In a receiver, the frequency-modulation components may be balanced out so as to leave a pure amplitude modulation which can be detected in the usual way. Transmitter.-A frequency-modulated input of frequency WS at terminals 10, Fig. 1, is combined with oscillations of frequency Wo in frequency-changer 11, the resulting currents of frequency WS+Wo passing through a band-pass filter 13 to a second frequency-changer 14 where they are combined with part of the initial input to produce currents of frequency 2WS+Wo, which pass to the output 17, the frequency deviation having thus been doubled. Receiver; Fig. 2.-The received currents of frequency WS are amplified at 22, Fig. 2, and part are combined with local oscillations of frequency Wo in frequency-changer 23 to produce difference currents of frequency WS-Wo, which pass through a "hybridizer" 25 and are combined in a second frequency-changer 26 with part of the received currents to produce amplitudemodulated currents of frequency Wo, which are detected and amplified in the usual way. The "hybridizer" 25 comprising resistance 31, inductance 32, and capacitance 33 converts the frequency-modulated signal into a hybrid signal having both frequency-modulation and amplitudemodulation components, the former being removed in frequency-changer 26. Receiver, Fig. 3.-The received currents of frequency WS are amplified at 22 and combined with local oscillations of frequency Wo in frequency changer 23 to produce different currents of frequency WS-Wo which are combined with local oscillations of frequency Wo2 in a second frequency changer 26 to produce sum and difference frequencies which pass to the " hybridizer 36. This comprises a band-pass filter 39, 40, 41, 42 tuned to the sum frequency WS-Wo +Wo2 and connected to the first grid of a third frequency-changer 37, and a band-pass filter 43, 44, 45, 46 tuned to the difference frequency WS-Wo-Wo2 and connected to the third grid. The transmission characteristic of filter 39-42 is uniform over its pass-band, whereas that of filter 43 ... 46 has a constant slope over its passband. In order to derive a hybrid signal having amplitude modulation as well as frequency modulation, the voltage across circuit 43, 44 is added via condenser 18 to that across circuit 45, 46. The output circuit 47, 48 of the frequencychanger 37 thus comprises only an amplitudemodulated carrier of frequency 2Wo2 (i.e. twice the frequency of oscillator 35) which is rectified and amplified in the usual way. Receiver, Fig. 4.-This comprises a balanced " hybridizer" adapted to balance out noise or residual amplitude modulation, such as transmitter hum, when the frequency modulation is substantially zero. The received currents are combined with local oscillations in frequencychanger 51 to produce sum and difference frequencies to which the circuits 53, 54 and 55, 56 are respectively tuned. The circuit 55, 56 is coupled to the circuits 60, 64, also tuned to the difference frequency and connected to the grids of frequency-changing valves 65, 66, whose anodes are differentially connected to the output circuit 73. The circuit 53, 54 is also connected to the grids of valves 65, 66 by way of phaseshifting circuits 61, 62 and 57, 58 respectively. The sum and difference frequencies are thus applied to the frequency changers 65, 66 and the frequency-modulation components balance out as before, leaving an amplitude-modulated signal without a carrier component. The carrier component may be introduced from oscillator 52 via a selector 70 (tuned to twice the oscillator frequency) and a phase-shifting network 71. The Specification as open to inspection under Sect. 91 describes the application of the invention to convert frequency-modulated signals into phase-modulated signals. For this purpose, the band-pass filters shown in Fig. 3 would have similar amplitude-frequency transmission characteristics but unlike frequency-phase characteristics. This subject-matter does not appear in the Specification as accepted.