Patent GB-869353-A (GB)
Title: Improvements in or relating to modulating arrangements for frequency or phase modulation communication systems
Inventor: JACOBSEN BENT BUELOW
IPC: H03C3/06
Filed: 1958-07-25
Abstract: 869,353. Frequency phase modulation. STANDARD TELEPHONES & CABLES Ltd. July 25, 1958, No. 24007/58. Class 40(5) A frequency or phase modulated wave is passed via a hybrid circuit 2 through two paths having different delays, and one having a frequency changer 4, to an amplitude modulator 5 with filter output deriving the sum or difference frequency, to convert the wave to a different mean frequency and frequency swing. It is shown that the original frequency deviation a sin. #.t. is changed to 2a cos. #d/2 sin. (#t- #d/2) or to -2a sin. #d/2 cos ( #t- #d/2), dependent on whether the upper or lower sideband is selected, where co is the modulating frequency and d the delay. The resultant signal, being dependent on co and d, may therefore be used to compensate for non-linearity of a succeeding frequency discriminator. An additional discriminator, in parallel, for higher modulating frequencies, may also be used preceded by a circuit as in Fig. 1 for producing a sine factor for the frequency swing, i.e. attenuating low modulating frequencies. Two frequency swing modification circuits as in Fig. 1 may also be used in cascade. Alternatively, the circuits of Fig. 1 may be used for multi-channel FM links to produce attenuation at undesired frequencies. The circuits of Fig. 1 may be applied to produce pass and stop band filters for an FM signal, of the type (Fig. 7) giving alternately an output between # and zero, and represented by the Fourier series To this end (Fig. 8) a series of above circuits 18, 19, 20 with delays d, 3d, 5d ... are employed and their outputs alternately added and subtracted from an initial FM wave from source 1 by modulators 27, 29, 31, with output filters alternately selecting upper and lower sidebands to produced frequency addition and subtraction. In addition each envelope, i.e. frequency swing modifying, circuit 18, 19, 20 has its output frequency divided by 1, 3, 5, 7 . . . to produce the above ratios 1, <SP>1</SP>/ 3 , <SP>1</SP>/ 5  . . . . The constant term # / 2 is produced by adding the original signal (#cŒa sin. #t) multiplied by #/2 at 26 to the series by means of a modulator 27. For example the output frequency of divider 30 is where # 5  is the frequency change produced by each frequency changer in circuits 18, 19, 20. Since the final carrier frequency and deviation has been multiplied by #, a frequency division by # may be performed at 32. A delay line with delays d/2, d, d between terminals 25, 24, 23, 21 equalises the phase changes n#d/2 produced by circuits 18, 29, 20. The latter may use the same delay line. Alternatively, an additional frequency division by #/2 after circuits 18, 19, 20 may be used in place of multiplier 26. Stop and pass bands in Fig. 8 may be transposed by selecting the opposite sidebands from modulators 27, 29, 31. In a further filter circuit, stop and pass bands are produced by using delays d/2 , d/4 respectively, then subtracting the outputs in a final modulator circuit with lower sideband output; (Figs. 9, 10 not shown).