681,673. Frequency modulation distortion measurements. STANDARD TELEPHONES & CABLES, Ltd. April 10, 1951, No. 8266/51. Addition to 678,533. Class 37. [Also in Group XL (c)] Arrangements for measuring non-linear distortion or group delay in frequency or phase modulation systems of communication are described comprising developments of the invention claimed in the parent Specification. As shown in Fig. 1, which is a modification of Fig. 2 of the parent Specification, two similar oscillators 1, 2 generate carrier waves of frequencies f<SP>1</SP>, f<SP>2</SP>. Two signal wave oscillators 3, 4 generate low-frequency waves of frequencies p, q. The wave p modulates the frequency of the oscillator 2 through two hybrid coils 5, 6. The oscillator 1 is frequency-modulated with the frequency q from the oscillator 4 through an amplitude modulator or frequency changer 7 and with the frequency p from the oscillator 3 through the hybrid coils 5 and an amplitude and phase adjuster 8. The output from the modulator 7 is a wave of frequency q accompanied by two " comparison side-bands " of frequencies q+p which are used for determining the level of the distortion components produced by the elements 11, 12 under test. The level of the modulating waves applied to the oscillator 1 is adjusted by means of a variable attenuator 9. The outputs from the oscillators 1, 2 are combined in a frequency changing modulator 10 and the sidebands are applied to a filter 11 and amplifier 12 which are the elements under test. The filter 11 may select the lower side-band of mean frequency #<SP>1</SP> -<SP> </SP>#<SP>2</SP> which has modulation components corresponding to the frequencies p, q and q+p. Both the frequencies p, q are together represented in the test elements 11, 12 and if non-linear distortion is present there will also be generated interaction side-bands corresponding to q+p. The comparison side-bands derived from the modulator 7 are also present in the test devices and are adjusted by means of the adjuster 8 so that the comparison sidebands neutralize the interaction side-bands. A percentage modulation tester 13 connected to the output of the modulator 7 then gives the relative level of the interaction side-bands produced by the test elements 11, 12. Before the test can be made it is necessary to remove the frequency modulation components corresponding to the frequency p from the output of the amplifier 12 and this is done by means of an oscillator 14 generating a frequency f<SP>2</SP>. The wave p from the oscillator 3 is applied through the hybrid coils 6 and an adjuster 15 similar to 8 to frequency-modulate the oscillator 14, the output of which is applied to a modulator 16 together with the output of the amplifier 12, the lower side-band of mean frequency f being selected by a filter 17. The frequency modulation components corresponding to frequency p present at the output of the amplifier 12 can be neutralized by those produced by the oscillator 14, so that at the output of the filter 17 there will be no frequency modulation components of the mean frequency f<SP>1</SP> corresponding top. The output of the filter 17 is applied to a modulator 18 supplied by an oscillator 19 generating a frequency #<SP>3</SP>, one of the side-bands #<SP>3</SP> +#<SP>1</SP> being selected by a filter 20 and passed to a frequency discriminator 21. The modulation components consisting mainly of frequencies q, and q+p are selected by a filter 22 and are rectified by a rectifier 23 to recover the frequency p, which is selected by a filter 24, tuned to frequency p, and supplied to headphones 25. The adjuster 8 is adjusted until there is no output at frequency p from the filter 24, so that there is no component of frequency, q+p at the output of filter 22, i.e. the comparison side-bands have neutralized the interaction side-bands produced in the elements 11, 12. The adjustment of the adjuster 15 to cancel out the p components from the output of the modulator 16 can be effected by temporarily cutting out the elements 22, 23 so that the filter 24 selects the frequency p directly from the output of the discriminator 21. If the frequency f<SP>1</SP> selected by the filter 17 is chosen to be suitable for the discriminator 21, the elements 18, 19, 20 may be omitted, a direct connection 26 being made between elements 17, 21. If p is an audio frequency and is very small compared to f<SP>1</SP> or f<SP>2</SP>, the elements 6, 14, 15 may be omitted and replaced by direct connections 27,28. Fig. 2 shows an arrangement for measuring group delay in a high-frequency network in order to avoid errors due to non-linear effects in the measuring circuit. The group delay of a network 29 over a range of high frequencies is to be compared with the phase shift produced by an adjustable phase shift network 30 in a comparison wave having a low frequency q, e.g. 1000 c.p.s., which is represented by a highfrequency side-band in the network 29. The frequency q is supplied by an oscillator 32 through a hybrid coil coupling 33 and an adjustable attenuator 34 to the phase shift network 39, and also to modulate the frequency of a high-frequency oscillator 35 generating a mean frequency f<SP>1</SP>. A very low frequency p, e.g. 35 c.p.s., is supplied from an oscillator 36 through a hybrid coil coupling 37 to frequency modulate a second high-frequency oscillator 38 generating a mean frequency f<SP>2</SP>, and also to synchronize a sweep circuit 39 for an oscillograph 31. The oscillators 35, 38 correspond to the oscillators 1, 2, Fig. 1, and are connected to a frequency changing modulator 40, from which a filter 48 selects the lower side-band of mean frequency #<SP>1</SP> -<SP> </SP>2# and supplies it to the network 29 under test. The frequency deviation of the frequency f<SP>2</SP> produced by modulation at frequency p is such that the high-frequency supplied to the network 29 varies over the whole frequency range to be covered by the network. The output of the network 29 is supplied to a modulator 42 which is also supplied with frequency f<SP>2</SP> direct from the oscillator 38. The lower side-band of mean frequency f<SP>1</SP> is selected by a filter 43 and passed to a discriminator 44 from the output of which a filter 45 selects the frequency q. The two waves of frequency q are combined in a balanced modulator 46 arranged so that it suppresses waves from the filter 45. The output of the modulator 46 is applied through a D.C. amplifier 47 to the verticallydeflecting plates of the oscillograph 31. In order to adjust the phase shift introduced by the network 30, the connection between the hybrid coil network 37 and the oscillator 38 is temporarily broken so that the oscillator generates the mean frequency f<SP>2</SP>. Then the phase shift is adjusted until the D.C. output of the modulator 46 is zero, as indicated by a D.C. voltmeter 48. On restoration of the connection between the elements 37, 38, the trace on the oscillograph is a graph of the variation of the group delay with respect to the frequency in the range #<SP>1</SP> - #<SP>2</SP> + 10 megacycles per second.