Patent GB-760757-A (GB)
Title: Equalizer-adjusting circuit
IPC: H04B3/10
Filed: 1954-08-31
Abstract: 760,757. Attenuation frequency and phase/ frequency equalizer adjustments. WESTERN ELECTRIC CO., Inc. Aug. 31, 1954 [Sept. 4, 1953], No. 25262/54. Class 37. Apparatus for adjusting the transmission characteristics for equalizer in an electrical signal transmission channel to minimize errors of the equalization thereof comprises means for applying to the channel input a test voltage of constant amplitude and repetitively varying frequency or frequencies; one or more weighting impedance networks adapted for connection at their input ends across the output end of the channel, such weighting networks having transmission characteristics adapted to produce a change in the average transmission through the combination of channel and weighting network indicative of the required equalizer adjustment necessary to minimize the equalization errors; and voltage detecting and indicating means associated with the outputs of the weighting network or networks for determining the change of the average transmission over the channel which is due to the weighting network or net. works. In a wideband transmission channel comprising an equalizer for attenuation or phase delay composed of N multiple independently adjustable sections, each of the latter has a frequency response characteristic F n  (f), and a proportion k n  of each characteristic is adjustably inserted into the channel to give the latter a resultant response characteristic for a range of M discrete frequencies from f to f m , which is equivalent to the channel equalization error for correct equalization over the frequency range. It is shown that if u weighting function impedance networks, having frequency response characteristics G u  (f) suitably chosen that the average value G u  (f) F (f) = 1 where m = n and the average value G u  (f) F n  (f) = 0 where m # n, are connected in series with the channel output, the average of the resultant signal from the weighting network over the frequency range of the nth equalizer represents the proportion k n  of that equalizer which must be inserted for correct equalization of the channel. In Fig. 1 a sweep signal source 1 is alternatively switchable through a wave filter 5 limiting the equalization band, to a transmission channel 7 followed by an adjustable attenuation (or phase delay) equalizer 8; there being included in the switchable circuit a net. work 11 of predetermined attentuation/frequency response for attenuation equalization or a balanced modulator 12 followed by a network 13 of predetermined delay/frequency response for phasedelay equalization. The sweep signal source 1 comprises (Fig. 2) a known generator 27 of a symmetrical triangular voltage/ time waveform (Fig. 3) repetitive at frequency f, which energizes through series resistance 28 and amplifier 19 a frequency modulating circuit 20 comprising (Fig. 6), an oscillator triode 44 whose reactive circuit is tunable by reactance triode 45, grid energized by the output of amplifier 19 to produce frequency modulated signals which are further modulated in mixer 46 by fixed frequency oscillator 51, freed from side-bands by low-pass filter 47, and fed back through an optional low frequency attenuating and distorting network 22 (Figs. 4, 5, not shown) to a half-wave diode rectifier 23 developing a feedback signal across the input of amplifier 19. The resultant signal at the output of the complete sweep source is of constant amplitude but varies cyclically in frequency from minimum to maximum to minimum in synchronism with the triangular input voltage wave form, either triangularly in the absence of network 22, or sinusoidally when the network is inserted (Fig. 15). For attenuation equalization, net. work 11 is preset to the inverse of the required attenuation/frequency characteristic of the equalized channel, being flat when a flat characteristic is required, and the output of the attenuation equalizer 8 having independently adjustable control elements 53, 54, 55 (which may be of the kind described in U.S.A. Specification 2,348,572) is connected to switch 77 selecting the inputs of one of three distinct pairs of weighting-averaging impedance networks 70, 70<SP>1</SP>; 71, 71<SP>1</SP>; or 72, 72<SP>1</SP>, of characteristics determined as above, whose outputs are respectively selected by switches 78, 79 ganged with switch 77 and applied to positive and negative diode detectors 73, 74 (Figs. 7, 8, not shown) whose unidirectional outputs are compared over equal resistances 94, 95 and common voltmeter 75, which may have a centre zero. Initially switches 77, 78, 79 connect to the equalizer output the weighting function networks 70, 70<SP>1</SP>, introducing in combination the respective positive and negative portions of an attenuation/frequency characteristic appropriate to the corresponding characteristic of equalizer section 53 as described below, and the response of the voltmeter indicates the sense and direction of the required adjustment of the proportion of the equalizer characteristic which must be effected to zeroize the voltmeter reading. The switches are adjusted to connect weighting function networks 71, 71<SP>1</SP> appropriate to equalizer section 54, which is similarly adjusted, and finally the process is repeated for weighting function networks 72, 72<SP>1</SP> and equalizer section 55. The transmission channel is then equalized with a close approximation to the required characteristic; the respective equalizer section adjustments being substantially independent. When the weighting functions G (f) have values of one polarity only, networks 70<SP>1</SP>, 71<SP>1</SP>, 72<SP>1</SP> may be dispensed with, while weighting functions having both positive and negative values may be introduced using networks 70, 71, 72 only by giving network 70 a characteristic 1 + G (f) appropriate to the nth equalizer characteristic and positive over the entire frequency range, network 71 a characteristic #G = 0 (merely an attenuation characteristic) and network 72 a characteristic appropriate to each additional equalizer characteristic ; the difference between the meter readings for the insertion of networks 70, 71, 72, &c., representing the proportion of the appropriate equalizer characteristic to be introduced. The equalizer characteristics may approximate to the terms of a Fourier series over the range to be equalized, corresponding to the orthogonal harmonic cosine curves 59, 60, 61 (Fig. 10) which characteristics are separately adjustable by the appropriate controls 53, 54, 55, or over. lapping orthogonal non-harmonic curves 106, 107 (Fig. 11); or orthogonal non-overlapping curves 113, 114, 115 (Fig. 12) or overlapping non-orthogonal curves 118, 119, 120 (Fig. 13). The harmonic curves of Fig. 10 may be extended to a series of e.g. 25 successive equalizer characteristics for wideband equalization, adjustable individually as to the proportion of each individual curve which is to be inserted into the overall characteristic by controls similar to 53, 54, 55. Since the phase/frequency characteristic is linear, a triangular input frequencymodulation characteristic such as 65 (Fig. 15) is appropriate. Where the phase/frequency characteristic is non-linear (Fig. 14, not shown) a sinusoidal input frequency-modulation characteristic 66 is preferred (Fig. 15) obtained by the introduction of network 22 into the feedback path of the sweep signal source. The equalizer attenuation/frequency characteristics shown in Fig. 10 are given by n is the order of the equalizer, # is the fundamental phase angle f is the fundamental frequency, A o  is a constant, k n  is the proportion of the equalizer characteristic inserted and it is shown that the weighting networks 70, 70<SP>1</SP>. have frequency characteristics 1 + cos n# and 1 - cos n#, so that the transmission channel and the equalizer together have characteristics 1 + k n  cos n# + k m  cos m# where m # n. Then the detectors 73, 74 develop outputs given by (1 + cos n#) (1 + k n  cos n# + k m  cos m#) and (1 - cos n#) (1 + k n  cos n# + k m  cos m#) so that voltmeter 75 reads proportionately to the value required for k n , since the average value of cos n# cos m# = 0 when m # n. When the equalizer response curves are overlapping and non-orthogonal (Fig. 13) the weighting network characteristics are chosen with relation to the characteristics of the unequalized transmission channel, and may have composite characteristics embodying linear combinations of the equalizer response curves 118, 119, 120. For delay equalization, the sweep signals from source 1 are switched to a known balanced modulator 12 which also receives a fixed frequency from oscillator 58, and generates a pair of signals at frequencies spaced by twice the fixed frequency which together cyclically sweep over the operating range in similar manner to the single frequency of source 1. After passing through network 13 whose delay/frequency characteristics are preset to the inverse of the required delay equalization characteristics, the signals are switched to channel 7 and delay equalizer 8, and the output is applied over switch 77 to respective pairs of weighting networks 70, 70<SP>1</SP>; 71, 71<SP>1</SP>; and 72, 72<SP>1</SP> as before, whose characteristics are similarly chosen with regard to the phase delay/frequency characteristics of the respective equalizer sections. The outputs of the pair of weighting networks are respectively connected over switches 78, 79 to positive and negative delay detectors each comprising (Fig. 9) a diode 102 whose output comprises the difference between the two input frequencies applied to the equalized transmission channel, repetitive with period t 2  (Fig. 3) and phase modulated by the delay characteristic of the channel and equalizer, so that the diode rectified signal comprises a carrier and sidebands spaced at 1 1 intervals of - or - (Fig. 3), from which narrow t 1  t 2