Patent GB-745716-A (GB)
Title: Equalizer-adjusting circuit
IPC: H04B3/10
Filed: 1954-02-05
Abstract: 745,716. Measurement and equalization of attenuation/frequency and phase delay/frequency characteristics of a transmission line. WESTERN ELECTRIC CO., Inc. Feb. 5, 1954 [Feb. 6, 1953], No. 3441/54. Class 37. Apparatus for equalizing a transmission line for attenuation or phase delay comprises an equalizer network having plural independently adjustable attenuation or phase delay control elements, each of which regulate the magnitude of a distinct one of a plurality of harmonically related component attenuation/frequency or delay/frequency equalizer characteristics, enabling the overall attenuation or delay characteristics of the equalizer to be variable as desired, together with a sweep oscillator for applying to the line and the equalizer a test voltage of constant amplitude which is triangularly and repetitively frequency modulated; the output voltage of the line and equalizer being rectified to a direct voltage containing components of different harmonically related frequencies which are separated by tuned filters for independent measurement of the magnitudes thereof to obtain indications of the maladjustments of the respective harmonically related component equalizer characteristics, which may be severally readjusted for correct equalization, at which the corresponding harmonic frequency components are minimized. In Fig. 1 a sweep signal source 1 is alternatively switchable to a transmission channel 7 followed by an adjustable attenuation (or phase delay) equalizer 8 followed by an indicating receiver 9, through a network 11 of predetermined attenuation/ frequency response for attenuation equalization, or through a balanced modulator 12 followed by a network 13 of predetermined delay/frequency response for delay equalization. The sweep signal source 1 comprises (Fig. 2) a known generator 27 of a symmetrical triangular voltage/time waveform (Fig. 3, not shown) 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 sidebands by low-pass filter 47, and are fed back through an optional low-frequency attenuating 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 waveform either triangularly in the absence of network 22 or sinusoidally when the network is inserted (Fig. 9). For attenuation equalization, network 11 is preset to the inverse of the required attenuation/ frequency characteristic of the equalized channel, being flat when a flat characteristic is desired, and the output of the attenuation equalizer 8 having independently adjustable control elements 52, 54, 55 (which may be of the kind described in U.S.A. Specification 2,348,572) is connected to the input of a receiver 9 (Fig. 10) wherein a signal rectifier 70 (Fig. 13, not shown) and amplifier 71 alternatively energize over ganged switches 92, 93 a telephone 85, a power meter 86, or an A.C. voltmeter 90 over selected band-pass filters 87, 88, 89 tuned to frequencies # 1 ,2# 1  and 3# 1  respectively. The attenuation/. frequency characteristics of the equalizer approximate to the first three terms of a Fourier series over the range to be equalized corresponding to the harmonic cosine curves 59, 60, 61 (Fig. 7), which characteristics are separately adjustable by the appropriate equalizer controls 52, 54, 55, which give a flat equalizer response when each is set to the centre of its adjustment range. The equalizer has a linear phase/frequency characteristic appropriate to a triangularly frequency-modulated input but where the phase/frequency characteristic is non-linear (Fig. 8, not shown) it is desirable to utilize the sinusoidal frequency modulation characteristic (Fig. 9) obtained by the insertion of network 22 in the feedback path of the sweep signal source. In general the channel is at first unequalized for attenuation so that the input signal frequency-modulated at frequency f 1  is shown to develop at the output of amplifier 71 (Fig. 10) a cosinusoidally varying signal at frequency f  1  (Fig. 15, not shown) corresponding to maladjustment of the equalizer section having response wave 59 (Fig. 7) together with similar signals at frequencies 2 f  1  and 3f 1  corresponding to maladjustment of the equalizer sections having response curves 60, 61. Filters 87, 88, 89 are respectively tuned to f 1 , 2f 1 , and 3f 1  so that equalizer controls 52, 54, 55 are successively coarsely adjustable to reduce the power output of the channel to telephone 85 or power meter 86, and are then successively finely adjusted to minimise the output of each filter in turn to the voltmeter 90. When this is achieved the channel is correctly equalized for attenuation. In modifications the filters may be connected selectively in a feedback path across the receiver amplifier (Fig. 11, not shown) or may be replaced by a modulator and adjustable oscillator whose output is selected by a narrow bandpass filter feeding the A.C. voltmeter; the response frequency being selected by the modulation oscillator frequency (Fig. 12, not shown). Further variable equalizer sections having cosinusoidal response curves corresponding to higher multiples of f 1  may be added, together with additional filters of appropriate pass frequencies in the receiver. 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 applied to channel 7 and delay equalizer 8 as before, at whose output the indicating receiver 9 -comprises instead of the signal rectifier 70 (Fig. 13, not shown) a diode rectifier 79 followed by a wide band-pass filter accepting signal carrier and side-bands over the whole input frequency sweep, and a narrow band-pass filter 83 accepting only a signal of the frequency of the spacing between the pair of sweep signals (Fig. 14). The outputs of the filters are applied to a known phase-sensitive rectifier 82 to develop voltage signals on terminals 73, 74, which are applied to amplifier 71 for analysis in a similar manner to the corresponding output signals in the attenuation equalization procedure, whereby the control elements 52, 54, 55 of delay equalizer 8 may be adjusted for correct delay equalization.