Patent GB-940115-A (GB)
Title: Improvements in and relating to the measurement of random noise in electrical signals
Inventor: HOLDER JOHN ERIC
IPC: G01R29/26
Filed: 1960-12-12
Abstract: 940,115. Voltage frequency measurement. BRITISH BROADCASTING CORPORATION. Jan. 8, 1962 [Dec. 12,1960], No. 42689/60. Heading G1U. Apparatus measuring random noise power in a signal such that it includes transmitted power in plural bands of frequency separated by further frequency bands containing no transmitted power, comprising sources supplying at least two carrier oscillations of differing frequencies to a circuit modulating the signal with the carrier oscillations which are of such frequencies that the modulation produces in one or in part of one of the further frequency bands modulation components of the noise which is present in a plurality of others of such further bands and no other modulation components; there being provided a filter for selecting the noise modulation components from other components generated by the modulation and a power measuring device energized by such noise modulation components. A video signal at 10 (Fig.3) comprising line synchronizing pulses at frequency F L  separated by constant amplitude levels is connected over switch 11 alternatively with a calibration random noise generator 18 to filter 12 feeding modulator 13 receiving carrier frequencies from oscillators 15,16; the modulated signal passing through second filter 14 to energy measuring device 17. It is shown that the signal comprises a series of harmonics F L , 2F L , 3F L  &c. of the line frequency each surrounded by a group 20 of sidebands of bandwidth 2f 1  the groups being separated by frequency bands 21 of width f 2  containing only noise (Fig. 2). The carrier frequencies are integral multiples of F L  and a first carrier e.g. 3F L  produces for the sideband group surrounding harmonic 3F L  modulation signal components in band A-B from zero to f 1  while noise in the bands 21 (FG, HJ) adjacent to harmonic 3F L  produce modulation noise components in band B-C from f 1  to (f 1  + f2) where f2 is the width of each band 21. A second carrier e.g. 12F L  produces modulation signal components in band A-B for the sideband group surrounding harmonic 12F L  and modulation noise components in the range B-C from f 1  to (f 1  + f 2 ). Other carrier oscillations may be added to produce from other sideband groups 20 signal modulation components in band A-B and noise modulation components in band B-C. Filter 14 is thus turned to pass only band B-C and filter 12 to remove from the input signal those components which would beat with carrier harmonics to produce components in band B-C. It is shown that carrier oscillations 12F L  and 3F L  also produce modulated signal components in range C-D from (f 1  + f 2 ) to (3f 1  + f 2 ) and modulated noise components in range D-E from (3f 1  +f 2 ) to (3f 1  + 2f 2 ), and filter 14 may also be arranged to pass these bands of frequency to measuring device 17. The carrier oscillations may be obtained by filtration from harmonics of a pulse waveform synchronized to the line frequency and plural modulators may be utilized in parallel, or the carrier frequencies may alternatively be made equal to the midfrequencies of successive bands 21 containing noise only. Signal/noise ratio of the signal may be determined from total random noise applied to the input measured by inductor 17 and the peak to peak video signal amplitude.