ier. The signal path, also through the diodes, is thus inverted on alternate half cycles of the carrier. If the input signal is represented by i (t) then the resulting The current flowing, i (t), due to the carrier signal is close to a modulated waveform at the output is given by Equation 20.2. i.e. the square wave due to the clipping action of the diodes and by Fourier output frequency spectra is formed by the upper and lower side- analysis is given by Equation 20.1, where k k3, k5 5 etc. are circuit bands about the carrier frequency and odd harmonics of the carrier constants. frequency. SUPERGROUP MASTERGROUP TRANSLATING TRANSLATING Supergroup 312 552 kHz Supermastergroup 500 17 500 kHz 3 600 Channel System Mastergroup Assembly 10 x Supergroups 564 3 084 kHz Note 1 SG BANK For previous Translation Television Stages see Figure 20.2 Translating 1 860 Channel System 3X MG Assembly 312 8 284 kHz 600 Channel System 60 788 kHz 10 x Supergroups 10X SG Assemby 2X MG 1 200 Channel System Assembly SG BANK Coaxial cable, Radio, and Satellite Links. Figure 20.6 Bell FDM 'long haul' hierarchy. The Bell System Supergroup carrier frequencies do not always correspond to the CCITT supergroup carrier frequencies Carriers 20/7 NUMBER Baseband signal Output signal spectrum. spectrum. fc-fsh fc+fsl 3fc-fsh 3fc+fsl 5fc-fsh 5fc+fsl fc-fsl fc+fsh 3fc-fsl 3fc+fsh 5fc-fsl 5fc+fsh Figure 20.7 Ring bridge modulation Table 20.1 Filter Fa requirements sin 5w t + .... (20.2) Product Level at Requirement Filter modulator suppression Note that neither the carrier frequency, carrier frequency har- output monics, nor the original baseband signal is present at the output. i.e. Carrier leak +20dBmO -50dBmo the modulator is 'balanced'. Signal leak -30dBmO -80dBmO 20.4.1 Ring bridge modulator/demodulator Upper -30dBmO -80dBmO design considerations sideband 20.4.1.1 Carrier compression. With this type of modulator the ratio of the change in carrier power to the change in signal loss of the modulator, known as 'carrier compression', is approximately 1