An experimental study has been undertaken in an endeavour to achieve an understanding of the mechanism of carrier-leak in shunt modulators, to elucidate some of the factors affecting the attainment of low leak levels (in the range 40 to 80 dB below the carrier voltage), and to investigate the possibilities of d.c. balancing of the rectifier bridge. With a square-wave carrier voltage in a non-reactive circuit (or at low frequencies) conditions approximate to those of ideal switching; carrier-leak is a function of the unbalance of the rectifier resistances for two specific voltages (one positive and one negative) and is quite calculable. A very low total leak voltage (i.e. including all frequency components) can be obtained by means of a simple resistance balance. The correlation of d.c. and a.c. components of the leak is excellent if rectifiers with high or alternatively well-balanced back-resistances are used. With a sinusoidal carrier supply, carrier-leak arises during the transition period of rectifier switching (i.e. during the change from forward to backward resistance and vice versa), and can therefore no longer be completely eliminated by a simple resistance control. Complete suppression of any one component (usually the fundamental) is theoretically possible in the non-reactive case, but in practice, unbalance of rectifier and stray reactances gives rise to a finite leak voltage. Results, even at low frequencies (less than 5 kc/s), are thus inferior to those obtained using a square-wave carrier, and in the experimental circuit it was difficult to obtain, by resistance balancing, fundamental components of the leak voltages lower than 0.1% of the carrier forward peak potential difference. Correlation between d.c. and a.c. balance conditions is also inferior, and zero d.c. leak might correspond to anything up to 0.4% leak of fundamental a.c. component. It is shown that the use of a capacitive as well as a resistive balance control can reduce leak voltages to a v-nery low order, approaching?and with care equalling?zero on a short-term basis. At higher frequencies, reactance unbalance plays a very large part in the production of carrier leak. It is demonstrated that the minimum fundamental leak voltage obtainable with a resistive balance alone rises linearly as frequency is increased, and that a combination of resistive and reactive balance controls can be used to achieve suppression of one component as good as, or better than, that obtained at low frequencies without a reactive balance. The a.c./d.c. correlation at high frequencies is quite reasonable, even with a reactive unbalance. With an unbalance of 50 ??F at 1 kc/s (equivalent to the probable value of 0.5pp??F at 100kc/s) correlation was such that for zero d.c. leak the a.c. fundamental leak did not exceed about 0.5% of the carrier forward peak potential difference. It appears to be a fairly safe general conclusion that control of carrier-leak adjustment by the use of a d.c. meter is likely to prove very useful in practice and, provided that efficient rectifiers are used, should enable a.c. leak voltages to be maintained at least 40 dB below the carrier voltage across the rectifiers. Under reactive conditions, the effect of carrier-generator impedance is complicated. If it is itself partly reactive, the capacitance balance becomes frequency dependent, i.e. a different adjustment of the balancing capacitor is required at each frequency. This dependence on the carrier-generator impedance is probably due to the fact that the rectifier bridge is not truly balanced over the whole switching cycle, but is merely adjusted to give a minimum output of one particular frequency. Some results are given for the type of shunt modulator having no transformer?a useful alternative to the Cowan type. Very low leak voltages are shown to be obtainable by adjustment of the relative value of the two coupling capacitors.......

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【期刊名称】: Proceedings of the IEE - Part B: Radio and Electronic Engineering

【期刊论文数据库】: [DBS_Articles_01]

【期刊论文编号】: 108,691,336

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