PubMed · 7382584
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M G Pepper. 1980. Pacemaker vector cardiograph.. https://doi.org/10.1007/bf02442488
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Using separate preamplifiers for the two outputs of a quadrature receive coil (and then combining the preamplifier outputs in a quadrature hybrid) provides a better signal-to-noise ratio (SNR) than is obtained by directly combining the quadrature outputs in a hybrid followed by a single preamplifier. The advantage of the two-preamplifier configuration increases when the body coil impedance changes and is no longer matched to 50 ohms. Using 0.4 dB noise figure preamplifiers, theory predicts 1.53, 0.42, 0, 0.42, and 1.53 dB SNR advantage of the two-preamplifier configuration over the one-preamplifier arrangement at body coil impedances of 12.5, 25, 50, 100, and 200 ohms, respectively. Experimental hot/cold resistor noise figure measurements indicate 2.86, 0.65, 0.36, 0.83, and 1.40 dB noise figure advantage for the two preamplifier configuration relative to the one-preamplifier configuration at those impedances. Empirical gains larger than theoretically calculated are attributable to insertion losses of various circuit elements, such as the quadrature hybrid, for the one-preamplifier configuration.
Crosstalk due to coupling produces noise correlation between receiver coils. It has been stated that this correlation reduces the signal-to-noise ratio obtainable from combining signals from the coils. In this paper, it is shown that the effects of crosstalk on the signal-to-noise ratio may in theory be eliminated by properly combining signals. Equations are derived which show how the signals from two coils should be combined in the presence of crosstalk in order to obtain the same signal-to-noise ratio as in an ideal case of no crosstalk. The deviation from optimum signal-to-noise ratio due to imperfect circuits and amplifiers is discussed. An experimental technique for achieving the proper combination of signals is presented.
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