Hi all,
some thougts with regards to the CMRR of a low-Z loop amplifier for an aperiodic broadband loop:
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- the desired differential reception voltage of a loop is the magnetically induced voltage across the loop clamps.? This voltage drives a current into low Z transimpedance amplifier (aperiodic broadband loop) or feeds a resonance circuit formed of the loop inductivity with an external capacitor (Hi-Z selective tuned loop)
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- the common mode voltage ist a voltage induced mainly by the electric field. The voltage occures equally at both clamps with reference to earth.
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- the Z of the source for the common mode voltage consists - equivalent to a short monopole - of a verly low resistance in series to a (frequency dependant) high capacitive reactance. (some milliohms in series to a few picofarads)
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- because of the latter, the CMRR measured with a source impedance of 50 Ohm must not be comparable to the CMRR voltage of a loop acting as a short monopole, as a reactive voltage source connected to a low -Z amplifier input impedance. A short monopole dummy antenna should be used for the measurement.
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What is your opinion?
regards, Fred
