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OK, maybe you're beginning to understand. Q can be calculated as
reactance (at resonance) divided by the effective series resistance, or as effective parallel resistance divided by reactance at resonance. For a loop where you know the series resistance, it's easiest to use that first relationship. If you put your 10 ohm receiver input in series with your 10 ohm reactance loop, you've ruined all that effort to get to a very low loop conductor resistance and obviated the need for high-Q capacitors. And we're all having a very hard time seeing how you will couple your 10-ohm receiver input to EITHER the parallel-tuned loop OR the series tuned loop, without having nasty consequences for your holy-grail Q. You might think it's best to impedance match ("conjugate match") to your load, so you transfer the most power to the load. However, that may not be optimum from a system design standpoint. If you already have enough signal (along with atmospheric noise) that the receiver doesn't contribute significantly to the overall SNR, then you may be better off by intentionally mismatching so that the Q remains high, if that's important to you. (I personally think you've overrated it, but that's up to you to decide.) But even if you're wanting to get the lowest noise contribution from your electronics, the appropriate match is generally not the conjugate impedance match that results in highest power transfer. For example, an MMBT2222 NPN transistor running at about 100uA collector current in a common-emitter configuration with no feedback will have a low-frequency (e.g. 60kHz) input resistance around 50kohms, but the optimal source resistance from a noise standpoint--the source resistance which will yield the lowest noise figure for the amplifier--will be about 2kohms. At optimal source resistance, you can get a noise figure well below 1dB from an MMBT2222--and from many other bipolars. One reason that people like to use FET amplifiers across their "parallel-tuned" loops is that the amplifier input resistance is quite high, but (using appropriate FETs) the noise contribution of the amplifier is negligible. And with proper design, the distortion contribution can be considerably lower than the distortion of your detector. For high source impedances, JFETs can give noise figures that are a small fraction of a dB. Cheers, Tom |
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