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HIGH Q CAPS FOR VLF LOOP ANTENNA?
On 28 Oct 2005 09:44:00 -0700, "K7ITM" wrote:
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. It appears to me that a 10 ohm receiver input impedance is dead center with regard to any loop configuration I could come up with...it doesn't work with anything! It's easy to see now:: 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. Is the MMBT2222 the same as a 2N2222, which I already have in my junk box? Also have 3904's, maybe they are just as suitable? Should I tune the output, or rely on a modest input tuned filter in the front end and just do a no tune in and out common emitter? Assuming a 50 ohm receiver input impedance, is it ok to take the output across a 50 ohm emitter resistor? 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. JFET's sound good too. Richard, which type of buffer amp has a lower distortion and better linearity in the presence of (potentially) large out of band signals? If I do use a buffer to preserve the Q, it has to be 'clean' in order to preserve the performance of the QSD receiver. Appreciate any enlightenment along these lines. I'm not opposed to using an op amp IF it provides cleaner output signals and if the power consumption is manageable. I must say it seems much cheaper (and probably more practical) to use a buffer to preserve the existing Q than it is to build higher Q into the loop...only to have it cut in half by the addition of a receiver front end. Regards, T |
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