On Thu, Jan 30, 2025 at 10:46 PM, Fred M wrote:
you measured the CMRR of the amplifier standalone, not the CMRR of the active-loop antenna.
Yes, pre-amp only.?
BTW, I thought the whole group knows Martin & I have been long trying to eliminate the loop aerial while characterizing the pre-amp, by using a dummy aerial.
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For the life of me, I can't understand why would anyone want to include the loop in the characterization when comparing different pre-amps.
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Folks are telling me 'horror stories' of how these GHz fT transistor devices like to oscillate.
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Hi Dhiru, I don't know if in Fred's mind that I fit the category of "folks who have no real practical experience". IMHO, the risk is real and GHz transistors deserve their notoriety. Most people don't notice because the instability can occur way above the limits of their spectrum analyzer, e.g. Ka band and in temperature extremes. Oftentimes, I discovered it only after having connected a power sensor (the sensor can respond to frequencies above its specified maximum, though un-calibrated).?
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I moved the exact point of connection of the capacitor by a few mm at most, to demonstrate how easy it is to obtain a change in CMRR. The variation is very noticeable, even with a slight alteration in layout, and in a fairly well controlled workshop environment
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Yes, I experienced the same variation too. A small difference in the length of the wires connecting the test source to the balanced input terminals resulted in a different reading. In the case of M0AYF and PA0FRI, I minimized the variability by widely opening the red crocodile clip so that it could contact both terminals (photo below)
/g/loopantennas/photo/300229/3881034
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