There's a YouTube video, "Demo of a DIY digital L/C Circuit Q-Meter" <>
where the author, demonstrating a Q meter, tests an inductor using a number of different variable capacitors. Wasn't a rigorous or well controlled test, but a considerable difference in Q with different caps.
John
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On 8/9/2022 6:04 AM, Dr. David Kirkby, Kirkby Microwave Ltd wrote:
On Tue, 9 Aug 2022 at 13:02, Steven Greenfield AE7HD via groups.io <> <alienrelics@... <mailto:[email protected]>> wrote:
I don't think I've seen anyone mention the impact of the variable
capacitor on Q. I understand that for lower Q, the losses in the
inductor swamp losses in the capacitor. But at what point do
capacitor losses begin to impact the measurements?
In other words, will the garden variety tin coated, brass, or
aluminum plate variable do? Does it need to be silver plated for
lowest losses?
Steve Greenfield AE7HD
Somewhere I read that the capacitor loss can become significant above 10 MHz. So cheap capacitors would be yet another source of error. I would guess that a butterfly capacitor, where there¡¯s no sliding contact would be preferable. Likewise silver plating.
I can¡¯t help feeling that the answer to the losses *might* be to introduce a further known loss, to determine the original unknown loss.
Take for example an analog multimeter (eg AVO). If that is measuring a voltage from an unknown source impedance, and the impedance of the multimeter is unknown, you don¡¯t know the source voltage, as there are two errors. I would have thought that using a couple of known resistors, one could deduce the internal resistance of the multimeter and the source resistance, allowing the source voltage to be determined.
Dave
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Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@... <mailto:drkirkby@...>
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John Kolb