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Clapp oscillator

I found this surrey.ac.uk past paper whilst surfing:

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I'm a bit confused by the 0.5 ohm loss resistance shown in figure A1 on page 2. The capacitive reactances add up to -j160. If the inductor was +j160 to resonate, the unloaded Q would be 160/0.5 = 320, which sounds a bit high even for unloaded Q. It says Q=80 on the diagram, which is more like it. Did they calculate 80/160=0.5 by mistake? Is

80 supposed to be the loaded Q? What am I missing??

BTW if you go up a few directories on that URL, you can browse all their past papers.

Reply to
Andrew Holme
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I don't see where the .5 ohm resistor is labeled as being a loss resistance. I think it is probably the internal series resistance of the varactor.

Reply to
John Popelish

Hi,

By giving you the inductor's Q (= 80), they are indirectly telling you its total loss resistance (RF and DC) which I make to be 2 ohms. Add- in the 0.5 ohm parasitic and you finish up with Q(tot) = 64. This value doesn't include any loading by the bias and emitter resistors but then that is the beauty of the Clapp. This oscillator dates back to the 30's but J. K. Clapp published an article on it in QST as late as October 1948.

What I love about these old exam papers is that they remind one just how much has been forgotten over the years and forces you to dig out the books once again - good old Terman, et al... Cheers - Joe

Reply to
Joe McElvenney

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