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Clapp oscillator

A

Andrew Holme

Jan 1, 1970
0
I found this surrey.ac.uk past paper whilst surfing:

http://www.ee.surrey.ac.uk/Teaching/Exams/pastpapers/01-02/Spring/Level4/EE4.rfs02.pdf

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.
 
J

Joe McElvenney

Jan 1, 1970
0
Hi,
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??

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

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
 
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