Good day Steve,
The historic definition of Q is as Quality factor for a coil used in a
parallel-resonant circuit as the load of a vacuum-tube amplifier. The
bandwidth (selectivity in a receiver) was set by the coil loss. Thus, a
high-quality coil has a high Q=X_s/R_s, where X_s is the coil reactance at
the operating frequency and R_s is the effective series resistance. In a
tetrode or pentode circuit, the load is driven by a currentb source so
HPBW=1/Q. This is the definition given in Terman's "Radio Engineers'
Handbook", 1st ed., 1943. In the same book, he uses the bandwidth-Q
relation to define the Q of a resonant cavity, and then gives, as an
alternative, the electromagnetic definition
Q=2*pi*(stored energy)/(energy lost per cycle)
Why is the 2*pi in this definition? Because it's needed to make the
bandwidth right and work for an inductor.
In Krauss, et. al., "Sold-State Radio Engineering", 1980, the Q concept is
used in designing resistance-matching networks in which the reactive
elements are assumed lossless. Q describes the relationship of theelements
with the terminating resisors, and is still Q=1/HPBW. In this case, the
bandwidth is determined by both the source and load resistances. In Randall
Rhea's book on oscillator design, he talks about loaded and unloaded Q of a
resonator. You may have a rsonator with a high Q cnsidering only its
losses, but the additional loss of the connected circuit may cause the
effective Q (loaded Q) to be low, which in turns means the frequency
stability of the oscillator will be poor.
My point in all this is that one needs to be specific about the definition
and context of Q. Q is most useful when one can say Q=1/HPBW for the
circuit in which the device is to be used.
Doug Miron
-- The NEC-List mailing list <nec-list_at_gweep.ca> http://www.gweep.ca/mailman/listinfo.cgi/nec-listReceived on Tue Dec 31 2002 - 16:33:13 EST
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