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#10
November 17th 07, 02:26 PM
posted to rec.radio.amateur.boatanchors
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BC-453 IF tuning
"Richard Knoppow" wrote in message ... I tried to find an illustration of the effect I am talking about on the web but could not. It would make things simpler. Q is a measure of the ratio of inductive reactance of an inductor to resistance. the higher the value of Q the better the inductor but there are circumstances where the Q may be delibrately limited. The bandwidth of a resonant circuit at resonance is affected by Q, in fact, the definition of Q is the ratio of the half-power bandwidth to the resonant frequency. Varying the Q of a resonant circuit also varies the amplitude, the lower the Q the greater the losses and the lowe the amplitude. Varying bandwidth by varying the mutual inductance of a transformer behaves in a different way. Up to a value of coupling known and critical coupling the bandwidth of the transmission curve does not change significantly but does increase in amplitude. If coupling is increased beyond critical the transmission curve becomes double peaked. Where there is no other coupling than magnetic the two peaks are symmetrical around the center frequency. Their deviation from the center frequency increases as coupling is increased but the amplitude does not decrease until very large values of mutual inductance are reached. The Q of neither side of the transformer is affected. There are many variations on the idea of providing for variation of mutual inductance. The Hammarlund method, using a physically moving coupling coil, allows the coupling to be varied without introducing variations in capacitance. Other methods, such as the one used in the well known Hallicrafters SX-28, vary both mutual inductance and capacitive coupling so that the two peaks gotten with more than critical coupling are not symmetrical about the center frequency. In fact, one tends to stay about at the center frequency while the other moves. It is possible to get symmetrical variation without using a moving element and this is done in some later variable coupling IF tranformers. Again, there is no effect on the Q of either circuit. Now, the bandwidth of an IF or RF transformer at critical coupling _is_ affected by the Q of the component coils which also affect the efficiency of the transformer. However, the variation of this Q is not generally used to vary the bandwidth of the transformer. All of this stuff is covered in many books on receiver design and basic circuit theory. The trick is finding one which is not overly mathematical. -- --- Richard Knoppow Los Angeles, CA, USA I agree with Richard's concise explanation. Pete |
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