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Figure 33A. Determination of Inductor L<br />
be 100 times that of coil L. The inductance of Lc would then be<br />
950 microhenries. The series resonant frequency of the anode<br />
choke must be determined to assure that it is outside the normal<br />
operating frequency range of the amplifier. This can be checked<br />
by shorting the ends of the choke together using a low inductance<br />
strap and measuring the resonant frequency by means of a griddip<br />
meter or impedance bridge. Building an amplifier that covers a<br />
wide range of frequencies (for example 2 to 30 MHz) may present<br />
such a demand on the anode choke that the easiest and most<br />
effective solution is to use two rf chokes in series, shorting out one<br />
section on all but the lowest frequency band. Varying the pitch of a<br />
portion of the solenoid windings is another technique for avoiding a<br />
series resonance at an undesired frequency. Pi-wound chokes are<br />
often riddled with resonances and are generally not used in high<br />
power amplifiers.<br />
51
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