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in the radio. The L/C ratio, in turn, determines their Q and so their selectivity, because the rest of the circuit - the aerial or amplifier feeding the tuned circuit for example - will contain present resistance. For a series resonant circuit, the higher the inductance and the lower the capacitance,
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will be relatively high compared with the series source/load resistances). For a parallel resonant circuit the opposite applies; small inductances reduce the
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tuned circuits for selectivity. Fixed tuning eliminates the requirement that multiple tuning stages accurately match while being adjusted.
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stray capacitance, and capacitance within the transistors or valves of associated circuitry, may become significant (and vary with time)
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large inductances imply physically large (and expensive coils) and/or thinner wire (hence worse internal resistance).
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the series resistance internal to the wire in the coil, may be significant (for parallel tuned circuits especially)
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There are practical limits to the increase in selectivity with changing L/C ratio:
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The
American Radio Relay League: "The Radio Amateur's Handbook, 1968", page 112
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165:"Practical RF circuit design for modern wireless systems", volume I, by
66:. If the signal is at the adjacent channel of the selected signal, this
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Therefore other methods may be used to increase selectivity, such as
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tuning capacitors of large values can be difficult to construct
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173:; chapter 3.2.6, "Receiver selectivity" (p. 113),
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received against that of a similar signal on another
108:of the inductance, L, and the capacitance, C, at
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104:the narrower the filter bandwidth (meaning the
81:Selectivity also provides some immunity to
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50:Selectivity is usually measured as a
35:) and reject other signals nearby in
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95:("Quality" factor) determines the
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72:adjacent-channel rejection ratio
91:are often used as filters; the
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1:
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152:allow use one or more fixed
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116:of external circuitry (see
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31:it is tuned to (such as a
150:Superheterodyne receivers
27:to respond only to the
154:intermediate frequency
146:regenerative receivers
220:Broadcast engineering
215:Electrical parameters
118:electronic oscillator
58:(dB), comparing the
19:is a measure of the
110:resonant frequency
39:, such as another
70:is also known as
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45:adjacent channel
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60:signal strength
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85:interference.
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179:1-58053-521-6
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171:Rowan Gilmore
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144:circuits and
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101:tuned circuit
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142:Q multiplier
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29:radio signal
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99:of each LC
89:LC circuits
68:measurement
21:performance
17:Selectivity
209:Categories
186:References
167:Les Besser
83:blanketing
106:reactance
97:bandwidth
64:frequency
41:broadcast
37:frequency
160:See also
56:decibels
114:damping
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43:on an
52:ratio
23:of a
175:ISBN
169:and
76:ACRR
120:).
78:).
54:in
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93:Q
74:(
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