CSP1027 Datasheet, PDF (42/64 Pages) Agere Systems – CSP1027 Voice Band Codec for Cellular Handset and Modem Applications

CSP1027 Voice Band Codec for
Cellular Handset and Modem Applications
Data Sheet
December 1999
7 Application Information (continued)
Arbitrarily set trap resonance to geometric mean of f1 and f3. Since f1 = f3/3, the geometric mean would be:
fT = --^f--³-- = ⁴----⁰------^M-----^H-----^z- = 23 MHz
3
3
At the third overtone frequency, f3, it is desirable to have the net impedance of the trap circuit (XT) equal to the
impedance of C2 (XC2), i.e.,
|| XT = XC2 = XC1 (XC3 + XL1)
Selecting C3 so that XC3 << XL1 yields,
|| XT = XC2 = XC1 XL1
For a capacitor,
XC = ---–---^j--
ωC
For an inductor,
XL = jωL
2
Solving for C1, and realizing that L1C1 = 3/ω3 yields,
C1 = ³-- C2
2
where ω = 2πf.
Hence, for C2 = 10 pF, C1 = 15 pF. Since the impedance of the trap circuit in this example would be equal to the
impedance of a 10 pF capacitor, the negative resistance and supply current curves for C1 = C2 = 10 pF at 40 MHz
would apply to this example.
Finally, solving for the inductor value, L1,
L1 = -------------¹--------------
4π²f ²TC2
For the above example, L1 would be 3.2 µH.
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