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Table 8.2.
General Inverters, Resonators, and End Couplings 301
Analysis Topology Code for the
Combline Filter in Figure 8.22 (Example 8.4)"
Type
4
314.16E6
-3
3
4
314.16E6
-3
3
4
314.16E6
-3
aUnits: ohms, pF, and fiH.
Value
23.68
45.00
97.66
36.76
8.8801
45.00
269.72
51.98
11.8403
45.00
216.86
Name
z",
Wo and° 0
C,
C"
Zo,
Wo and 8 0
C,
C"
z",
Wo and 9 0
C,
resistance R" is completely arbitrary; setting R" equal to 600 ohms increases
Z02 to 71.04 ohms. Changing all capacitances for this circumstance and
analyzing the network confirmed that the selectivity was indeed independent
of R'2' (Try it.) However, it turned out that C 23 = 13 pF, which is near the
minimum practical capacitance range. The trade-offs in this procedure are
quite visible. The solution for the unacceptably low value of ZOI appears in
Section 8.3.5.
Slope equivalence (8.45) is a means for estimating passband behavior, and
(8.52) estimates spurious passband frequencies for capacitively loaded, shortcircuited
transmission line resonators. Stopband selectivity estimate (8.27) may
be applied to the loaded-line case if the CK,q/C k ratio in Figure 8.20 is
determined for equal prototype and actual resonator susceptance at a given
stopband frequency. Replace CK by CK,q in (8.44) and equate this to (8.46).
Substitution of (4.27) and (8.48) yields the requirement for equal stopband
susceptances for Figure 8.20:
CK,q [w tan 110 ] I
c:;;-= W
o - tan(lI ow/w o ) F'
For the second harmonic, this reduces to
(8.55)
CK