Passive RF · Filter Design

FM Band-Stop Filter

A seven-element elliptic band-stop that deletes the 88 to 108 MHz broadcast band before it reaches anything with gain in it. 60 dB down across the band, and 1.1 dB of loss at 137 MHz.

Tools

LTspice · Altium

Board

2-layer, all passive

Insertion loss

1.1 dB at 137 MHz

Rejection

60 dB, 88 to 108 MHz

Schematic of the seven-element elliptic band-stop ladder: four shunt LC branches to ground alternating with three parallel LC branches in the signal path — Seven-element ladder, four shunt and three series branches

Branch values and resonance
C1 / L1 · shunt	18 pF + 150 nH · 96.9 MHz
C2 ∥ L2 · series	47 pF \| 56 nH · 98.1 MHz
C3 / L3 · shunt	33 pF + 82 nH · 96.8 MHz
C4 ∥ L4 · series	43 pF \| 62 nH · 97.5 MHz
C5 / L5 · shunt	33 pF + 82 nH · 96.8 MHz
C6 ∥ L6 · series	47 pF \| 56 nH · 98.1 MHz
C7 / L7 · shunt	18 pF + 150 nH · 96.9 MHz

Rejection, 88 to 108 MHz	60 dB spec · 67 dB simulated at the worst point
Insertion loss at 137 MHz	1.1 dB measured · 1.12 dB simulated
Notch centre	97.5 MHz, all seven branches within 1.4 MHz
Order	7-element elliptic, symmetric ladder
Board	2-layer, all passive, SMA at opposite edges

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