1
BANDPASS FILTER
Drawing References: Figure 1 and Figure 4.
The LARCAN bandpass filter implementation consists of a cascaded series of coupled helical resonators. A
helical resonator is essentially a self supporting high Q coil (the helix) mounted inside a metallic shield enclosure.
One end of the coil is solidly connected to the shield enclosure and the other end is open circuited except for a
small trimmer capacitance to ground. The dimensions of the coil are critical as to frequency of operation; the
assembly behaves as though it were a quarter wave coaxial transmission line resonator. Several sizes of coils
and enclosures are necessary to cover the desired frequency ranges. Figure 4 indicates the generic assembly of
a coupled helical resonator bandpass filter.
The referenced drawing in Figure 4 is a low band filter, but the high band unit is laid out identically and appears
almost the same, except the high band helixes have fewer turns of coarser winding pitch, and their shield
enclosure dimensions are somewhat smaller.
The desired response shape is presented as Figure 1, and the filter electrical equivalents are presented as Figure
2. When we examine the assembly, and take capacitances into account, the equivalent circuit of a helical
resonator becomes simply a parallel resonant LC tank circuit having low (trimmer) capacitance and relatively high
inductance. Adjustment of the trimmer produces a change of capacitance, and the trimmer's moveable slug is
shaped to appear as a shorted turn, which alters the inductance of the helix.
Matching from and to 50 ohm transmission lines is accomplished with taps on the input and output helixes.
Coupling between sections is electrically a bridged T network of capacitors, and is made up of the small
capacitance between the free ends of the coils, controllable by the amount of capacitance to ground that is
introduced by the coupling adjustment screws; the coupling is maximum when the screws are backed out fully
from the enclosure. Shielding partitions placed inside the enclosure between helixes, produce fixed area
apertures which affect the coupling capacitance between helixes. Helix #3 in Figure 4 has taller partitions on both
sides of it, giving lower capacitance and less coupling than the others.
For system use, the tuning and coupling is adjusted for a flat topped response with steep sides, and the desired
shape is such that f
V
- 4.5 MHz and f
V
+ 9.0 MHz are both 30 dB down, but the carriers must be f
V
< 0.6 dB and
f
A
< 0.7 dB departure from flatness. Input and output return loss must be 20 dB or better over the full 6 MHz
bandwidth. These sweep curves are shown below as Figure 1A.
There are nine screw adjustments and two I/O matching (with soldering iron) adjustments that need to be made
simultaneously. Factory adjustment is never attempted without the aid of a network analyzer, and for this reason
we say the unit is not user-adjustable.
PUB96-26 Rev 1 September 13, 2005
26-2
RF Output: BP Filter & Directional Coupler
