User Manual

1. Bandpass Filter: (continued).
LARCAN bandpass filter implementations generally consist of a cascaded series of
coupled resonators. Some use helical resonators; 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 to the 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.
Fold-out Figure 4 indicates the generic assembly of a coupled helical resonator
bandpass filter.
The referenced drawing in Figure 4 happens to be a low band filter, but the high
band unit is laid out identically and appears almost the same as Figure 4 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 below, and the filter
electrical equivalents are presented on the next page 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 the Figure 4 drawing 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 idyllic sweep curves are shown
below as Figure 1.