User Manual

3. 20B1118G1 Low Pass and Notch Filter for Low Band:
Please refer to Figures 6 and 7. Ten adjustable inductors, four fixed inductors
(which also can be adjusted slightly by spreading their turns with a suitable
tool), and twenty-one fixed capacitors, make up the complete filter.
Functionally, it consists of two sections of shunt m-derived low pass network,
followed by four bridged-tee notch networks. Its signal direction is intended to
be one way only, as indicated in Figure 7.
The low pass section (L1, L2, C1 thru C5) of the filter is designed to cut off at
a frequency lower than the second harmonic, so that only the fundamental is able
to pass through, but harmonics are blocked.
The four notch sections are tuned to reject f
V
_9, f
V
_4.5, f
V
+9, and f
V
+13.5 MHz
(down typically 18, 30, 24, and 26 dB respectively - sometimes one of the tuned
circuits at f
V
_9, either L3 or L5, is readjusted to f
V
_4.5 in order to make that
notch deep enough that the f
V
_3.58 MHz component is sufficiently attenuated).
L4, L7, L10, and L13 are adjusted to set the width and depth of the notch. A
typical network analyzer transfer function plot of the filter indicates the four
notches at the values stated above, and the band pass of the f
V
and f
A
frequencies and the channel between, having less than 0.1 dB of ripple.
If your particular filter appears to be missing some of its capacitors, don't be
concerned. Probably the "missing" ones are chip capacitors soldered to the pads
on the underside of the board. The circuit board is a mature design which was
made before surface mount components were in widespread use; occasionally we run
out of low value capacitors having leads. Most RF parts used these days are
surface mount.
4. RF Directional Coupler:
A directional coupler is based on the principles of inductive (magnetic) coupling
and capacitive coupling.
In the LARCAN quad directional coupler implementation as shown in Figure 3
(schematic equivalent) and Figure 8 (assembly), the RF to be sampled passes
through a microstrip transmission line that is connected between the transmitter
output filter at J3 and the antenna system at J4. The magnetic field surrounding
the hot conductor of this transmission line induces a small RF current flow in
other conductors situated parallel to it. One end of each sampling conductor is
terminated by a resistor to ground. Sometimes small capacitors are connected
across these resistors to provide a termination that remains resistive over the
band. The other end of each sampling conductor connects to an external load,
usually a 50 Ù input of something such as an RF detector for AGC, the station
demodulator, or an RF detector for VSWR sensing.
If the sampling system as described in the forgoing paragraph were dependent only
on magnetic coupling and absolutely no capacitance were present, the external
loads would be driven with RF samples regardless of the direction they came from.
Omnidirectionality is not wanted; our objective is that the system should be
directional, that is, a signal coming from the transmitter should be seen by the
"forward" ports, and a signal reflected back from the antenna should be seen by
the "reflected" ports, but at the same time as little as possible of the forward