Communication Concepts FL1 User Manual
FL1 LOW PASS FILTERS
Broadband amplifiers, by definition, provide little, if any, suppression of harmonic
energy. The output of the amplifier will contain harmonics of the input signal.
Thus, if direct operation into an antenna is expected, filtering of the amplifier
output is necessary to meet FCC regulations for spectral purity. A five element,
low pass filter will provide more than sufficient harmonic attenuation. The low
pass filter will attenuate signals above the desired output frequency.
Filter Design
The five element, low pass filter design is derived from information contained in
the ARRL Handbook. The filter schematic is shown in Figure 1. The various
filter parameters are shown in Table 1. The capacitance values derived for C1
and C2 are not standard values for some of the filters. In order to achieve the
closest value for the filter, standard values are placed in parallel. Provision has
been made on the PC board to accommodate the parallel values. When a
capacitance value requires parallel values, the capacitors are identified as C1A
and C1B for the parallel combination of C1. C2A and C2B are the parallel
combination of C2. These combinations are shown in Table 2.
Figure 1 - FL1 Schematic Diagram
Table 1 – FL1 Filter Parameters
BAND Fcutoff L1,L3 L2 C1,C2
(meters) (MHz) (uH) No. of Turns Toroid (uH) No. of Turns Toroid (pf)
160 2.1 8.1 23 T-106-2 11.4 28 T-106-2 1653
80 4.1 4.1 16 T-106-2 5.8 20 T-106-2 847
40 7.4 2.3 12 T-106-2 3.2 14 T-106-2 470
20 14.450 1.18 9 T-106-6 1.65 11 T-106-6 240
15 21.550 0.79 7 T-106-6 1.11 8 T-106-6 161
10 29.8 0.57 6 T-106-6 0.80 7 T-106-6 117
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Table 2 – Parallel Capacitance Values
Desired Value Parallel Values
BAND C1, C2 C1A C1B C2A C2B
(meters) (pf) (pf) (pf) (pf) (pf)
160 1653 1500 150 1500 150
80 847 820 27 820 27
40 470 470 -- 470 -20 240 240 -- 240 -15 161 110 51 110 51
10 117 100 18 100 18
Construction Hints
The effective inductance of a toroid coil depends in part on the distributed
capacitance between the coil turns and between the ends of the winding. The
distributed capacitance should be kept as low as possible. The pictorial
illustration in Figure 2 show the inductor turns distributed uniformly around the
toroid core, but a gap of approximately 30 degrees is maintained between the
ends of the winding. This method is recommended to reduce the distributed
capacitance of the winding. The closer the ends of the winding are to one
another, the greater the unwanted capacitance. Also, in order to achieve the
desired toroid inductance, the winding should be spread over the core as shown
in Figure 2.
Figure 2 – Toroid Winding Pictorial
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