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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

Summary of content (6 pages)

PAGE 1
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.
PAGE 2
BAND (meters) 160 80 40 20 15 10 Table 2 – Parallel Capacitance Values Desired Value Parallel Values C1, C2 C1A C1B C2A (pf) (pf) (pf) (pf) 1653 1500 150 1500 847 820 27 820 470 470 -470 240 240 -240 161 110 51 110 117 100 18 100 C2B (pf) 150 27 --51 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.
PAGE 3
The proper method for counting the turns on a toroidal inductor is shown in Figure 3. The core is shown as it would appear when stood on its edge with the narrow dimension toward the viewer. In this example, a four-turn winding has been placed on the core. Figure 3 – Toroid Turns Counting Pictorial Filter Construction The construction of the filter is fairly simple but requires some care. A component layout pictorial is shown in Figure 4.
PAGE 4
The capacitors C1 (or C1A and C1B) and C2 (or C2A and C2B) should be mounted on the PC board first. Refer to Table 2 for the proper values and the component layout pictorial in Figure 4 for proper placement. Next wind the toroid with the proper number of turns using the #18 AWG enameled wire included. The wire should follow the contour of the core and be snug. Refer to Figure 2 for the proper number of windings and the construction hints for the toroid winding procedures.
PAGE 5
Filter Parts List 3 each 2 each 2 each 3 each 3 each 3 each 3 each 1 each 160 Meter Filter Board Part Number FL1-160 T-106-2 Toroid Cores 1500 pf Silver Mica Capacitors 500 WVDC 150 pf Silver Mica Capacitors 500 WVDC 60 inches #18 AWB enameled wire 4-40 x ¾” bolt 4-40 Kep Nut Fiber Washer PC Board (FL1) 3 each 2 each 2 each 3 each 3 each 3 each 3 each 1 each 80 Meter Filter Board Part Number FL1-80 T-106-2 Toroid Cores 820 pf Silver Mica Capacitors 500 WVDC 27 pf Silver Mica Capacitors 500 WVDC 40 inches
PAGE 6
3 each 2 each 2 each 3 each 3 each 3 each 3 each 1 each 15 Meter Filter Board Part Number FL1-15 T-106-6 Toroid Cores 110 pf Silver Mica Capacitors 500 WVDC 51 pf Silver Mica Capacitors 500 WVDC 18 inches #18 AWB enameled wire 4-40 x ¾” bolt 4-40 Kep Nut Fiber Washer PC Board (FL1) 3 each 2 each 2 each 3 each 3 each 3 each 3 each 1 each 10 Meter Filter Board Part Number FL1-10 T-106-6 Toroid Cores 100 pf Silver Mica Capacitors 500 WVDC 18 pf Silver Mica Capacitors 500 WVDC 15 inches #18 AWB enameled wire