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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

1 2 3 4 5 6

FL1 LOW PASS FILTERS 1

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Page 1 - FL1 LOW PASS FILTERS

1FL1 LOW PASS FILTERSBroadband amplifiers, by definition, provide little, if any, suppression of harmonicenergy. The output of the amplifier will con

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2Table 2 – Parallel Capacitance ValuesDesired Value Parallel ValuesBAND C1, C2 C1A C1B C2A C2B(meters) (pf) (pf) (pf) (pf) (pf)160 1653 1500 150 1500

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3The proper method for counting the turns on a toroidal inductor is shown inFigure 3. The core is shown as it would appear when stood on its edge wit

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4The capacitors C1 (or C1A and C1B) and C2 (or C2A and C2B) should bemounted on the PC board first. Refer to Table 2 for the proper values and thecom

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5Filter Parts List160 Meter Filter Board Part Number FL1-1603 each T-106-2 Toroid Cores2 each 1500 pf Silver Mica Capacitors 500 WVDC2 each 150 pf

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615 Meter Filter Board Part Number FL1-153 each T-106-6 Toroid Cores2 each 110 pf Silver Mica Capacitors 500 WVDC2 each 51 pf Silver Mica Capacitor

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Communication-concepts FL1 Low Pass Filter Manuel d'utilisateur

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