LowPass Filter - Ham Radio Projects

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LowPass Filter

DOUBLE DDS
Signal from the amplifier is further modified by the LPF in every section. For each band (except of 17+15 and 12+10 meters)  separate LPF is using. Schematic was divided into several parts:


For LPF design I used very good PC software LADPAC 2002 (taken from "Experimental Methods in RF design", published by ARRL), which allows to check graphically and numerically all of important parameters during design. First and last inductors of every LPF were selected from a range of E12 (size 1210), inductance in the middle of the LPF was combined from two values of E12 in series. Inductors L1 and L4 for 6m LPF were made by hand. See the next table for more details. Meaning of each element is apparent from the sample scheme. Depending on the final inductance values capacitors they had to be corrected to achieve the desired parameters. At the end positions I used variable capacitors during LPF adjustment (C*, C**).


BandL1, L2+L3, L4C1, C2, C3, C4C2=C3 usedC1, C4 used
1604700, 5380, 47001198, 2640, 2640, 1150120+1200+1200+120C*+560+560+C**
802700, 3200, 2700872, 1640, 1640, 880820+820C*+390+390+C**
401500, 1600, 1500344, 858, 858, 37839+390+390+39C*+150+150+C**
301000, 1200, 1000320, 593, 593, 32027+270+270+27C*+150+130+C**
20680, 859, 680256, 412, 412, 24056+150+150+56120+120+C*
17+15470, 53, 470128, 276, 276, 12318+120+120+1856+56+C*
12+10390, 472,39095, 188, 188, 94120+10047+39+C*
6200, 235, 20049, 99, 99, 4847+47+4.722+22+C*
 
 

My LPF settings priorities in adition to the basic requirements:

1. Harmonics suppression. 
2. Impedance matching. To set as close as possible to real 50 Ohms LPF Input Impedance (from both sides) inside the amateur        bands range. 

Measurement results from N2PK VNA are visible in the following pictures.
 
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