PA3CSG website

5 pole Cauer filters for the HF bands

 

To build a solid state amplifier today, one has the availability of many fets. Preferably Ldmos as this is the most sturdy and the most forgiving to mistakes from the operator. The normal attenuation of a push pull amplifier on the HF bands is about 30-40dB. Thus requiring a filter.

 

The choice has been made for elliptic filters. These provide the steepest stop band attenuation.

 

 

 

The graph above has been taken from the Wikipedia website. One can adjust the notches for the 2nd and 3rd harmonic to obtain extra attenuation.

I never had my play with these filters. I started with the design in the ARRL 2010 handbook. But as this was not to my likings I designed my own. One filter for one band. The crosstalk on the original design was rather high on 6M so I decided to try a different approach. A filter for each band and all on separate PCB boards with two relays on each PCB. This would make the filter fully flexible and extend-able. The same PCB with relays can be used to make a bypass for the filter.

PCB is not too scale.

The PCB is a double sided epoxy FR4 type. The relays are soldered on the bottom part. The relay used here is the Omron G5LA. It is available in different voltages. The solder dots are 1mm brass rivets used as grounding contacts.

 

; }

All the rest of the components are soldered to the top layer. Each relay is bypassed with a 100nF capacitor and a 1N4148 diode.

 

 

 

 

Alignment

For the alignment of the filters, the home brew network analyzer from Jack, PE1KXH was used. This fine piece of equipment is available from SDR-kits. For more information check the references chapter or Google for DG8SAQ

 

First the L1-C2 and the L2-C4 have to aligned to match the second and third harmonic. If you increase the inductance of L1 and L2 too much your matching of the filter will go bad. Adjust the air wound inductors by bending, pulling them apart or pushing them together.

The coils wound on the toroids are more difficult to tune. Move the winding on the toroid. This has only minor effects. If this does not result one has to adapt the capacitor value. Fortunately these are the lower frequencies and therefore the less critical.

Results

Below are some measurements

The free software LTspice does a great job also simulating this kind of RF problems: See the screenshot below for the 7 MHz filter.

 



 

 

5 pole Cauer Low pass filters

Frequency

C1

C2

L1

C3

L2

C4

C5

160M

680pF

130pF

4,2uH

2000pF

3,3uH

560pF

560pF

80M

560pF

100pF

2,0uH

1000pF

1,5uH

330pF

470pF

40M

330pF

56pF

1,06uH

560pF

831nH

130pF

220pF

30M

150pF

22pF

765nH

330pF

603nH

80pF

100pF

20M

100pF

20pF

578nH

253pF

455nH

56pF

150pF

17M

100pF

15pF

443nH

200pF

349nH

68pF

56pF

15M

68pF

22pF

382nH

180pF

301nH

47pF

47pF

12M

68pF

12pF

334nH

150pF

263nH

33pF

47pF

10M

68pF

12pF

307nH

150pF

239nH

56pF

56pF

6M

36pF

6,8pF

157nH

70pF

125nH

30pF

27pF

 

 

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5 pole Cauer Low pass filters

Frequency

C1

C2

L1

C3

L2

C4

C5

160M

680pF SM

130pF SM

22wdg op T94-2 rood 1.5mm dia

2000pF SM

20wdg op T94-2 rood 1.5mm dia

560pF SM

560pF SM

80M

560pF SM

100pF SM

16wdg op T94-2 rood 1.5mm dia

1000pF SM

13wdg op T94-2 rood 1.5mm dia

330pF SM

470pF SM

40M

330pF SM

56pF SM

11wdg op T94-2 rood 1.5mm dia

560pF SM

10wdg op T94-2 rood 1.5mm dia

130pF SM

220pF SM

30M

150pF ATC100B

22pF ATC100B

10wdg op T94-6 geel 1.5mm dia.

330pF SM

9wdg op T94-6 geel 1.5mm dia.

80pF

33pF ATC100b 47pF ATC100B

100pF SM

20M

100pF SM

10pF SM 10pF SM

9wdg op T94-6 geel 1.5mm dia.

220pF SM

33pF ATC100B

8wdg op T94-6 geel 1.5mm dia.

56pF ATC100B

150pF ATC100B

17M

100pF ATC100B

3,9pF ATC100B

12pF ATC100B

8wdg 15mm dia. 20Mm lang 1,8mm dia

100pF SM

100pF SM

7wdg 15mm dia. 20Mm lang 2mm dia

68pF SM

56pF SM

15M

68pF SM

22pF ATC100B

7wdg 15mm dia. 20Mm lang

180pF SM

6wdg 15mm dia. 20Mm lang

47pF ATC100B

47pF ATC100B

12M

68pF SM

10pF SM

2,2pF ATC100B

7wdg 15mm dia. 20Mm lang

150pF ATC100B

6wdg 15mm dia. 20Mm lang

33pF ATC100B

47pF ATC100B

10M

68pF SM

12pF ATC100B

3,3pF ATC100B

7wdg 8mm dia. 1,3mm

150pF ATC100B

6wdg 8mm dia. 1,3mm

56pF ATC100B

56pF ATC100B

6M

33pF ATC100B

3,3pF ATC100B

6,8pF ATC100B

5wdg 8mm dia. 1,3mm

22pF ATC100B

47pF ATC100B

3wdg 8mm dia. 1,3mm

27pF ATC100B

3,9pF ATC100B

27pF ATC100B

 

 

 

5 pole Cauer Low pass filters

Frequency

C1

C2

L1

C3

L2

C4

C5

160M

680pF

130pF

4,2uH

2000pF

3,3uH

560pF

560pF

80M

560pF

100pF

2,0uH

1000pF

1,5uH

330pF

470pF

40M

330pF

56pF

1,06uH

560pF

831nH

130pF

220pF

30M

150pF

22pF

765nH

330pF

603nH

80pF

100pF

20M

100pF

20pF

578nH

253pF

455nH

56pF

150pF

17M

100pF

15pF

443nH

200pF

349nH

68pF

56pF

15M

68pF

22pF

382nH

180pF

301nH

47pF

47pF

12M

68pF

12pF

334nH

150pF

263nH

33pF

47pF

10M

68pF

12pF

307nH

150pF

239nH

56pF

56pF

6M

36pF

6,8pF

157nH

70pF

125nH

30pF

27pF

 

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