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 2^nd and 3^rd 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 analyser 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 screen shot 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

 

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