Minifilters for the Mobo

The Mobo can take 8 filters at a time. Two different filter sets are available for you to use. So you actually need EITHER SET A or SET B. Thus, one set of 10 filter boards will suffice.

However, if you love to experiment with filters, you might like to have both sets to play with. In that case, you will need 2 sets of filter boards (ie total of 20 boards).

Minifilter Set A - the Lazy Man's Set

SET A has parts for making 6 filters, which will provide a good continuous coverage from 160m to 10m. In the Minifilter SET A.xls spreadsheet, there is a plot of the filter response of the 6 filters. It provides good pass band for Tx in all the amateur bands, and it can receive all short wave broadcasts. All the inductors are SMT parts so no winding of toroids required :-)))

All the 3 inductors in a filter are of the same value, so it miminizes chances of mixing them up during construction. Therefore SET A will be much easier and faster to build. It will get you up and running in no time.

Minifilter Set B - the Experimenter's's Set

SET B has parts for making 10 filters, which provide narrower band coverage for individual amateur bands: 160/80/60/40/30/20/17/15/12/10m. However, as there are only 8 slots on the Mobo, you have to select 8 out of the 10 filters, depending on your particular band preferences - eg. whether you are a top bander, whether you need top performance in WARC bands etc.

The filter shape is assymetric, such that the 40m filter provides fairly good coverage for 60m, and 15m for 17m, 10m for 12m etc.

SET B kit also has BOTH the SMT inductors and the toroids/wire provided, so you can choose whether to build with SMT inductors or with toroids.

• The toroids based filters have better passband performance (so in Rx it is more sensitive, and in Tx, higher power etc.) because of the higher Q.
• However, the SMT inductor based filters have better out of band ultimate rejection (probably because there is less cross inductor coupling).

Therefore SET B is more suitable for keen experimenters.

Roll Your Own Set

Of course, you can design you own minifilter. The minifilter pcb is very flexible and allows many different filter topologies to be implemented, not just the Cauer BPF topology that we use in SET A and SET B. Even using the same topology, the component values can be tweaked for varied response.

The beauty of the Mobo design is such that these minfilters are pluggins. So you can make many minifilters and mix and match them . The filter cross over points are configurable via software (Fred PE0FKO's CfgSR.exe tool), so it is very easy and convenient to change.