Winding the 36:1 Transformer on a torroid form
Monolythic Crystal Filters are use in two different circuits of the SSA. The MCF from different suppliers and in different frequencies ranges and bandwidths have different input / output impedances. I chose to use a transformer to match the impedances, the transformer offering the possibility to change the winding ratio to accomodate different MCF devices. The target impedance for the selected MCF is 1800 ohms. The input impedance to output impedance in a transformer is related by the formula Zp = Zs*(Np / Ns)^2 . Therefore, to get a 36:1 impedance ratio a turns ratio of 6:1 is required.

The torroid is a FT37-43 which was chosen for operation at 10.7MHz. In the future these instructions may be expanded to use a FT23-43 core which is quite a bit smaller (about 5/8 diameter).

Making a transformer
Start by cutting two lenghts of wire. The first is 3.0" (7.6cm) of emameled xxAWG wire for the primary and the second wire is 8.5" (21.6cm), used for the secondary winding. At this time srtip and tin 0.325 to 0.5" (1cm) each end of both wires.

In the following instructions: 1)hold the core by the one side of the torroid with the hole exposed, 2) except starting the first turn of each winding, all motion of the wire will be from the center of the torroid around the outside of the torroid and back through the center from the bottom of the core. This constitutes one turn. On each turn, draw the wire tight to the torroid but do not stress or strain the wire.

The primary winding
Put the end of the primary wire through the center and allow it to extend approximately 0.5", then add two turns according to the instruction 2, above. The winding should proceed around the torroid in a clockwise fashion, taking care not to over lap turns or kink the wire. The transformer should look like this picture at this point.

The secondary winding
As before, put the end of the secondary wire through the center an allow it to extend approximately 0.5", then add seventeen (17) turns according to the instruction 2, above. This winding should proceed around the torroid in a clockwise direction. Take care not to overlap or kink the wire. Now the transformer should look like the picture below.

The finished transformer
The phasing of the transformer is such that in both the primary and secondary finish by coming over the top of the core. In all cases this is the end of the winding that goes to ground.

The finished product soldered into the NARROW_FILTER_Rev.0 Board
Notice in this picture that the dotted lead comes from under the transformer an solders to the pad. The end of the winding that comes over the top is soldered to the ground pad. After the circuit is proven I would suggest using a small amount of non-corrosive RTV to affix the transformer to the board. Hot glue might work just as well, but I don't use that stuff very much, myself.