Add_LPF

On most Softrock RXTX models it is necessary to use an additional LPF on the lowest frequency of each band group that is used. (Not the 15-12-10 option). This is to provide some suppression of the second harmonic and some additional suppression of the third harmonic. Without this LPF the transmitter will not reach the specification for spurious emissions required by many authorities.

As an example if a Softrock is built for 40-30-20 metres then a 20m LPF is fitted. This LPF offers no suppression of the second harmonic of 40 metres, 7MHz x 2 = 14MHz. The same applies to an 80-60-40 metre build, an 80m LPF must be used. And to 30-20-17 metres, a 30m LPF is needed. 

This may be built as an external unit, maybe neater than this.

I built a 30 metre LPF into my30-20-17 Ensemble using a double pole two way switch to bypass it when not required.

Schematic shows the switch towards the bypass position. On the picture below the two top pins of this switch are shorted, being the bypass position.


The switch is mounted next to the on-board 17 metre LPF. A small piece of double-sided PCB material is used. The top is divided into three lands, two soldered to the switch, the third is the centre of the LPF where L200, L201 and C 201 meet. The underside of the board is grounded to the Ensemble ground plane. The three capacitors are soldered to their respective lands and to this grounded underside. A thick blue wire connects this to the Softrock ground.
The primary of T6 is lifted and connected by the purple wire to a switch pole. The other pole goes via the black wire to L3/C26 where T6 was originally connected. As the LPF is symmetrical either way is correct.
Connections should be short, if the LPF cannot be mounted close to the Softrock output then screened 50 ohm cable should be used. Maybe the antenna socket could be removed, a coax lead to the LPF switch from there and an antenna socket mounted elsewhere. 


An 80m LPF mounted on a switch. 
A relay could be used allowing the operating switch to be placed anywhere convenient. Something similar could be devised to connect the LPF to the relay pins.
 

The pictures give some idea of my homebrew enclosures. The Softrock board is mounted so access to both sides is possible without removing it. The double-sided PCB is easily put together and CAN be made very presentable but I'm more interested in getting things working than making them look good.

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