SM7OVK - Simple LF-filter


The idea with a filter is to make it easier to receive a weak signal, or a weak signal among several strong signals. Since there originally is no CW-filter in the FT225 I thought it could be nice to try this simple filter. The filter was published in QTC 6/2000, written by SM4INV. There is nothing magical about this filter and it would be easy to make another design. The nice thing about this filter is simplicity since it's made of only a capacitor and an inductor.


I used a Schaffner RN112-0,4/02, a 2x39mH inductor, together with a capacitor of 0,47uF. The two inductors should be connected in series and this will give a total of 156mH since the two inductors are wound on the same core. The capacitor and the inductor-package are put in parallel to form a bandpass-filter, ie all other frequencies but the resonance-frequency, will be "connected" to ground.
When used, the filter is connected from ground to the LF-line from the volume-control, ie between the volume-control and the AF AMP-unit, PB1764 - pin13. You can hear the difference here.

Calculation example

The resonance-frequency will be;

f=1/2*pi*(L*C)0,5=1/(2*pi*(156*10-3*0,47*10-6) 0,5)

The resonance-frequency is about 600Hz. I think that 600Hz is a little to low and it would be easy to change the capacitor if you would like a higher frequency. (The received signal is SK4MPI, a beacon in JP70 about 550km from my QTH. I tried to compress this file but since it contain a lot of noise it is hard to compress it, therefore it is quite short and I am working on a way to show this in a better way.)


Before you start with the modification, make sure the AC-cord is disconnected! Turning the radio off is not enough! Lethal voltages are present inside the radio and YOU might go QRT!

To switch the filter on/off I use the "AUX"-switch on the lefthand-side of the front-panel. (I have never heard of anybody using this feature so the decision was easy.)
I cut the PCB tracks and soldered a thin coax from this switch to the volume-control. The inductor and capacitor is soldered together and I put them behind the AUX-switch, connected to the switch with the same type of thin coaxial cable as to the volume-control.
NB! On the schematic there is only shown three connections to the AUX-switch but I had five wires! Cut the wires and isolate carefully!