New England QRP Club : The Excitement is Building....

Hey Doug,
Thanks for the encouragement. Actually, you wouldn't need two filters for CW/SSB. Some people have used the filter as-is for SSB, just crank open the bandwidth control. It can be helpful, though to shift the filter center frequency up for SSB.

For instance, a common center freq for CW might be 600 Hz. For SSB, about 1.5-1.6 KHz would be more appropriate. In the NEScaf, C6 (.001 uF, NP0) works with the trimmer pot and front panel center freq pot to set the clock frequency from the 555 timer. In our circuit, the clock runs at 100 times the center frequency desired. So, what we could do is replace C6 with two capacitors. One fixed on the board for SSB range, and one that is switched in parallel for CW use. The added capacitor lowers the clock frequency, and hence the filter center point.

If we start with the total overall value of 1000 pF (.001 uF) and do some quick math, we find that a fixed cap of 270 pF and another one of 730 pF switched in, will add up to the original 1000 pF, and give us the ratio we want (1600/600 Hz or about 2.7/1)). 730 pF is not a standard value, but could be approximated by a 680/47 parallel combo. These should all be NP0 or similar, low temp coefficient types.

One thing to keep in mind is that a switched capacitor lead must be kept short, so as to not radiate the clock signal into the rest of the rig. Keeping the SCAF board close to the front panel, and perhaps even shielding it may be needed. Another possibility is to switch in the added capacitance using an FET, controlled by a front panel switch. This means the leads to the switch would only carry DC, not your clock signal.

As always, your mileage may vary.

72/73 have fun es GL,
Bruce N1RX