Just to give you a heads up, I just ordered a NEScaf CW audio filter from the New England QRP Club . I will be using it with the FT-817ND. I figure it will be more helpful than the optional Yaesu or W4RT CW Filter module, and at a small fraction of the cost. $31 for a full-featured switched-capacitor audio filter kit is a very good deal. It is not an IF filter, but it will sure help a lot on the CW copy. Its variable bandwidth and center frequency will be a plus. I intend to carry it around when going portable-QRP.
Update (30/08/2011): Just received the kit yesterday and have started assembling it. Instructions are very clear. It total, it should take me about 90 minutes of assembly time before firing it up. I will report back on the unit as soon as it is assembled. So stay tuned!
Update (31/08/2011): I managed to find time to wrap up the assembly process and power up the NEScaf board. Here are my first comments on the assembly process and the filter operation.
The assembly process took me about 90 minutes, not including the enclosure work. I am not a beginner on these things, so a beginner will likely take more time than that.
The assembly instructions are very clear, with lots of additional notes on component substitutions, pin bending, potentiometer wiring, etc.
I used i.c. sockets on all three chips. For what they cost, it may save some work in the future.
When you follow the instructions, you get a bandwidth control knob that operates in the reverse manner compared to what I would have expected. Turning the control clockwise reduces the filter bandwidth. So I flipped the wiring on the potentiometer so that turning the control clockwise increases the bandwidth. A matter of preference...
It is better to use a metallic enclosure to properly shield the unit from the outside world. See the comment below.
It is very effective in eliminating unwanted CW stations. Of course, if the unwanted station is pounding in compared to a small signal you are trying to pick up, the radio AGC will play tricks on you. But that was expected since it is an audio filter. Garbage in, garbage out...
It sounds good, generally speaking. There does not seem to be any ringing (self-oscillation), that is if you keep your fingers away from the potentiometer contacts or the filter board. Touching the circuit components will turn this filter into a broadcast radio receiver or an oscillator! I presume using a metallic enclosure is a good idea, especially for portable operation on mountain tops.
The narrowest filter setting is quite sharp. It is about 40 Hz for a 3dB bandwidth. The widest setting bandwidth is about 1500 Hz. See the plots below. I have not tried the latter setting with an SSB signal yet, but will do this soon. Since the skirts are not that sharp, I think it is going to be OK. We don't really care because this is a CW filter after all.
The center frequency adjustment is a bit tight when in the smallest bandwidth. It will probably be better once the knob is installed on the potentiometer shaft.
I am not sure if I like the detent feature on the center frequency pot. If you are trying to fine tune to a station close in audio frequency to the center detent position, it is difficult to stay out of the detent position. I may decide to replace that pot with a regular one in the end.
The one thing definitely lacking on this filter is an audio bypass path for when you are transmitting. If the filter center frequency is not set to the CW sidetone frequency of your radio and the filter is at the smallest bandwidth, you will not hear your sidetone! So I will design in a bypass path kicking in when the CW key is pressed.
All in all, a very capable filter for its low price. If you are willing to assemble this kit, you will get a biggest-bang-for-the-buck filter. What is left for me to do:
Agree ! This is the Best Analog Filter Kit available.
Nearly ideal. Two Analog Filters on a chip, with support circuitry. Terrific Final Stage to my own Butterworth design audio filter for my HW8 and Kenwood TS830.
I have made a modification to the NESCAF filter that allows to bypass the filter when transmitting. This allows the user to hear the CW sidetone, which may otherwise be filtered out if the filter center frequency is not lined up with the CW sidetone generated by the radio.
See my website at http://ve2zaz.net/NESCAF_Filt_Mod/Nescaf_mod.htm .