Sunday, December 16, 2012

Analog Signal Processing

In the November issue of CQ magazine, there was a great article on an interesting kit. The kit is a regenerative audio filter. The filter, according to the designer, Phil Anderson W0XI, removes or scrubs galactic and man made noise from the audio, leaving only the CW note. At first I thought, interesting, but really??

First, some background. Band noise on some receivers can really get bad, not only bad but teeth grinding, chalkboard scraping annoying. I got rid of a transceiver which was nearly unusable on 40 meters. The noise blanker did nothing to touch the noise. Currently, I am now using a Yaesu FT-817ND which a month or so ago, I was in the backyard with a wire in a tree checking into the NAQCC Rocky Mountain QRS net. It was a rugged contact. The noise level that day was really bad. It started me thinking about different options to deal with the noise level. My two choices were the NEQRP SCAF filter (Temporarily out of stock) or the W4RT installed BHI DSP filter. The SCAF is about $30-35, and the BHI is $169 installed. I was pondering the two when I bought the November CQ magazine to read a review on the Elecraft KX3 and found the article on the CW regenerative filter by Midnight Science.


I walked into my wife's sewing room who looked at me and said: “You look bored”. I replied, “I am”. She then said, “You need a kit” First rule of marriage, do not argue with a woman, especially when she is right. Between the CQ article and the need to melt solder, I ordered the regen kit. I went the full meal deal and got the case along with it. It is a basic through hole component kit with 5 chips, resistors, capacitors, all the hardware and a voltage regulator. It is a high quality kit with everything included and integrates nicely in the case.

Now the performance, I was blown away by the ability of this filter to remove noise from a signal. After completing the kit, a random wire was connected to my FT-817 and the filter switched in. You have the option of bypassing the filter, if you wish, by pressing a button. I was amazed at how much static and junk was removed from the band noise. Indoors at the time, when I switched on a fluorescent light, I got a load of extra noise. When the filter was switched in, the lamp noise, along with most of the static, was just......GONE. It was flat amazing. I went to 20 meters and found some CW signals to tune. When you cross the designed frequency (You have the choice of building for 500, 600 or 700 Hz) the signal noticeably peaks. The noise is gone and stronger signals are a pleasure to listen to. I then tuned to a weak signal which was going in and out of the noise. I brought up the regeneration setting until the filter broke into oscillation and then backed it off a skosh. The signal had a slightly hollow, slight echo quality to it at the sweet spot of regeneration. I switched the filter out and listened until the CW started into a deep fade, I then immediately switched in the filter and the signal popped into my headphones. From completely in the noise to easily copyable, the filter is flat amazing. Even if you are not listening to a weak signal, back off the regeneration and use the filter just to clean out the noise, you can listen for longer periods without fatigue. This is one of the best purchases I have made.