by Don Cantrell, ND6T
As much as I love the native BITX40 VFO with it's clean signal and simple design, our rural mountain home has room temperatures that vary widely and the VCO just proved too much for me to stabilize to my taste. I caved and raided the junk box for a Direct Digital Synthesizer solution.
Finding an Arduino Nano®, an AD9850 board, and a generic I2C 1602 LCD display, I cut a slot for the display in the front panel of my Radio Shack® #270-235 cabinet that I found in a swap meet (5 ¼”W X 3” H X 5 7/8” D) and mounted it with 4 short stand-offs.
I used a small individual-hole-plated piece of vector board to mount the Nano® and DDS board using IC sockets. A 7805 regulator was then soldered to small piece of un-etched printed circuit board and glued to one edge. A hole was drilled through both for a support bracket. I also mounted the filter capacitors along with a couple of resistors for a voltage monitor feature. This daughter board is now bolted to the rear panel of the cabinet with an “L” bracket, therefore using that previously wasted space and also heat sinking the regulator (as if it needed it!).
Not having an encoder, I decided to use the original tuning pot in a “shuttle” arrangement. (That was what we called the control on video editing boards back in the 1960's). Instead of spinning the knob, you twist it one way or the other. In the center position, it is idle: No tuning, the frequency is stable. Turn it clockwise slightly and the frequency starts to slowly increase. Counterclockwise, the frequency decreases. As you twist farther it tunes progressively faster. No buttons, no pressing. Tune any frequency in mere seconds and in single Hertz steps when you near your target.
It's like having a power tuner. The amount of twist is like the amount of throttle. I have mine set up so that it takes a leisurely 15 seconds to tune from 7.0 to 7.3 full throttle but I can still tune down to the single Hz frequency. That's actually much faster than tuning any commercial rig when set to single Hz resolution. Turn to the 11 o'clock or 1 o'clock position and slowly scan for signals. When I get around to it, I intend to install spring loading to provide haptic feedback on the knob. In the meantime I display a tuning indicator after the frequency reading.
One big knob. No clicking, no pushing, no range switch. Best of all, no boggle like you get if your knob is not-quite-in-the-middle of a stepped value. It is one big range with the tuning speed adjusted at an exponential differential rate. If you don't like the rate or the idle zone size, just plug in the USB cord from your computer and change it to suit. Create band limits. It's an Arduino®!
Although Shuttle Tuning requires just a few lines of code, this blog isn't probably the place for it. If anyone is interested in reading my poor programming style they can email me at firstname.lastname@example.org or at ND6T@ARRL.net and I will gladly attach a copy of the simple software and answer questions.
Yesterday I ordered a second BITX40, this one with the Raduino. I can hardly wait!