The Techno Whizzy Part I

Build a direct digital synthesis (DDSj radio.

by John Welch N9JZW

Direct Digital Synthesizer Old Circuits

Photo A. The Techno- Whizzy I DDS transmitter.

Photo B The TW-l DDS transmitter consists of three separate PC boards. The RF amplifier is shown on the left, the diode matrix for frequency selection is shown at the upper right and the DDS synthesizer hoard at the lower right.

I've grown tired of the same old designs being rehashed over and over again. 1 wanted something new, something state-of-the-art, but my wallet balked at paying over $2,000 for a new rig. Nobody offered a cheap Direct Digital Synthesized (DDS) radio but, folks, it is not only possible to design one, ii is both fun and easy.

No, it doesn't have all ibe bells and whistles yet, What it does offer is a chance to build a DDS radio of your own, for a lot less than buying a whole new rig. It's modular, so as 1 (or you) design new boards [hey can be added to the basic unit without throwing out all your work. I call it the TW-L for Techno-Whizzy, model 1,

The TW-1 is a modular multiband CW QRP transmitter using a new Dinect Digital Synthesis (DDS) chip from Qualcomm (1055 Sorrento Valley Rd.t San Diego CA 92121; Tel: (619) 597-5005). It has I] diode-programmable channels and one dip switch variable frequency channel, and it puts out 2 watts from I MIH/, through 2L5 MHz (that's 160 meters, 80 meters, 40 meters, 30 meters, 20 meters, 17 meters and 15 meters. FULL COVERAGE, plus many MARS and CA!> bands to boot). With some modifications to the amplifier stage it can even cover the 1750 meter Experimenters Band. I'm planning to eventually add a receiver board, a linear amp, a doubler to gel full HF coverage, a digital signal processor to handle SSB and a digital frequency display wiih keypad input.

Instead of a VFO, the heart of this transmitter is the new Q2220 DDS chip from Qualcomm. It runs at 55 MH/ clock input, and has ll 24-bit phase accumulator. In English, this means that the minimum frequency change is 55,000,000 / 2(two raised to the 24th power), or just about 3 hertz. Above I/8th the clock frequency, the quality of the output signal falls off, but light filtering can remove the harmonics. Betueen 1/3 and i/2 the clock frequency is the maximum output frequency you'll get w ithout a tot of fancy filters, I've designed the TW-1 to run at any frequency from 3 hertz to over 21 MHz in 3 Hz steps while maintaining a decent sine wave output to the amplifier. With the additional filtering after the amplifier, it's FCC legal—the spurious emissions are all more than 30 dB dow n as measured on a spectrum analyzer.

How It Works

The frequency is set using pull-up resisiors and diodes. The input to the Q2220 chip is binary (don't panic I II explain this later), with Ps being a 5-volt input and 0*s being grounded inputs. There is a line of pull-up resistors, one for each input line, and a place to insult pull-down diodes. When you select a channel, it grounds the cathode end of that row of diodes, pulling those lines low.

The Q2220 reads the input lines from the frequency selection board 55 million limes a second (once per clock cycle), ant: adds thai value to its current value in the phase accumulator. (If the result is larger than 224, it subtracts I2* from it to keep the phase angle between 0 and 224}. This number, expressed as a percent of 2-J, tells the DDS how far around a circle it currently is. It then calculates the sine at that position, and outputs the sine's value to the Digital-co-Analog Converter (DAC).

The DAC then takes this value and translates it into a voltage between DC and 1 volt. At this point, the signal doesn't look like a sine wave—it looks like a staircase with up to 256 steps built on a sine wave. To remove the "staircase/" the signal is passed through a seven-pole low-pass filter that deans it up, The now -clean sine wave (at 2 milliwatts > is sent to a lou -level class A linear RF amplifier.

If the key is on, power is applied to the class A driver amplifier, which then amplifies the signal to 80 milliwatts and passes it to the final amp. The final runs class A also, for maximum linearity rafter all, if you've gone through all the trouble of making a clean sine waveT why mess it up with a nonlinear amp?]. The final outputs 2 walls into a 50-ohm load (either a dummy load or your antenna).

Switching to another frequency is easy; just turn the channel knob. The DDS chip reads the nev* value, and starts all over from

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