Early Development Prototype - CW84

I'm calling it the CW84 because it's a refinement of an 80-metre cw transceiver I made about four years ago, which I called the CW80. Not only is it four years younger, but the true wavelength of a 3.550MHz signal is 84 metres. It's a direct-conversion receiver, very straightforward in design, with an equally simple transmitter tacked onto it. A 700Hz offset is added to the transmitted signal by adjusting the control voltage fed to the tuning varactor, which is a forward-biassed LED.

Today, I achieved some satisfactory results with an initial development prototype. It's cobbled together on stripboard, and the picture shows the diagram of the circuit as it stands. There is as yet no RF input to the SA602 mixer / oscillator, and I haven't tried to recover any audio from the thing. It does, however, tune it's oscillator sweetly between 3.550 and 3.583MHz, which is exactly the 30kHz I wanted; this little band covers the 3.555 and 3.564 slow Morse code (QRS) 'sandpits', and the 3.560 low-power (QRP) centre of activity.

The front end may be a simple bandpass filter and a 1k-ohm pot attenuator, or I may include a FET preamp as I did in the CW80. I'll try to avoid complexity wherever possible. The whole idea is one of simplicity; very little setup, few wound components and even fewer trimmer capacitors. Today, I eliminated a padder from the oscillator tuning, and that's the kind of change which I welcome. There will be no AF gain pot. The original CW80 doesn't use one, simply because the RF attenuator does all the gain-setting required. It also stops a novice operator from setting the RF gain too high, backing off the AF gain and wondering why the mixer is getting swamped with powerful signals. The SA602 is a lovely device, if you respect the fact that Gilbert cell mixers are easily overloaded, but have good conversion gain and hence work well at low input levels. Another simplification is in the AF output stage. The LM386 is noted for its low ancillary component count, but I've taken the liberty of doing without the bypass on pin 7 and the Zobel network on the output at pin 5. If they prove necessary, I'll add them; but only if.

Tomorrow (hopefully), I'll add a front end and a pair of 'phones. I may even try listening to my gate-dip oscillator later tonight. Whatever happens, and whenever I get around to doing it, I'll post it right here. Remember; as always, although I assert copyright on my work, please feel free to try these things for yourself. I intend to market kits for these projects, and support them with this blog and other web-based materials, but use your junk-box stock or the shopping-lists which I publish with the designs.

Witlessness and woe

If I've learned anything this evening, it's how to waste three hours work on one simple detail. The new radio was laid out three nights ago, and it looked elegant, compact and concise in its prototype stripboard form. The resonators, 1K pots and 100p COG capacitors arrived today, so it all looked good for a first build. Thursday evenings are free of Sea Cadet obligations, and nothing was planned by the family. I settled down in my untidy position at my desk in the extension, and carefully loaded the stripboard. My youngest son came by to wish me goodnight at around 2100, and I was just hooking-up the tuning pot. The tuning LED glowed and dimmed, the volts were where they were meant to be, but no output from the SA602. I'd put a 100p cap on pin 7 (the oscillator emitter), to take the signal to a keyed buffer for the transmitter. I was patiently listening with my old Realistic DX-392, looking for signs of life. I searched the board for open joints, solder whiskers, anything. I changed the ceramic resonator for another type, I removed the bandsetting padder, I changed the LED coupling capacitor, nothing.

Then it hit me, like a huge, soggy mattress. I am so used to using chips upside-down on a piece of copperclad, that I'd laid the board out with the two chips back to front. The entire layout is scrap, and I face a fresh piece of quadrille paper to start afresh. Ho-hum.

It's wonderful to watch your ideas take shape, and weird to watch the shape change as the ideas mature into workable products. The latest radio is no exception. I went through a panic stage four days ago, and the direct-conversion receiver / cw rig turned into a Pixie II, and back again. Do a search for the Pixie II, it's a fascinating exercise in minimalism. The PA transistor doubles as the receive mixer; keying the PA emitter shorts the receive path and puts the transistor into full drive. On key-up, recovered audio is passed to an LM386, and into a pair of 32-ohm headphones. Usual power source is a 9V PP3, and the housing can be anything from an Altoids tin to al fresco.

I did say it turned back again, and it remains a variant of the Rev. George Dobbs G3RJV's famous 'Sudden' receiver. I'm using Micrometals iron toroids in the receive preselector, and a 3.58MHz ceramic resonator as the frequency controlling element. I need tighter control than George's VFO original, because it's also transmitting. I've settled (for the time being) on a 30kHz band, from 3.55 to 3.58. This includes the QRS 'sandpit' at 3.555 and the QRP centre of activity at 3.560. The radio has a 700Hz transmit offset, given by arranging for a pair of resistors to be switched in and out of the frequency control voltage 'totem-pole', which feeds the LED varactor. Not only am I using a LED as a varactor, but I find that forward bias gives more linearity. This method is not original, but has been used with success in the radio home-brewing world for some years. The other feature of the forward bias is that the LED can be brought out to the front panel, adding interest as the radio is tuned. The LED dims and brightens, glowing well at the lowest frequency.

I'm currently waiting for parts, and I have two evenings ahead of me where I'm committed elsewhere, so it'll be just ideas until the latter part of the next weekend. With a following wind, the prototype may be running inside a week, and taking reports from Southern England. We'll see.

Morse Code Advocacy

This Saturday gone was hard work, but great fun. I delivered a talk and teach-in to forty-eight 10 - 12 year-old Junior Sea Cadets, along with their twelve adult and senior Cadet team leaders at an Adventure Camp. I presented a potted history, a practical demonstration of current amateur radio usage and then let them loose with keys and oscillators. The kids loved it. They were given a copy of the code, on an A5 sheet, and they sent me their names. I divided them into three-member sub-teams and gave one group the task of composing a distress message, while the other three were shown more about amateur radio. When ready, the distressed group sent their message, and the 'helpers' decoded it and responded...

There was a wide spectrum of interest; some Cadets wanted to know if they could talk to aliens, others wanted to know where they could get keys and beepers. I think a couple of them may be hooked, and we might hear them in future when they have their Foundation Licences.

I had to return home late Saturday evening for domestic reasons, but I've had reports that there were flashing-light Morse signals passed from tent-to-tent after lights-out. Seeing how Morse can capture the imagination of these youngsters, I'm wondering if more could be done to raise the profile of what remains one of the most efficient ways of moving information around our planet. I wrote a short pamphlet about it, and I'm considering writing an extended monograph for just this purpose.

As I often say in this blog, more follows...

A New Station - M6BMO

My youngest son, Billy, recently qualified as a Foundation licensee. He studied with the Worthing and District Amateur Radio Club, to which we are both very grateful for their help and support. Now Bill is an M6, he needs a radio, and there lies a problem. I can't afford a 'black box' rig for him, and as a Foundation ham he can't use my homebrews. He can, however, use a radio made from a commercially available kit. Some of Tim Walford G3PCJ's kits ( www.users.globalnet.co.uk/~walfor ) are suitable, and maybe I'll buy Bill one for Christmas. Until then, I'll simply have to become a vendor myself!

What I have in mind is a simple kit based on Billy's immediate desires, and which may prove useful to others in the same boat (impoverished, Foundation licence). He wants to use 20m, and cw. This fills my heart with gladness; 20m is my favourite band, the rig is technically unchallenging and can be made in a variety of ways. First thoughts are with either a Pixie II clone or a Sudden / VXO / Pebblecrusher mix. What ever happens, I hope to have a prototype going quickly, and a kit advertised soon after. Then, Bill can build a kit bought with his pocket-money (I'm hoping it'll be that cheap), and get going.