SUBTITLE: “…Adding a form factor to circuits with this (almost) self-contained HF radio station.”
PART 1 – INTRODUCTION
This whole idea started way back before solar cycle 23! I was working with some interesting circuits for the US Navy at the time including a new DDS chip that I knew would provide a stable VFO for such a unit. (The now famous Analog Devices DDS series) It was one of those daydream projects that never fully materialized. I started to design and analyze various transistor RF circuits with the Radio Designer software [1]. This project fell behind as it was impossible to design a working miniature SSB transceiver after I began working with a struggling startup company full time. I briefly hit on an idea during that period to simply repackage an existing transceiver into a hand held format. I even bought a WM-20 [2] to kit bash but it was just too cool not to build it intact. I eventually did interface the WM20 to a DDS VFO [3], in a 19” rack format - not a very hold-able format to be sure... The solar max came and went and my dream of this radio faded too.
My interest was recently again peaked by a question from my dad; “…what’s the deal with these coast-to-coast ‘walkie-talkie’ phones? It sounds a lot like ham radio to me.” After explaining some the cellular technology and marketing behind those phones I remembered this project once again. Could my dream of a transcontinental handheld HF radio be revived? It looked like I could use surface mount components (after all SOT transistors are real small) to make a cool mini version of some transistor only design? I wanted to use components that could make this reproducible, no obsolete components (like the MC1350) and simple design controlled by a small microprocessor. Then there is the upcoming solar max... Can it be done in time for Cycle 24, if it ever arrives, when the high ends of the bands are again active?
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| Radio Control |
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| Meter Disp functions |
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| RF Block Diagramme |
CONCEPT
The concept for this design would be a single band transceiver; the RF elements could be re-populated and re-program the controller to make different band versions on all of the HF bands, 20 meters and up, hence Model 620! [5] This transceiver is designed for USB operation so it is compatible for the upper HF band plans. The radio for this project may seem by some almost as an afterthought but the point of this effort was intended to be the re-packaging of some “proven” existing design. It turned out to be a bit of a cut & paste design, so you may see one or two of your favorite circuits included. I tried to give all the credit where it is due for these circuits; some are just public domain by now so often reproduced. There is a lot of design work involved in designing even a simple transceiver from the ground up. As well as all of the mechanics of packaging a working transceiver in this form factor. It turned out; 3 separate PCBs would be needed; RF Board, Controller Board (initially with a DDS-60 [6] daughter board), and PA Board. This project presents some unique problems and challenges, not only stuffing a QRP transceiver into a pre-defined form factor, but some lessons in RF compatibility as well. The control panel shares not only the majority of the user controls but the antenna RF connector. Then the issue of making the unit totally self contained with batteries – a whole slew of problems as well.
DESIGN
With the basic features nailed down, the next question was: how big? It would be possible to squeeze the radio into this recognizable format using thru-hole components, or would I need to use “the dreaded” [7] Surface Mount Technologies? (SMT) package sizes for some of the passive components available are so small that it is not practical to expect to hand solder and assemble the 100’s of components needed for this design. My personal size limitation for PCB work is 0602, even then it is very difficult to manage to install and solder these small parts by hand. You may be surprised that ICs do not have the same limitations. I have personally successfully hand soldered some of the finest pitch ICs available using a technique using solder wick to soak up the excess solder bridging the pins, but I knew this design would probably also need to use a number of discrete components as well.
So how does SMT work for these nearly microscopic parts? Typically, in SMT a paste mask is developed, along with the PCB, which is basically a thin sheet of metal has holes (called apertures in PCB terminology) for all the pads on a board, the paste is spread, kind of like peanut butter, over the mask and board sandwich and when the mask is removed the paste remains only on the pads. Then all the small components are placed and temporarily held by the sticky solder paste. After all the components are placed; the board is heated, in a temperature controlled oven, until the solder paste is melted and then it is cooled down, effectively soldering all components in place at once!
This process is not too practical for “one-off” prototypes as the mask needs to be developed and machined, not to mention the extra soldering equipment involved. On the other hand, by using the larger 0805 and larger component sizes; although hand soldering is possible, the actual board space used is achievable using through-hole components, if they are mounted vertically. [8] The problem with standing axial components is the extra lead lengths cause unwanted inductance, but if you open up many “older” radios they successfully use this technique. As it turned out; the project ended up as a hybrid, using the thru-hole components except where SMT was the only alternative.
I decided to give an initial PCB go to see how many of the radio components could be squeezed into a small board, using thru-hole components, so at the very least it could be used as a drilling and placement guide for a hand built proto. (I call that the “pretty ugly” technique) even if an actual PCB was not made from it. The board size itself was determined by measuring several VHF handhelds and scanners I had on hand. Some of the older models dimensions seemed likely to be workable so I used these measurements as a starting point. Also, for the chassis, I liked the extruded models with slots for boards and end caps as they also tend to have rounded, more comfortable looking sides, and provide a better control and antenna mounting surfaces as they are perpendicular to the rest of the chassis. [9]
My project is not the original HF handheld either; Mizuho Radio had several handheld models in production [4]. They were hard to find in the US
