Monday, May 31, 2010


One my search for detailed information about the Sangean ATS-909 (aka Radio-Shack DX-398, Siemens RK777 and Roberts R861), I was able to locate schematic diagrams.

The reader may ask what that's got to do with a MEPT... well, nothing up to now... BUT:

 When checking the receiver's control PCB schematics, I found something, the main micro-processor generates the PLL reference signal. Maybe it is always done like that, I don't know, but it looks remarkably simple to me.

In the upper right corner, that would be the micro-processor. The chips leads X1 and X2 are connected to a 4.332MHz crystal. This oscillator is the classical Pierce configuration, as we know it from so many digital applications.
The crystal's cold end is provided with a trimmer (TC301) for frequency tuning.
The crystal's hot end is connected to an RC low-pass filter (R389,C363) which further feeds the signal into the PLL chip.
BTW: Can someone explain the function of C359 to me? It seems to couple GND to GND.... hmmmmm(!)

So, here's the MEPT idea (actually, this was done before, sort of, by Clayton from Down Under): clock the keyer (e.g. 12F629) by its high speed built-in oscillator, using a crystal for a QRSS frequency and couple the signal out in the way the professionals do :-))
There is just one down-side to this, the controller's clock is now at a somewhat odd frequency, odd for controllers that is, so timing could be off. I will check if compilers (I am programming in C, not in assembler) are able to compensate for that...

Now that we got the signal, what shall we do with it? Well... amplify?!
For another project, I happen to study switch-mode amplifier, aka class E or F. F seems a little bit too much work, but E seems fine for a MEPT, as Hans proved with his Caribbean beacon. Hence, a class E amplifier seems appropriate; I guess, the crystal's hot end will have enough voltage to switch a FET.

The chip could serve for than just the oscillator and the keying device.
During my time at various physics experiments I experienced the difference between fast control and slow control. In our experiment slow control was taking care of temperatures...
So, with seeing the keying as "fast" control (well, sort of fast), controlling the crystals temperature could be integrated as slow control. Those PICs got ADCs, which could easily be used for measuring temperature.

My present vision for mini-MEPTs is presently:
  • one PIC clocked by a QRSS-xtal
  • one FET as PA
  • oven control by the PIC
  • telemetry?

The whole story could have some other advantage.... I realized that, depending on the TX frequency, some PICs stall when RF leaks in. If the controller is clocked with the same frequency and phase, this should not occur.

Next step: design something! Stay tuned, I will let you now as soon as I got more. For now you may click on one of the "sponsors" to find your way out of this blabla...