Tuesday, December 8, 2009

new low frequency ideas (TX)

Working with dividers seem to have advantages to me, amongst: greater stability and easy digital design. The only question is, are there enough crystals in a close range?
Well, what about old CB crystals? And how would those get us onto the 136kHz band?

First things first!

Here is what I came up with and how I started the process:

There is an industrial crystal 27000kHz crystal, divide this by 196, get us to 137.76kHz, just 60Hz off the QRSS COA (center of activity).
How to divide by 196, well, that actually easier than I though:
  1. 196/2=98
  2. 98/2=49
  3. 49=7*7
The recipe is hence, two counters to 7 in series (so that they multiply) and two FlipFlops. There we go, a 50% duty cycle with a cheap crystal in the middle of the QRSS range. Drift should not be an issue here, assume, the oscillator drifts by 1kHz, that would result in a 5Hz drift on long wave.
I think, there is even a canned oscillator of that frequency, making the design in particular simple.

And now to the added bonus: CB-XTALs!

I reach in my junk-box to see what crystals I got from old CB radios. Some calculation revealed the following:

  1. 26600/196 = 135,71
  2. 26610/196 = 135,77
  3. 26620/196 = 135,82
  4. 26630/196 = 135,87
  5. 26650/196 = 135,97
  6. 26660/196 = 136,02
  7. 26670/196 = 136,07
  8. 26680/196 = 136,12
  9. 26780/196 = 136,63
  10. 27000/196 = 137,76
  11. 27005/196 = 137,78
This covers essentially the whole 136kHz band with junk crystals from CB radios. Even better, look at the channel count, to me that looks like I will be using the casing and the channel selector too ;-)
I missed a couple of frequencies, since I got no crystals for those in my junk box.

The internet discloses that there are RX crystals for the CB channels 8 to 40 ranging from 26600kHz to 26950kHz with 10kHz increments, a few channels skip however. 10kHz steps will result in 51Hz steps in the 136kHz band.
Additionally, there are TX crystals for CB radios. CB channels 1 to 4 (26965kHz to 27005kHz) would be interesting here.

One may consider to experiment with VXOs, since old CB radios have those anyway, ranging +/-5kHz, which would the cover essentially the whole band, with just a few gaps due to the lack of crystals.

The same principle could be serving for the 500kHz band. Here, one would require a division by 54, which would be two counters 9 x 3 = 27 and one flipflop => 54. CB TX crystals from upper channels could be used. Since I don't hold a licence to that band, I will not look any deeper into this. Nevertheless, I hope, that someone finds this useful.