Wednesday, February 22, 2012

472kHz Phasing Transmitter

We have seen that a 1.8432MHz oscillator will provide us with a 460.8kHz I/Q-SDR LO.
This is very much in a comfortable range for of the new amateur radio MF band, i.e. 11.2kHz to the lower band edge and 18.2kHz to the higher band edge.
Now, how to generate the modulator signal? Phasing style, the easiest would be to build an oscillator for the 44.8 to 72.8kHz and use two Flip-Flops to generated the 90 degrees phase shift.

  1. Such an oscillator could be a rather simple function generator. Other solutions could be based on micro-controllers such as PICs, PICAXE, AT-Tiny, etc. With such a controller, it would also be possible to program features like memory channels, frequency display, beacon-keyer...
  2. Another approach would be to build a crystal oscillator, using cheap industrial xtals, and divide it down. Some ideas could be crystals from the XMHz range divided by N (by means of a binary counter) before feeding the Flip-Flops:
    • 3.000MHz / 64 = 46.88kHz resulting in 472.5kHz
    • 3.072MHz / 64 = 48.0kHz resulting in 472.8kHz
    • 3.2768MHz / 64 = 51.2kHz finally resulting in 473.6kHz
    • 3.579545MHz / 64 = 55.93kHz resulting in 474.78kHz
    • 3.6864MHz / 64 = 57.6kHz resulting in 475.2kHz
    • 3.93216MHz / 64 = 61.44kHz resulting in 476.16kHz
    • 4.000MHz / 64 = 62.5kHz resulting in 476.4kHz
    • 4.096MHz / 64 = 64.0kHz resulting in 4768kHz
    • 4.194394MhZ / 64 = 65.54kHz resulting in 477.2kHz
    • 4.433619MHz / 64 = 69.28kHz resulting in 478.1kHz
  3. In the light of the above, ham-radio crystal such as (in MHz) 3.530, 3.535, 5.540, 3.550, 3.555, 3.560, 3.575611, 3.880, 3.885 can fill in gaps. Those crystals are found at "expanded spectrum systems".
  4. With some luck, one finds tons and tons of surplus crystals in the range of 2.8672MHz to 4.6592MHz. As I recall, there where channelised commercial transceivers (e.g. military, maritime etc.) making use of crystals in that range.
  5. Similar to the crystal approach, one could consider to use 3.58MHz, 4.0MHz, 4.19Mhz, 4.50MHz and 4.91MHz ceramic resonators for a VFO. The 6.00MHz, 6.50MHz and 8.00MHz resonators would require one additional division.
  6. The deluxe version of it all would be a DDS for the range 2.8672MHz to 4.6592MHz. I wonder is there is any kit in which the LO offset can be easily programmed to (f/256)+460800Hz. Maybe a project with the DDS60 board.
When modulating the phase shifted AF signals, one has to consider that those are essentially square waves. In order to reduce harmonics, it would be required to do some severe low pass filtering at about 19kHz before injecting the signals into the I/Q mixers.

As to receiving, the 11.2kHz to 18.2kHz is in the comfort zone of any 48kbps sampling sound card.

There you have it, my presently preferred solution for the new 600m amateur radio band.

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