Additionally, there are some approaches mixing different frequencies, I will not go into this in the present note.
The trick is to end the chain of dividers with a division by two, creating a 50% duty cycle.
I would like to point your interest to some frequencies for which ceramic resonators are available. Those could be nicely pulled, given the ratios, the stability of a ceramic resonators oscillator seems OK-ish to give some results still. Assume 1kHz drift @ 5.5MHz. This would resulting in a 25Hz drift @ 136kHz, not desirable, would allow for first tests however. It is clear that care should be taken to avoid drift all together.
For crystals, one could consider moderate super-VXOs.
At the end of this message, a band plan is attached. Some frequencies I listed are actually outside the band, consider this as a hint to SDR-receivers, in particular when the last division could be by 4.
There are two candidates for QRSS marked in red. However, both require a division by 13 :-(
Simple solution "ripple counters"
- 138550 = 4.433619 / 32 (=8x4=2x2x2x2x2)
- 138550 = 8.867238 / 64 (=8x2x4=2x2x2x2x2x2)
One decade counter + ripple counter or flipflops
- 136533 = 9.8304 / 72 (=9x2x2x2)
- 137500 = 22.000 / 160 (=10x8x2)
- 138240 = 22.1184 / 160 (=10x4x4=10x4x2x2)
- 136533 = 4.9152 / 36 (=9x2x2)
- 138888 = 5.000 / 36 (=9x4=9x2x2)
- 138888 = 10.000 / 72 (=9x2x2x2)
- 144444 = 5.200 / 36 (=9x4=9x2x2)
- 136533 = 6.5536 / 48 (=6x2x2x2)
- 136533 = 7.3728 / 54 (=9x3x2)
- 136533 = 14.7456 / 108 (=9x3x2x2)
- 136533 = 16.384 / 120 (=10x3x2x2)
- 136550 = 2.4579 / 18 (=9x2)
- 136533 = 24.576 / 180 (=10x9x2)
- 136533 = 3.2768 / 24 (=6x2x2)
- 136533 = 3.6864 / 27 (=9x3)
- 136533 = 4.096 / 30 (=5x3x2)
Two decade counters + ripple counter or flipflops
- 136364 = 3 / 22 (=11x2)
- 136364 = 6.000 / 44 (=11x2x2)
- 137675 = 3.579545 / 26 (=13x2)
- 137675 = 14.31818 / 104 (13x2x2x2)
- 137255 = 14.000 / 102 (=17x3x2)
- 136364 = 15.000 / 110 (=11x5x2)
- 136364 = 18.000 / 132 (=11x3x2x2)
Ceramic resonators
- 137500 = 5.50 / 40 (=10x2x2)
- 138889 = 5.00 / 36 (=9x4x2)
- 138889 = 10.00 / 72 (=9x8x2)
- 136389 = 4.91 / 36 (9x2x2)
Now, what to select, taking into account the band plan:
- 135.7 - 136.0 kHz
- Station Tests and transatlantic reception window
- 136.0 - 137.4 kHz
- Telegraphy
- 137.4 - 137.6 kHz
- Non-Telegraphy digital modes
- 137.6 - 137.8 kHz
- Very slow telegraphy centred on 137.7 kHz