Friday, August 14, 2009

New QRSS xtal combinations

Following the idea having the QRSS region about 100Hz below the WSPR region on every band, new considerations had to be made in order to allow for MEPTs controlled by cheap crystals.

For the bands 40m and 30m the situation is very simple:
  1. 10,140MHz crystal available from N4ESS (no change anyway)
  2. 7,040MHz QRP crystal available
  3. 7,038MHz QRP crystal, which could serve as a basis for a d.c.-receiver
Design ideas... well... I will leave that up to you!


80m slightly more tricky, but no big deal at all!
  1. 3,595 = 7,190 / 2
Design ideas: The crystal is available at the same source as the 30m-crystal (N4ESS). The division by two can easily be done with a Flip-Flop. In order to get the signal into the QRSS range, one has to pull down the oscillator by 2kHz, which should not pose any further problem.


For the bands 160m, 20m, 17m, 12m and 10m, a combination of two crystals will do the trick to generate a frequency in the QRSS range.
The following frequencies (in MHz) are reached by mixing of two fundamental crystal frequencies:
  1. 1,8372 = 8,8672 - 7,030
  2. 14,096 = 10,000 + 4,096
  3. 14,096 = 18,096 - 4,000
  4. 14,097 = 12,000 + 2,09715
  5. 18,106 = 8,000 + 10,106
  6. 24,926 = 14,7456 + 10,180
  7. 28,125 = 18,000 + 10,125
Design ideas: the easiest way of generating the wanted signal appears to be using a 74HC86 as demonstrated here: http://www.smoke.com.au/~ic/ham/xtal_synth.html.
It seems appropriate to note that the 12m combination under point 6 uses a 10,180MHz crystal which can be found in older crystal-synthesized CB radios.


The 15m band can be reached by means of multplication and division:
  1. 21,096 = 4,096 + 2 * 8,500
  2. 21,097 = 2,09715 + 38,000 / 2
Design ideas: Solution #1 involves a 8,5MHz crystal (available from DigiKey) which is doubled obviously. In terms of filtering, this combination is advantageous since the primary mixing products will be nicely separated. However, if doubling is not done perfectly, contributions of the crystal's fundamental will spoil the fun! E.g. second harmonic 3 * 8,5 = 25,5 --> 25,5 - 4,096 = 21,404.
Solution #2 looks more promising to me for the following reasons. 38,0 divided by two gets us to 19,0. Mixing products will be 21,097 and 16,903. We certainly would need a band trap for 16,9MHz, which should be doable. As divider I would propose the usual Flip-Flop. The best part is, there are canned oscillators available for 38MHz.

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