Tuesday, January 17, 2012

600m with CB-XTALs follow up

Hi there... I am back!
Presently, amateurs in The Netherlands are really happy, not only have we got the permission to transmit on 4m, we also may transmit on 600m again. For the time being between 501 and 505kHz.
For this range (well, to the upper QRG of 504kHz) this article could lead to a cheap signal source.

However, in about a month's time, we will know is we need to redesign our exciters, i.e. to the range 472 to 480kHz. So, lets have another look at the cheap crystal solution.
The new range is not that easy really. One solution would be the mixing of two standard xtal frequencies:
  • 4915.2 - 4433.6 = 481.6 somewhat high, could be pulled into the range
A spot on solution would be a 10m QRP crystal (28.060MHz) on its fundamental mixed with a 9.8304MHz standard crystal:
  • 9830.4 - 28060/3 = 9830.4 - 9353.3 = 477.1
Another spot on option: a 40m QRP crystal, mixed with a 6.5536MHz standard xtal.
  • 7030 - 6553.6 = 476.4
Starting from lower frequencies, crystals from box73.de:
  • 14745.6 - 14270 = 475.6
  • 7025 - 6553.6 = 471.4 (pull!)
  • 3550 - 3072 = 478
  • 4000 - 3530 = 470 (pull!) (*)
  • 3555 - 3072 = 483 (pull!)
Subractive mixing has the advantage that some thermal effects may cancel. Assume to us several standard crystals in a super-VXO.
Mind you, the higher the frequencies, the easier to pull, i.e. create a nice tuning range. However, lower frequencies will give better stability.

(*) Note, there is a 4.00MHz ceramic resonator, which will allows for a nice VFO. Use a Pierce oscillator to obtain a QRG above the ceramic resonator's series resonance.

5 comments:

  1. Have a similar problem but for 488Khz reception, i'm doing some tests with 5Mhz crystal for filter and a 4.5Mhz ceramic filter (from a vcr) as oscillator since it's easier to pull than the crystal.

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    1. Yes, ceramic resonators are easier to pull, but, they will not not be as stable as quartz-crystals will be.
      What about 4915.2-4433.6=481.6 with a little pull to achieve an LO of 486kHz? Use a Pierce oscillator of the 4915.2 and pull down the 4433.6 and you will be just fine.
      73!

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  2. Hi Joachim, thanks for the idea, I normally use a Colpitts but for sure will try the Pierce. you're right on the ceramic, they are not as stable. Local electronics store doesn't stock the 4.915, or near, but found out now that they stock 3.579 if I can pull 67Khz from one (or super VXO with 2) I get 488Khz using a simple 4Mhz filter.
    The idea on the SDR is also nice!
    73!

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    Replies
    1. Hi Ricardo,
      the trick with the Pierce oscillator is that is oscillates above the crystal series frequency. It is relatively easy to pull upwards by a couple of kHz. The second oscillator should therefore not be a Pierce type, but rather one the can be easily pulled down.
      A source for 4.9152 xtals could be www.reichelt.de
      You could also find them in old computers, check out the particular sources http://en.wikipedia.org/wiki/Crystal_oscillator_frequencies
      Bear in mind that a pulling the crystal makes the oscillator less stable. The more pull, the less stability...
      73, Joachim

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