Tuesday, October 05, 2010

Updates on the VHF transceiver I

The VHF FM receiver part is done! I made some more tests trying to put the APRS packet decoder working with this receiver but no success. Will leave only the discriminator out and them with more time in the future will see what's the problem. Never the less, with the computer sound card I can decode nicely the APRS packets.
Here's how it looks line now:
The top PCB is the transmitter one that will occupy me for the next times.
I found a 4.5 Mhz cristal in an old car radio that will fit nicely in the PLL transmiter board...what a luck since the part was getting dificult to find and I was making tests with an ceramic filter doubling as oscillator.

Now some other things...

I still think in making a simple, standalone NAVTEX receiver. After some sucessfull experiments my only problem is to get a cristal combination (for filter and LO) that allows reception of 490 and 518Khz transmissions, I want to avoid PLL, DDS or any other more complex method for the LO frequency. In the past I made some experiments dividing frequency but doesn't hurt to try some more like the ones bellow (external site image):

PA1GSJ blog has also some topics on this subject of finding crystal combinations, and also subharmonic mixers (easier than making a frequency divider). You can read more at: http://draaggolf.blogspot.com/

If you have any idea on a crystal combination that migh work on 490 and 518Khz I would like to ear... preferebly those frequencies plus 1.5Khz to avoid inverting mark and space. Thank you.

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