Tuesday, March 30, 2010

Divide by "n"

Still trying to find a way of getting a stable navtex receiver using crystal oscillator /IF combination decided to make a quick test on frequency dividers, my idea was not to make a direct conversion receiver but I guess I have no other choice unless I go to a pll system or find a combination of IF VFO with crystals matching.
With this schematic I can get 487Khz from a comon 10.245 crystal (used particulary in FM sets). If I find one 3615Khz crystal dividing by 7 one will get 516.5Khz the other NAVTEX frequency...

If you intend to use this schematic, remember I didn't looked at the output wave form but for shure it's going to need a lot of filtering. One of this days I will connected it to the navtex dc receiver block to make some tests...see below the dc receiver block with an LC vfo.

Some time ago someone gave me an old military direction finding equipment, at least the main body but not the accessories and since it's an heavy item I decided to dismantle to get some components, Inside there were some boxes I suspect were vfo's for each channel, with this looks:

Now I have some trim caps and some strange coils that I used to make an vfo for one of the navtex receiver, probably will build one of the receivers (decided to make 2) inside the aluminum box since the variable capacitor is already in place, so is one of the coils. I think this set was supposed to work around 3-4Mhz range due the size of the coils. With some extra caps I placed the resonance at 500Khz.

Testing the coil and cap as vfo:

I replaced the second NE602 working as preamplifier to an LM741 before the LM386.... now a classic DC receiver block. The combination of the coil and caps is quite stable but not enough to unatended operation.

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