Tuesday, April 13, 2010

Universal NE612 PCB

N3ZI offers nice little PCBs (click here) for superhet receivers.
It appears that those little receivers can easily be turned into transmitters. I got some ideas in my mind, which I will successively add to this post, so, stay tuned.

For a 10MHz I.F. and a crystal controlled LO, please see my earlier post. For QRSS I would favor 7000800Hz, 14000800Hz, 21000800Hz and 28000800Hz, since crystals (oscillators) are available for 3MHz, 4MHz, 11MHz and 18MHz.
More cheap crystal combinations could be found on my web-page. Note that only pairs with M=1 are "drop in" compatible with N3ZI's PCBs.
One nice option appears to be a dedicated 17m WSPR receiver, or maybe even a transceiver. This design would make use of a 8.000MHz I.F. and a 10.106MHz crystal (30m QRP). A tiny bit of pull and the carrier frequency should be 18.1046MHz.

600m is easily available with the following crystals: 5.000MHz - 4.5000MHz - 4.000MHz - 3.500MHz - 3.000MHz - 2.500MHz - 2.000MHz.
Remember, the subtractive mixing product compensates for thermal drift of the oscillators.

Transmit?? you may ask yourself... Yes, I believe that Douglas' PCBs are equally suitable for generating a single side band signal. Please have a look to the schematics on N3ZI's webpage.
A modification to a transmitter, in my view, could be the following:
  1. feeding an AF signal to pin 1 of U1, the input filter and transformer make place for an AF network
  2. X6 should have the same frequency as the filter-crystals
  3. U2 is now a converter, thus, X5 serves the LO
  4. Q2 a buffer, C17 should therefore be replaced by a some means for linearizing Q2
  5. C22's capacitance should definitely be reduced
I figure that this PCB, modified for transmit, together with a simple MP3-player, makes an excellent base for a MEPT in combination.

    Two PCBs will provide a nice and easy transceiver. One PCB just like intended, the other modified for transmit as shown above. Here, some additional considerations seem appropriate:
    1. feed the BFO from the RX board to U1 of the TX board, alternatively use an external BFO
    2. feed the LO from the RX board to U2 of the TX board, alternatively use an external LO or VFO

    As soon as I got the first results, I will write an update on this site. Stay tuned....