Friday, January 4, 2013

Easy 472kHz Superhet

Just a thought, a 4.000 MHz (industrial xtal) signal mixed with a 3.530 MHz (80m xtal) signal would result in 470kHz, somewhat shy of our band.

The 4MHz would make a nice intermediate frequency, with a cheap ladder filter. Tweaking QRG is hence restricted to the 3.5x MHz frequency

A couple of options
  • pull 80m xtal down
  • pen the 80m xtal down (super VXO)
A thought outside the box could be to get dividers into the equation. Douple 3.530 and you will get 7.060. As stated above, the 3.53 are just a bit too high.
Expanded Spectrum Systems sells 40m crystals, which, divided by 2, would result in in-band frequencies.
  • 7.042 MHz => 479kHz
  • 7.050 MHz => 475kHz
  • 7.055 MHz => 472.5kHz
Also here, a super VXO would be an option.

For a WSPR transceiver, the 7.050 MHz option seems ideal. A small downward pull of some hundred Hz should be easily doable without compromising stability. Mind you, WSPR need a "USB dial frequency" of 474.2kHz.