[Lowfer] New LF RX Conv Published

[Johan H. Bodin]

Zack Widup wrote:

> I've been thinking of playing around with a VLF-LF
> direct-conversion receiver using the Rick Campbell KK7B
> R2 design.  Has anyone tried that yet? I think you'd do
> OK using SBL-3's instead of SBL-1's in it for that range.

Hi Zack,

yes, I built an I/Q front end with a pair of SBL-3s many years ago and
it worked well. Today I prefer using a QSD mixer (SoftRock, Tayloe,
Youngblood etc.). If you want a "bullet proof" mixer, check out the work
done by Martein PA3AKE:

  http://www.xs4all.nl/~martein/pa3ake/hmode/

However, you don't need a "fast bus switch" to make a QSD. Even a
junkbox 74HC4052 used as QSD will make a pretty good LF front end. You
may find some inspiration here:

  http://yu1lm.qrpradio.com/

Analog phase shifters are cute but the DSP version is much more
versatile. I2PHD's Winrad (www.weaksignals.com), for example, lets you
tune a bandwidth almost equal to the soundcard sampling rate without
changing the hardware LO. LF bands tend to be quite narrow so an XTAL
oscillator is often enough (and MUCH cleaner than a DDS wrt spurs!). If
you run the XTAL LO on 4 times the RX center frequency (not too hard at
LF...), you can use a 74HC74 (or 'AC74) as a 4:1 divider to get the
quadrature LO for the QSD.

NE612 is for toys ;-)


73
Johan SM6LKM