I needed a new LPF (the one used yesterday just offers the choice of 80m or 40m), so I looked at chapter 12 of the ARRL Handbook (mine's the 2009 edition - no doubt the shiny new 2010 edition contains the same info) and fired up "Elsie", the filter designer package that comes with the book. Here's the resulting design for a 7th order low pass filter with -3dB corner at 12MHz...
I worked up the filter in LTSpice - along with a parallel filter to check the effects of using preferred value capacitors and "practical" inductors (I think you can do this in Elsie - but I couldn't remember how, so it was easier to use Spice)...
The two simulated responses essentially overlay each other, so I won't waste time and space with a graph.
Whilst we roll our own inductors (and so don't have "preferred" values) we are stuck with an essentially quantized set of inductance values by virtue of the integer number of turns we can put on a toroid (of course, we can mess with the spacing to fine-tune the inductance but, at the "design" stage we have quantized inductance values). I refer to David g4coe's chart in SPRAT 135 when rolling an inductor...
Here's my finished LPF...
The entire rig has now been "smartened up" a bit for the camera...
I fired it up and got an immediate spot from Francesco, hb9afz. More news on my h/b efforts with WSPR to follow...
...-.- de m0xpd
No comments:
Post a Comment