RadioBanter - View Single Post

October 14th 05, 10:16 PM

wrote:
For VHF 3.2 would likely work well enough except if there is a
sporatic E opening and people tend to pile up. Then it's really wide!

For filters I use microprocessor crystals in the ladder configuration.
With the correct shunt C and 4-8 crystals you can make a very fine
crystal filter with bandwisths from 2-400hz to as wide as you may
want. The design process is documented elsewhere (see EMRFD
and the handbook as well as internet). I build for 6 and 2m and have
built very nice ladder filters this way. Also since microprocessor
crystals in the range of 4-20mhz are dirt cheap it's also a help.
Also the higher the frequency the less likely dual conversion is
required to avoid images and allows the selectivity to be closer
to the antenna (better overload performance).

My latest 6m rig uses 12mhz crystals, 8 of them for a 2.3khz
bandwidth at 6db and 3.9khz at 60db with symetrical skirts.

Allison
KB!GMX
I managed to buy over 400 pcs of 8.3886mhz crystals on ebay, for
just pennies each. I am planning on trying to build ladder filters
with them. These crystals are in the larger HC6/u size holders which
are supposed to work better than the miniature size used in the micro-
processor crystals.

First step would be to build the DDS vfo for
the radio since I can program the DDS to function as a sweep generator
for aligning the filter. With the DDS sweeping the output frequency
while providing a sawtooth ramp to drive the scope sweep in step
with the frequency sweep I could see the actual bandwith plotted on
the scope.