RadioBanter - View Single Post

October 15th 05, 12:38 AM

wrote:
On Fri, 14 Oct 2005 17:16:21 -0400, Ken Scharf
wrote:

wrote:

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
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.

The HC6 parts work fine as do the HC18, 49 and so on. The real trick
is doing the work to measure and check the crystals for use and then
calculate the capacitors and termination impedence based on that.
For a little work you get fine filters dirt cheap.

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.

While I have a DDS to do that with I found that using the first "high"
crystal in a VXO that gets calibrated worked as well with a lot less
fuss. Then I can use the same osc to sweep the filter later to test
it by adding a varicap doide.

That would work fine, but with the DDS, I can program the actual
frequency range to be swept and probably be able to calibrate the
scope face to read the actual frequency 'break' points on the filter.
Using the vxo method will get you a working filter quickly no
doubt, but I will still need the DDS vfo for the finished rig, so
I just figured I'd do that first.

How many 'rocks' did you use in an SSB filter? I've seen some designs
on the web with 6 crystals, would the shape factor be any better with
8 or more? (with over 400 crystals in the junk box I can go crazy,
but I'd still have to find the capacitors :-).