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.

Allison

"Ken Scharf" wrote in message
.. .
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.


Just sweeping a filter designed for SSB would be fine I suppose. I recently
had an opportunity to hear the difference between a stock FT-1000 CW filter
and one homebrewed with attention paid to group delay- the difference was
very clear to hear- in favor of the homebrew filter.
\
Dale

On Fri, 14 Oct 2005 19:38:37 -0400, Ken Scharf
wrote:


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.

Makes sense. I found the other way easy when DDS chips were 55$ each!
The varicap sweep is calabrated to the scope so that was not an issue
for sweeping the filter. I found that using wideband noise and a
sound card was better.

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 :-).

I'd say 4 is a useable minimum. With that I'll add the skirts at 40db
down are not very good though. I've used 6-8 to get a good 6-60db
shape (under 2:1). There is a problem if you go for too many. The
filter can have enough group delay that while it's shape is good, the
sound has a hollowness.

The caps, once you figured the qalues you will likely end up using
parallel values. IE: 232pf may be a 220+12pf or a 220 and a 4-20pf
trimmer.

Allison

On Sat, 15 Oct 2005 00:24:39 GMT, "Dale Parfitt"
wrote:

Just sweeping a filter designed for SSB would be fine I suppose. I recently
had an opportunity to hear the difference between a stock FT-1000 CW filter
and one homebrewed with attention paid to group delay- the difference was
very clear to hear- in favor of the homebrew filter.
\
Dale

My prefered filter has a gausian to 6db shape for less ringing and
group delay. I work for that goal.

However, try the KK7B Phasing rigs for sound. They are direct
conversion SSB (image rejecting) so all the selectivity is in the
audio bandpass. I use a miniR2 and T2 pair on 6m and filter
artifacts like group delay aren't there. Transparentcy is a good
word to describe it.


Allison
KB1GMX

A common mistake in years past was to try to put all of the selectivity into
a single super-deluxe crystal, mechanical or digital filter. These filters
quite often have "raspy" noise interference at the edges of the passband
(especially in CW mode) due to the enhancements of spectral noise peaks at
the very sharp band edges, caused by the conversion of the phase statistics
of noise to amplitude statistics (each edge of the filter acts like a phase
disciminator). This raspy noise interferes with weak signals.

The filter can be equalized for group delay, as mentioned, with improvement
in the problem. The band edges can be softened, with good results. A much
better way is to use two or more intermediate-performance filters in
cascade. This method softens the edges so that the effect is greatly
reduced. It also improves overall shape factor. A cascade in this manner
of identical bandpass or audio lowpass filters tends in the limit toward the
Bessel or even the Gaussian response. Digital filters can also use a method
called Transition Band Sampling (see Oppenheim and Schafer 1975 or Oppenheim
and Willsky 1983). All of these results are related in principle to the
Central Limit Theorem of statistics.

The cascaded filter approach is also very beneficial in other respects, in
particular the reduction of wideband noise in high-gain IF amplifiers. This
noise degrades AGC performance and adds audio frequency noise to the product
detector output.

Bill W0IYH


"Dale Parfitt" wrote in message
news:b%X3f.301$W32.225@trnddc06...

Just sweeping a filter designed for SSB would be fine I suppose. I
recently had an opportunity to hear the difference between a stock FT-1000
CW filter and one homebrewed with attention paid to group delay- the
difference was very clear to hear- in favor of the homebrew filter.
\
Dale

Receiver bandwidth .. dayton filter find!
 

"William E. Sabin" wrote in message
news:6J84f.437250$x96.418250@attbi_s72...

A couple of improvements in the following paragraph:

The filter can be equalized for group delay, as mentioned, with
improvement in the problem. The band edges can be softened, with good
results. A much better way is to use two or more intermediate-performance
filters in cascade. This method softens the edges so that the effect is
greatly reduced. It also improves overall shape factor. A cascade in
this manner of identical bandpass or audio lowpass filters tends in the
limit toward the Bessel or even the Gaussian response. Digital filters
can also use a method called Transition Band Sampling (see Oppenheim and
Schafer 1975)

Delete the incorrect second Reference to Oppenheim and Willsky 1983.

All of these results are related in principle to the Central Limit
Theorem of statistics.

See http://mathworld.wolfram.com/CentralLimitTheorem.html for this
interesting topic. The general idea that the theorem alludes to in this
example is that as sharp-cornered filters are cascaded the passband response
becomes noticeably more rounded at the corners, similar to Bessel and
Gaussian filters.

Smoothing and Windowing methods can be used to reduce sharp corners in
discrete sequences such as digital filters (see Oppenheim and Schafer 1975
and many other sources).

Bill W0IYH