Max nr of poles in CW-filter ?
 

Having built several ladder filters for SSB (8.. 10 MHz, 4 .. 8 poles)
with BW abt. 2 kHz (mainly for TX applications) I would like to try a
narrow filter (BW 500 or even less). One thing I am worried about is
the insertion loss. Has anyone any experience regarding the maximum
number of poles (= crystals) in such narrow filters. I have seen designs
for SSB with 14 poles but some doubts exist if this would be excessive
in a CW filter.

73 de oh2lzi (Heikki)

Heikki wrote:
Having built several ladder filters for SSB (8.. 10 MHz, 4 .. 8 poles)
with BW abt. 2 kHz (mainly for TX applications) I would like to try a
narrow filter (BW 500 or even less). One thing I am worried about is
the insertion loss. Has anyone any experience regarding the maximum
number of poles (= crystals) in such narrow filters. I have seen designs
for SSB with 14 poles but some doubts exist if this would be excessive
in a CW filter.
73 de oh2lzi (Heikki)

Heikki - See QST for 8/78, 12/78 and 11/80 for answers.
If I have room, I put a mini trimmer cap in series with each xtal
to tune all the xtals to the exact same frequency.
This helps the bandwidth and reduces the insertion loss.
73 W7ZFB

Heikki wrote:
Having built several ladder filters for SSB (8.. 10 MHz, 4 .. 8 poles)
with BW abt. 2 kHz (mainly for TX applications) I would like to try a
narrow filter (BW 500 or even less). One thing I am worried about is
the insertion loss. Has anyone any experience regarding the maximum
number of poles (= crystals) in such narrow filters. I have seen designs
for SSB with 14 poles but some doubts exist if this would be excessive
in a CW filter.
73 de oh2lzi (Heikki)

Heikki - See QST for 8/78, 12/78 and 11/80 for answers.
If I have room, I put a mini trimmer cap in series with each xtal
to tune all the xtals to the exact same frequency.
This helps the bandwidth and reduces the insertion loss.
73 W7ZFB

Hi Heikki et al

I have some info from Bill Carver, W7AAZ's article "Why Crystal
Filters" and he describes a 16 pole SSB filter and using a "meshing"
technique and ended up with an xtal filter with the following spec:
Thats impressive!

BW : -3dB @ 2.23 KHz
BW: -6dB @ 2.42 KHz
BW: -60dB @ 2.89 KHz
BW: -80dB @ 3.24 KHz

A Shape factor 1.19 ( 6:60dB)

And of main interest : 1.8dB insertion loss (50 Ohm)

Now obviously the main criteria are the initial "Q" of the
crystals chosen. Bill used DigiKey 4.433Mhz types and bought 100,
HC-49 that had an average "Q" of 112,000.

To determine the minimum Q xtal for the application required then
refer to the following:


Qmin = Fo / BW

where Qmin in the minimum xtal requirement - but that gives infinite
loss definitions!

What Bill has measured for example is that using the above for 2.1
KHz filter centered on 8Mhz then the minimum Q xtals from any batch
you have must be at least :

8,000,000 / 2100 = 3,810 and for the 14 pole version that relates
to about a 3dB insertion loss. Now for the CW filter! consider 500 Hz
at 8Mhz then Qmin would be 16,000 and even a 6-pole Chebychev with Q's
of 150,000 would give rise to the overall filter loss of over 12dB's.
Depending upon what ripple values you choose then if and a big if
these days!! ( as cheap xtals are getting worse Q's! whenever I test
them!!) back to the story!: If same filter at 500 Hz BW had xtals
having Q = 320,000 ( you need luck!! ) then expect overall filter loss
to be 4 - 5 dB.

I had been experimenting ( not recently ) with xtals on 6.144 Mhz
with unloaded Q's of around 100K - must get back to that project as I
spent hours checking batches of those two legged critters!!

......... Best wishes

Ken, G3UDA


wrote in message
...
Heikki wrote:
Having built several ladder filters for SSB (8.. 10 MHz, 4 .. 8
poles)
with BW abt. 2 kHz (mainly for TX applications) I would like to try
a
narrow filter (BW 500 or even less). One thing I am worried about
is
the insertion loss. Has anyone any experience regarding the maximum
number of poles (= crystals) in such narrow filters. I have seen
designs
for SSB with 14 poles but some doubts exist if this would be
excessive
in a CW filter.
73 de oh2lzi (Heikki)

Heikki - See QST for 8/78, 12/78 and 11/80 for answers.
If I have room, I put a mini trimmer cap in series with each xtal
to tune all the xtals to the exact same frequency.
This helps the bandwidth and reduces the insertion loss.
73 W7ZFB

Hi Heikki et al

I have some info from Bill Carver, W7AAZ's article "Why Crystal
Filters" and he describes a 16 pole SSB filter and using a "meshing"
technique and ended up with an xtal filter with the following spec:
Thats impressive!

BW : -3dB @ 2.23 KHz
BW: -6dB @ 2.42 KHz
BW: -60dB @ 2.89 KHz
BW: -80dB @ 3.24 KHz

A Shape factor 1.19 ( 6:60dB)

And of main interest : 1.8dB insertion loss (50 Ohm)

Now obviously the main criteria are the initial "Q" of the
crystals chosen. Bill used DigiKey 4.433Mhz types and bought 100,
HC-49 that had an average "Q" of 112,000.

To determine the minimum Q xtal for the application required then
refer to the following:


Qmin = Fo / BW

where Qmin in the minimum xtal requirement - but that gives infinite
loss definitions!

What Bill has measured for example is that using the above for 2.1
KHz filter centered on 8Mhz then the minimum Q xtals from any batch
you have must be at least :

8,000,000 / 2100 = 3,810 and for the 14 pole version that relates
to about a 3dB insertion loss. Now for the CW filter! consider 500 Hz
at 8Mhz then Qmin would be 16,000 and even a 6-pole Chebychev with Q's
of 150,000 would give rise to the overall filter loss of over 12dB's.
Depending upon what ripple values you choose then if and a big if
these days!! ( as cheap xtals are getting worse Q's! whenever I test
them!!) back to the story!: If same filter at 500 Hz BW had xtals
having Q = 320,000 ( you need luck!! ) then expect overall filter loss
to be 4 - 5 dB.

I had been experimenting ( not recently ) with xtals on 6.144 Mhz
with unloaded Q's of around 100K - must get back to that project as I
spent hours checking batches of those two legged critters!!

......... Best wishes

Ken, G3UDA


wrote in message
...
Heikki wrote:
Having built several ladder filters for SSB (8.. 10 MHz, 4 .. 8
poles)
with BW abt. 2 kHz (mainly for TX applications) I would like to try
a
narrow filter (BW 500 or even less). One thing I am worried about
is
the insertion loss. Has anyone any experience regarding the maximum
number of poles (= crystals) in such narrow filters. I have seen
designs
for SSB with 14 poles but some doubts exist if this would be
excessive
in a CW filter.
73 de oh2lzi (Heikki)

Heikki - See QST for 8/78, 12/78 and 11/80 for answers.
If I have room, I put a mini trimmer cap in series with each xtal
to tune all the xtals to the exact same frequency.
This helps the bandwidth and reduces the insertion loss.
73 W7ZFB

I missed most of this thread, so this might already have been covered.

When making a filter for CW, it's essential that you pay careful
attenuation to the time-domain response, and don't concentrate
exclusively on the frequency domain. Otherwise, you're apt to end up
with a filter that rings badly, and makes it difficult or impossible to
copy CW. If you're a very slow speed operator, you can put up with more
ringing, but at higher speeds, good transient response is vital.

Among the canonical filter types, I've found the Butterworth to be a
very good compromise for CW. It provides tolerable transient response
while having a respectable rolloff characteristic. In practice, a more
optimum design (i.e., equal sharpness and transient response with fewer
total poles) can often or always be realized with a combination of a
sharper filter type like a Chebyshev or even elliptical, with added
allpass poles to correct the phase response and therefore tame the
ringing. However, the latter approach can be a good deal more time
consuming -- you can look up the pole locations for a Butterworth in a
table, synthesize the filter, and have a reasonable expectation that
it'll work pretty much as designed. On the other hand, a phase-corrected
Chebyshev or elliptical filter can involve a lot of trial and error
modeling.

Roy Lewallen, W7EL

I missed most of this thread, so this might already have been covered.

When making a filter for CW, it's essential that you pay careful
attenuation to the time-domain response, and don't concentrate
exclusively on the frequency domain. Otherwise, you're apt to end up
with a filter that rings badly, and makes it difficult or impossible to
copy CW. If you're a very slow speed operator, you can put up with more
ringing, but at higher speeds, good transient response is vital.

Among the canonical filter types, I've found the Butterworth to be a
very good compromise for CW. It provides tolerable transient response
while having a respectable rolloff characteristic. In practice, a more
optimum design (i.e., equal sharpness and transient response with fewer
total poles) can often or always be realized with a combination of a
sharper filter type like a Chebyshev or even elliptical, with added
allpass poles to correct the phase response and therefore tame the
ringing. However, the latter approach can be a good deal more time
consuming -- you can look up the pole locations for a Butterworth in a
table, synthesize the filter, and have a reasonable expectation that
it'll work pretty much as designed. On the other hand, a phase-corrected
Chebyshev or elliptical filter can involve a lot of trial and error
modeling.

Roy Lewallen, W7EL