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