EMRFD Message Archive 10742

Message	Date	From	Subject
10742	2015-01-20 17:08:52	prose062	ladder filter question
I have an SSB crystal ladder filter that I built from a kit that I have a question about. The filter is a ladder filter built with 47pf capacitors and 12 matched 4.9152 MHz crystals. 10 of the crystals are connected the way I would expect for a ladder filter:  In series with each other, and with 47pf capacitors to ground between each crystal. The unexpected part ( for me ) is that the first crystal ( from the hot end of the input matching transformer to the first capacitor ) has an identical crystal in parallel with it.   The last crystal ( from the hot end of the output matching transformer to the last capacitor ) also has an identical crystal in parallel with it. The filter tests well, has low loss, a flat passband and steep (mostly) symmetrical sides.  But I haven't actually listened to it yet. My question is:  what is the purpose of the extra 2 crystal in parallel at the ends, and is there a name for this configuration?  Are there additional design rules, or does one design a regular ladder filter and then add the extra parallel crystals at the ends? Thanks, Paul Rose K0EET
10743	2015-01-20 18:43:21	farhanbox@gmail.c...	Re: ladder filter question
thats the qer filter. an evolution of cohn. most of the new handbooks carry details about these filters. they are due to dr. gordon. G3UUR  _f ------ Original message------
10744	2015-01-20 18:47:31	Neil Martinsen-Bu...	Re: ladder filter question
That crystal configuration is diagnostic for the Quasi-equi-ripple (QER) filter. VK2SJA gives a nice introduction to the filter and programs for their design in this blog post: <http://vk2sja.org/piffle/2014/10/crystal-ladder-filter-20mhz/>. Ashhar Farhan VU2ESE used a QER filter for the SSB filter in his Minima transceiver design that has generated a great deal of interest recently: <http://www.phonestack.com/farhan/minima.html>. -Neil N0FN
10745	2015-01-20 18:56:58	kerrypwr	Re: ladder filter question
I've only heard of this, not studied it. The configuration is called Quasi Equi - Ripple (QER); I think that it's attributed to Dave Gordon - Smith G3UUR. It's a variation of the Cohn topology with less ripple in the passband; I noticed that the latest version of DJ6EV's Dishal program that I recently downloaded supports QER filters. But I haven't played with QER filters (been a bit QRT here  :(  ); others may be more helpful. Kerry VK2TIL.
10746	2015-01-20 19:08:52	prose062	Re: ladder filter question
Thanks all.  QER it is.  I'll have a look at the links given. Paul