Crystal filter passband shape
ashhar_farhan <farhan@...>
 In BITX20@..., "n3ted" <tedkell@e...> wrote:
crystalDid somebody already plot the passband shape of a resulting 4 the shape is easy to figure out if you already know the crystalfilter?How do you do that? Figure out the shape that is? parameters. the crystal parameters are easier to measure now thanks to the G3UUR method. i have outlined it at http://www.phonestack.com/farhan/xcvr1.html. the essential idea is that a crystal filter looks like a capacitor and an inductor in series (these are called motional capacitance and motional inductance respectively). in addition to these, between the leads of the crystal you will also be able measure a parallel capacitance. using the G3UUR method, you put each of the crystals into the oscillator and measure its frequency. then you solder a 22pf or a 33pf in series with the crystal and measure the frequency shift. the shift gives you a good approximation of the motional capacitance and given the capacitance it is trivial to calculate the inductance (given that we know the crystal's frequency). the parallel capacitance is also approximated and the crystal is completely modelled. once you know the crystal parameters, you can spend a weekend understanding the butterworth filter design. or you can use the cookbook method in EMRFD like I did, or just use w7zoi program that comes with his other book Introduction to RF Design. using the my motional parameters, w7zoi did run it through his program GPLA.exe and i have just uploaded the results to the pictures folder. i am including his comments that go with the picture below: <snip> The first file, ashhar01, part A, shows the filter I designed with your motional L of 11.95 mH. I designed for 2200 Hz bandwidth and a Butterworth response. Then the filter at part B is a more practical version of the same thing. It is much like the filter that you ended up building. My simulations suggest that is really is worthwhile to add the tuning capacitors at the ends though, for it produces a much smoother, ripple free response. The first analysis I did used equal terminations of 200 Ohms (perhaps 220) at each end and the response was of course text book. But the question remained  what would the response be if the filter was terminated in other resistances. The file ashhar02 shows what you get with a 50 Ohm source and a 200 Ohm load. This is still pretty good. The response is not nearly so good with 100 Ohms per side, which is about the characteristic Z0 of your amplifiers. </snip>  farhan

