EMRFD Message Archive 809
Message Date From Subject 809 2007-05-29 15:53:12 kerrypwr AADE Filter Program I am trying to design a 5 MHz/5-pole crystal filter very similar to
the one used in the Measurement Receiver. I wanted to use 250 Hz
bandwidth but I cannot obtain high-Q crystals (80 000 is the best I
can find so far) so I have settled for 2 000Hz for the moment.
The difficulty is that the schematic shows the crystals as 15.8 MHz; I
don't understand this.
I have plugged my Cm (0.0064 pF) and Lm (15.831 mH) values (obtained
using the G3UUR method) into the standard resonance equation as a
reality-check; the answer is close to 5 MHz.
Why does the program show 15 MHz crystals?
What am I doing wrong? I'm sure that I will feel foolish when someone
tells me! :)
810 2007-05-29 16:04:23 larry allen Re: AADE Filter Program Can I make a soft suggestion?...
Rather than use a crystal filter, why not try some controlled IF
feedback.. a 'Q' multiplier... Just a thought....
----- Original Message -----
811 2007-05-29 19:13:30 kerrypwr Re: AADE Filter Program Thanks for the suggestion, Larry, but it's the filter that I want.
So, why does the schematic produced by the program show the crystals
as 15.8 MHz??
812 2007-05-30 02:09:15 sm5glc Re: AADE Filter Program Could it be because it DOES resonate 813 2007-05-30 02:28:34 kerrypwr Re: AADE Filter Program GLC; I think you may be correct.
I have calculated Cm as 0.0064 pF when it may really be 0.064 pF.
The first figure gives 15.8 MHz which is what the program has
calculated; the second figure gives 5 MHz which is what the crystals are.
Maths is not my strongest point and I often have trouble with those
I will go over it all in the morning but I suspect that you are
correct and that I have a dismal point out-of-place.
Thanks for the check.