Transceiver Architecture 2.13
Lets Build that 11.5 MHz Filter Part II
In Part 1, (2.12) we shared how we measured all 25 crystals and found a grouping of 9 that were essentially "carbon copies" of each other. I never play the lottery because of that exact situation. I suspect all lotteries are rigged and the likelihood of having nine exactly the same crystals is somehow rigged. So the first thing I will do is to re-measure all of those nine in hopes of finding 6 out of that batch that are exactly the same. That may happen yet today.
So once you have the crystals and the two frequencies that were measured on the G3URR oscillator proceed to the WA5BDU tutorial on how to calculate Lm and Cm. I made an assumption that the crystal holder capacitance was 3 PF --probably not good or bad. Regardless, using the equations provided by Nick Kennedy I found the Cm to be 16.356 Femptofarads (10 to the -15) and using that value you can calculate the Lm which is 11.7 MHy.
Now for the actual crystal filter calculator go to the ARRL QEX download page and for the year 2009 find the Zip file 11/09_Steder_Hardcastle and download to your computer. Do NOT place it in the Program Files or Application Files!!!!!!!! I have a directory called N6QW and in that directory I have a folder called Dishal --extract it to that folder. Milton Dishal wrote a piece of software to simulate crystal filters. Steder and Hardcastle take that software and turn it into "ham speak" so it can be used by people like me.
Now I had a problem with the QEX download --my computer kept telling me it contained a virus and promptly removed it from my computer no less than 3 times. The fourth time on the Zip file I used a tool called compatibility --it seems when I now use the Dishal software my computer thinks it is a windows XP machine and all works good.
Here is where the dishal software can save you lots of effort --if you enter the shifted frequencies it will actually calculate the Lm and Cm --and they match my hand calculation. After playing with it a bit I can see that it would be ideal to have six crystals in the filter and here is why. See below.
The first plus is the 3 dB bandwidth which is about 2.2 KHz and at 30 dB down is probably less than 3.0 KHz. The 20 dB numbers and 40 dB numbers look OK. It appears to have a more symmetrical shape ( a factor that improves by having more crystal in the filter), thus getting away from the idea of strictly an LSB filter.
I found out the pass band ripple is linked to the input impedance. In this case it is 102 ohms. 102/50 = 2.04 and if we did a 7 turn and 10 turn winding on a FT37-43 core we would have 100/49 = 2.04 --so a perfect match. But if you drop the ripple to 0.2 dB then the input impedance raises to 127 ohms and this is a ratio of 2.54 when transformed to 50 Ohms. So if we use an 8 turn and 5 turn winding on a FT37-43 core that transforms to 64/25 = 2.56 ---really close.
The software has a built in LC matching network calculator so that you can essentially design "L Networks" for the matching. I prefer the broadband match.
The 0.2 dB pass band ripple also "twizzles" the caps so that the values are much nearer to close tolerance standard value caps. I have some low value air trimmer caps (0-15 PF) that could be paralleled with the standard values so that the filter will be dead on.
I also believe this filter will have to be built in such a way where the crystal cases are flat to a ground plane and the crystal can soldered to that plane. I will use SMA connectors to the filter and it will be totally enclosed in a metal box. This will be bullet proof.
My plan is to update this posting with the re-measuring of the 9 crystals --to see if I should play the lottery today.