That dip on the high side of the passband was bothering me. The GPLA crystal design program predicted it, and indeed, when I measured the performance of the actual filter, there it was. The rig was working fine -- the receiver sounded fine and everyone tells me that it sounds great on transmit. But still, it bothered me.
So I started working out with the various crystal filter software packages.
This filter was -- sort of -- a Cohn Min-loss filter, but I had built it with four crystals and three shunt caps (80 pf each) and no series caps at the input/output. This morning I decided to see what would happen if I put the series caps in there. Here is what Wes's GPLA predicted:
Wow, that looked a lot nicer. And the 150 ohm terminations seemed to be just about perfect for the BITX design -- no need for impedance transformation. I heated up the soldering iron and searched the junkbox for suitable caps (I found 2 82 pf caps -- close enough).
Here is what the results looked like (I didn't measure insertion loss so the top of the curve is just the peak of the response curve).
Exactly as predicted! Thanks Wes! And thanks to Farhan for encouraging me to characterize my crystals and to use the available software
Now I have to go back and de-ripple the 5 MHz filter in my BITX 17.
In response to popular demand, "SolderSmoke -- Global Adventures in Wireless Electronics" is now available as an e-book for Amazon's Kindle.
Here's the site:
For the print version:
For shipping from a printer in the U.S. (probably better for N. American buyers) Click here: SolderSmoke USA Version
For shipping from a printer in the UK, Spain, or the USA (probably better for UK and other European buyers)
Click here: SolderSmoke EU Version
The two versions are identical, except for a minor difference in the paper used. That's why the prices are a bit different.
Bill's OTHER Book (Warning: Not About Radio)
Click on the image to learn more
W4HBK's QRSS Grabber: The Amazing Pensacola Snapper (Live!)