Four Pole Crystal Filters!
I hate homebrew crystal filters! They are difficult to build and unless you have:
- A decent process to build one and
- Decent equipment to measure what you have built you will be doomed to failure.
- A typical result failing 1 & 2, does and will sound like crap.
Given the recent article in the GQRP SPRAT by ZL2BMI for a tramping SSB transceiver I have given pause to building another small transceiver --- very small. It is a sort of challenge from ZL2BMI as in a recent round of emails between Eric & myself resulted in his stating: "Mine is SMALLER than Yours".
I shared that my shirt pocket SSB transceiver is only 16 cubic inches to which he shared that his rig is less than that and that it has an OLED display. Mine used a crystal switched VXO covering only about 60 kHz of 20M. His is a tri-bander for 160-80 -40 Meters.
So you now know where I am headed.
Firstly I did build a four pole filter using 4.9152 MHz crystals. You can see the plot below! Man that is text book and using the Antuino I now can find the Filter Center Frequency. It will also help me make a 1st run at the LSB and USB BFO frequencies. I used 47PF coupling caps so the BW is wider than I normally use --but it will have presence and not be yellowy! I also only had 47PF in SMD.
JFF (Just For Fun--that must be what the emperor (small e) did when he made his Syria decision.) I installed the filter in a working 80M SSB XCVR and also built an outboard Digi VFO. It works!!!!!!!
Stay Tuned as I work on the ZL2BMI SIZE challenge.