Hi,
I am currently in the process of designing a band pass filter for a homebrew CW radio I want to build. I was looking at the bandpass filter design for the QMX and was confused because the passband of the filter doesn't seem to match up with anything on the appropriate bands.
If I am reading the schematics properly, there are 15 and 23 turns on a T50-2 Toroid, which would have inductances of 1.1uH and 2.59uH respectively. Connecting them to the listed caps we have the following combinations:
- 1.1uH, 30pF: 27.7MHz
- 2.59uH, 56pF: 13.21MHz
- 2.59uH, 220pF: 6.67MHz
What confuses is me is the first combination which is clearly not in the 80-20 meter bands. I am pretty sure I am missing something but not sure what. I am not even sure if a single resonant band pass filter is the best option for my use.
Honestly I have been at my wits end trying to design a functional band pass filter, because when I simulate a filter in Elsie or RF tools, the passband on the built filter is off by a few MHz. Whenever I try to use recommended values from Experimental Methods in RF Design or Solid State Design plugging it into LTspice or Elsie yields a completely different passband. I eventually want to have a 5 band circuit made of SMD components so I can shrink my design into a portable package so I am avoiding the QRP labs Band pass filters since they use transformers.
Can anyone point out to me what is probably a blatent mistake I have made?
Thanks,
-Jonathan KM4CFT
Jonathan Kayne, KM4CFT