Band Pass Filter Board

The Band Pass filter is a key element ih having your signal at the right place so you avoid spurious emissions, out of band signals and avoiding a pink ticket from the radio regulatory authorities. Here in the USA it is the FCC.

So how do you go about designing an LC Band Pass Filter? One way is to simply research projects and see what others used -- but there is the nagging question is it a really good band pass filter.

Not to long ago Mouser and others sold this amazing miniature 10.7 MHz IF transformer known as the 42IF123. These transformers could be modified so that with but a few external components --instant Band Pass Filter covering the HF Bands. Sadly these are no longer sold by Mouser and most other sources have dried up as well. In my KWM-4 I had a whole bank of these that were relay switched depending on the band.



But given those are no longer available other methods must be used. If you have a Solid State Design for the Radio Amateur publication, in the Appendix is a detailed methodology for hand calcualting a Band Pass Filter. A couple of times through the math calculations and you can be an expert. In that appendix are also "canned filter values" for many of the ham bands. That is very useful information. EMRFD I believe has a similar aid for calculating BPF's. I have never used that resource as I find less useful than SSDRA.

There are two free software programs that will give you the appropriate values. One of those is LT Spice and the other is ELSIE. Once or twice through ELSIE and I quickly convinced myself that LT Spice was a better tool for my needs.

The sheer beauty of LT Spice is that you can take the values from SSDRA and insert them in the simulation. From There you can tweak the numbers to suit what is in the junk box or for your application. If you are going to work only 40M SSB, why go nuts trying to have a flat bandwidth down through the CW band?

The really desirable feature of LT Spice is that you can display a plot of the filter and that is helpful in setting the center frequency. Many times when you calcualte and inductance for a toroid core -- the number of turns are not an integer. Thus in the LT Spice I round up/or down to the nearest turn and then calcualte the inductance and then use the capacitors to trim the network to the desired filter shape.

Two filters that can be used in the Big Kahuna are shown below and were simulated in LT Spice. I know that the nitpickers will look at the photos and will quickly say those are T-68-6 cores in the build and the schematics show the T-68-2. Use the T-68-2 cores as the number of turns provide the required inductance.

The 164 PF caps for the 20M BPF are 150 PF NPO in parallel with a 50PF Trimmer cap. The 14.8 PF caps are 50 PF Trimmers. You could fudge this value by using 15 PF NPO caps for the 14.8 PF. The 1.5 PF coupling cap was a 1-3 PF Trimmer cap. You could also fudge this by using a 1PF NP0. After you have had some practice with LT Spice you will find that the flatest curve plots are achieved with the smallest value of coupling cap.







Below is the Band Pass Filters used in the Big Kahuna. As I mentioned earlier I used the T-68-6 cores; but the simulations used the T-68-2 cores which I recommend using. The lower set of coils are for the 40 Meter Band. The filters are very similar and the 40M can easily be made from a 20M PBF by adding the 150 PF NPO caps and using 50PF trimers at the other 4 places. Now you perhaps see the method to my madness!