Stop! Yes we want to Stop a band of frequencies from passing through one of our rigs!
Normally we tend to think about Band Pass Filters, but just as important are Band Stop Filters (BSF). Sometimes you may have a situation where an unwanted mixing product could slip through some part of the circuit. Enter the BSF.
I had an issue with a VFO mixing problem and that is when I researched the Band Stop Filter.
Above is the LT Spice Simulation of our BSF and
the main frequency of concern was 2.3 MHz. At 2.3 MHz the attenuation is -68dB. The -40dB bandwidth is from 2.239 MHz to 2.412 MHz, a total of 170 kHz. That is one hell of a band stop!
Now let me not fool you as I did not have some magic calculator app to come up with these values. About 10 trials of values let me pick values where you can have standard inductors and capacitors. By tweaking values you can see the impact of changing the series resonant elements versus the parallel element and how that impacts the center frequency of the band stop and the bandwidth of the stop.
Fortunately LT Spice is scalable and so you can half the values and the band stop moves up by a factor of two.
Above is the LT Spice Simulation and I simply halved the component values for the simulation. CAUTION as simply making the inductors half the turns i.e. 6 Turns and 17 Turns as 6T = 0.144uH and 17T = 1.156uH will not provide the proper values. Thus you will need to calculate those specific values. The Al value for the T-50-6 = 40.
The 4.6 MHz Band Stop Plot is almost a mirror of the 2.3 MHz BSF only shifted by a factor of 2. So if any of you readers remember how to do ratios and proportions you can shift the Band Stop to any frequency you want and the same levels of attenuation will result.
This LINK was 1st used by me on a transceiver project and is shown in that link. I have since revised those values in the link to give a deeper null and tighten up the bandwidth.
You have now been armed with another tool for your tool box and you should file this away in your reference folder. There may come a day when you want to notch out a band of frequencies and this could be the answer to that problem. The Z in/out has been terminated with 50 Ohms.
Friend N2CQR in his 10/15 Homebrew Transceiver was having an issue on 15M with a harmonic of his low frequency Analog VFO slipping through a band pass filter. It was something like the 6th Harmonic (3.58*6 = 21.48 MHz) of his 3.5 MHz VFO. This BSF operating at 21 MHz and following the VFO could possibly have resolved the issue.
The 4.6 BSF components were proportioned by a factor of 4.63 (21.3/4.6) and this BSF would have provided some 60+dB of attenuation to halt that pesky 6th harmonic. For those having trouble C1, C2 = 108 PF and C3 = 1100pF.
The inductor values for this BSF are not standard as you end up with half turns. So the the best approach is to make the inductors integer values i.e 4 Turns not 4.11T and then tweak the caps as you have more latitude including small value trimmer caps. Drum Roll: here is a place to use your Nano VNA which has an inch of dust on the box as it hasn't been used in a year.
There you just received a TKT (Tribal Knowledge Tip).
73's
Pete N6QW