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8640Jr Band 2 Lowpass Filter

 

8640Jr Band 2 Lowpass Filter

Filed in homebrew on May.19, 2012

After the first two filters of the original 8640Jr design had been built, I headed off in my own direction to add another two filters for lower bands.

I miscalculated, and designed the filters to cutoff just after the third harmonic of the lowest frequencies, rather than the second, which would have placed it near the high end of the tuning range. The results were a little dismal at first…

 
Band 2 Lowpass Filter - Low End Bad

Yeah, that is one ugly waveform. The high end of the band looked better…

 
Band 2 Low Pass Filter Output - High End Original design

I didn’t catch this until I had already built the Band 1 filter with the same mistake. Oh well, that’s what makes it fun.

So the original design had the cutoff frequency way out at the edge of the third harmonic frequency of the low end of the range. As a result the third harmonic was only being attenuated by a few db.

 
Original Band 2 Frequency response (Bad)

Once I figured out what I had done wrong, I designed a new filter with a cutoff frequency of around 3.5 mhz, rather than around 5 mhz. I used the free AADE Filter Design tool, and input my cutoff frequency, and the third harmonic frequency with an attenuation of 25 db for the third harmonic. The tool spit out a design, and I modified it slightly to use the inductor values that I measured on my AADE LC Meter. I also tweaked the capacitor values to ones that I had on hand. It turned out pretty reasonable, and was not too touchy. The output shows about 28 db down on the third harmonic frequency of 5.3 Mhz.


 
Band 2 Lowpass Filter Design - Corrected

 
Band 2 Lowpass Filter predicted bandpass

The actual results are pretty close...

 
Band 2 Lowpass Filter Passband
 
The spectrum at the low end looks reasonable here...

 
Band 2 Low Pass Filter Output Spectrum - Low End

 And as usual, the high end looks good…

 
Band 2 Low Pass Filter Output Spectrum - High End

The resulting waveforms are very nice…

 
Band 2 Lowpass Filter Output - Low End

And the high end looks good, too…

 
Band 2 Lowpass Filter Output - High End



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