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8640Jr – Band 1 Lowpass Filter


8640Jr – Band 1 Lowpass Filter

The final band I added to the 8640Jr Signal Generator was for 850 – 1900 khz. I wanted to be able to cover 160 meters, and this allows the generator to do that, with Band 2 covering the top part of 160 meters, as well.

Band 1 Measured Response - New Filter

As I built both additional bands at the same time, I made the same mistake on both. 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 disappointing…

Band 1 Low Pass Filter Output - (original Bad)

The high end of the band looked better, as it had for Band 2…

Band 1 Lowpass Filter Output - High End Original Filter

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.

Band 1 Original Passband (Bad) - Cutoff too high

Once I figured out what I had done wrong, I designed a new filter with a cutoff frequency of around 1.7 mhz, rather than around 3.5 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 2.5 Mhz.

Band 1 Lowpass Filter Final Schematic



Band 1 Predicted Response

The actual results are pretty close…

Band 1 Measured Response

The spectrum at the low end looks reasonable here…

Band 1 Spectrum Analysis - Low End of Band

And as usual, the high end looks good…

Band 1 Spectrum Analysis - Low End of Band

The resulting waveforms are very nice…

Band 1 Waveform - Final Filter

And the high end looks good, too…

Band 1 Waveform - Final Filter


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