wb0smx.net

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 reslts were a litttle dismal at first…

Band 2 Lowpass Filter - Low End Bad

[Read the rest of this entry...]

There is a little distortion on the waveform of the Band 3 Low Pass Filter Output. The Spectrum Analyzer shows only about 14 db of attenuation on the 3rd harmonic of frequencies near the low end of the Range. This agrees with the predicted results below.

Band 3 Lowpass Filter Passband

[Read the rest of this entry...]

There is a little distortion on the waveform of the Band 4 Low Pass Filter Output.  The Spectrum Analyzer shows only about 14 db of attenuation on the 3rd harmonic of frequencies near the low end of the Range. This agrees with the predicted results below.

Band 4 Filter Passband

[Read the rest of this entry...]

Band 5 on the 8640 Jr covers approximately 13.6 – 31 Mhz.  There are now low pass filters configured for this band, but the output of the VFO has similar performance as the low pass filtered output of the other bands.  Here is a photo of it’s waveform at the low end of its range.

Band 5 Waveform - Low End of Range

[Read the rest of this entry...]

I selected a small enclosure for the low pass filters.  It was a little challenging getting all 4 filters into the small box, but it turned out pretty good.  I started running out of room on the circuit board, so I started mounting the resistors standing up for the last couple.  This photo was taken before I installed the final coupling cap from the switch to the output connector.

Low Pass Filter Assembly

[Read the rest of this entry...]