A mid-life crisis

uBITx.net followers will remember the concerns that Allison KB1GMX raised about the spurs generated by the µBITx above 18 MHz on SSB (but not on CW), at least on her ‘Sample of One’.

“No one is going to want to hear this” says Warren WA8TOD, “But the design of the PA output filter is probably fatally flawed and it is likely due to board layout issues.”

Warren was puzzled how the high order harmonics from 80 meter operation, for example, could possibly survive a 7th order filter which should have well over 50 dB of attenuation at the 5th harmonic. So he set out to test the results with a 9th order filter, figuring more is better. The harmonics remained. But in troubleshooting that installation the source of the issue was revealed.

In the attached screenshot the purple trace is the 80 meter CW output with the harmonics clearly far in excess of the green -43 dBc legal threshold (NB The line on the screen is actually at -53dB).

The blue trace is the output with L20 removed which disables the 80 meter filter and blocks RF output from the finals from reaching the output connector. (Don’t worry….. the tests were carried out with the drive reduced so the output was only one half watt. And I have a bag of spare IRF510s on hand……). The harmonics remain! How can that be?

The circuit diagram shows the path 80 meter RF follows through all three switching relays in order to reach the filter. It is a very novel and inventive concept to use three relays to switch both ends of 4 filters. Most manufacturers would require 8 separate relays to do this job. And for a very good reason.

In order for a filter to provide the required 50+ dB of attenuation, the filter itself must have 60 dB or more of input-to-output isolation. If it does not then the unwanted products simply bypass the filter and go directly to the output. That appears to be exactly what is happening in the filter complex of this unit.

There is no easy fix for this. No amount of on board filtering and tweaking is going to improve harmonic suppression until the path around the filters is blocked. I suspect that path is via the relays themselves but board layout often is critical as well in such cases. Warren’s guess is that the only solution would be either extensive external filtering or a carefully laid out daughter card to replace the existing filters and relays.