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Howard, I get your point. And a six DB improvement makes the device a much better transceiver per the regulations
With simple unshielded wiring to three external relays, in my tests I found the fifth harmonic of 80 meter CW was basically obliterated, but the seventh only went down by about six dB. (I don’t have an explanation for the disparity ) I haven’t tried any
shielded wiring to see if that would make any better — but it certainly might.
In my case It might be simpler to add in a permanent low pass filter above 15 MHZ in my case. Your idea of lowering the capacitance is interesting. And it’s pretty simple also! For many people that might be the easiest solution to the harmonic problem.
On Sep 3, 2018, at 15:47, Howard Fidel <sonic1@...
The fifth and the 7th are at -45dB now. You are missing my point. The design of the filter directly effects the level of the harmonics. Yes, the stray capacitance creates a "sneak" path around the filter. However, the input and output capacitors of the filter
act as a divider to that "sneak" signal. If you double the output capacitance, you 1/2 the sneak signals amplitude, reducing it 6 dB. If you create a model of each of the filters in a simulator, and put 8 pF from input to output, you can clearly see the improvement
in the attenuation of the filter for the higher output capacitance.
All the data I have seen shows that a 6 dB improvement would just make the uBitx legal.
On 9/3/2018 2:51 PM, Warren Allgyer wrote:
When I measured 80 meter harmonics the particularly troublesome ones were 5th at 18 MHz and 7th at 25 MHz. In both cases the attenuation of the stock uBitx at those frequencies was limited by the layout of the board and by the coupling of relays sharing both
filter input and output in the same frame. In such cases the characteristics of the filters, both existing and any modifications are immaterial. The attenuation of the stock filter supplied is more than adequate. But the harmonics bypass the filter and go
via I/O coupling straight to the output. Please measure the 5th and 7th harmonics on CW. I think you will find you have not done much to improve the stock situation.