Warren:
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.
Howard
On 9/3/2018 2:51 PM, Warren Allgyer wrote:
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Howard
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.
WA8TOD
