I’ve been spending a lot of time trying to track down the
source of a buzzing noise on the Tx signal as heard in a receiver across the
shack. I’ve put caps just about everywhere
that I could think of, although I did not put in place several at
once. I just added different values at
certain locations. There is permanently
installed caps on the power points for the dds and nano boards. As well I moved, to a certain degree, boards
around to see if I could detect any variances in the signal but again no
differences were noticed.
The next step in my quest, was to remove the cable for the
LO signal that is being supplied by the Si5351 board. I substituted in a signal
generator at the IF frequency of 4.9152mhz and injected the signal a 2
locations. The first one was to input the signal into the IF crystal filter and
the result was a clean signal. Next I
injected the signal into the LO of the mixer and a clean signal was heard there
as well. As the signal at this point was about 100mv, and the output of the dds
is around 4vpp, I increased the voltage to check if that was an issue. The signal was still clean at that level as
well.
So I’m fairly confident that the problem exists with the
dds. Now how to figure out how to clean that up. Especially since the signal from the signal
generator appears to be not that much different than what the dds was supplying.
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