Tim,
Were your screen shots done with spur reduction on or off? With spur
reduction on, I do not get the nine spurs you see on 28.581, I get one big
spur about 5.9 khz above. Spur situation does not change @ 28.582 except
the big spur is now only 4.9 khz above. For my setup with spur reduction
on, the DDS is running at 28.570102 MHz for these frequencies. (This
probably depends on the DDS clock offset currently set, in my case it's 7500)
@ 28.582661 the DDS goes to 28.573154 and I now have three spurs visible.
They march across the screen as I tune up and. At about 28.5843 on of the
spurs begins to intrude on a 2.6 khz passband (one case where an alternate
tuning algo would be useful)
@ 28.585711 the DDS goes to 28.576206 and no spurs are evident.
If I turn spur reduction off, I get something that looks like your 28.581
pic at about 28.5838, and about 1 khz above that I see a 4 spur pattern
somewhat like your 28.582 pic. I suspect the difference in the exact
frequencies we're seeing are probably due to different clock offset settings.
I have no doubt what you're seeing are DDS spurs. The point of my post was
that I think they can be avoided with a more aggressive spur dodging
tuning algorithm I believe the current spur reduction algorithm simply
tunes the DDS to a spur reduced pint that is within about 3 +/- khz of the
frequency of interest. With +/- 24 khz of IF tuning capability I think we
can dodge most (if not all) of the spurs with a cleverer tuning algorithm
-- 'Better Radio thru Software".
Regards,
Bill (kd5tfd)
