Another thing, you need to terminate the antenna input with a 50 ohm resistor (Assuming they are 50 ohm filters of course) and then you know for sure what you are seeing is internally generated
JR --- In [email protected], "Janne Pulkkinen" <[EMAIL PROTECTED]> wrote: > > I just finished my AD9954 board and I'm getting spurs on audio > frequencies. I haven't been able to test the board with spectrum > analyzer since there's no hams around with one... > > You can see the problem at: > http://www.saunalahti.fi/johku76/pic/spurs.jpg > > Pictures of the project can be found www.oh1gtf.com > > First quick note about the DDS setup: > I'm in the process of building a 425MHz reference oscillator, but > since it's not finished, I'm using only a 50MHz crystal "can" > oscillator and 4 X multiplier on the DDS. So, this way I'm having a > 400MHz system clock. > > In the picture above, the antenna is not connected. Some kind of > leakage can be detected, since when I connect the antenna some of the > "spurs" disappear. By the way, some of the spurs, which don't > disappear when connecting the antenna, don't have image rejection, so > they are both visible on the screen, getting away off from the zero > beat when adding frequency from the DDS. Some times when going with 1 > Hz steps the "spurs" kind of jump in and disappear with every Hz. > > Is those audio frequency spurs a result from bad IQ signal or some > kind of imbalance, or am I just having problems with grounding loops etc? > > 73 Janne, OH1GTF >
