DSP56362EVM "birdies"

Started by Unknown February 1, 2001
Hi folks,
I've spent a little over a year on the above platform (modifying BOTH
the 'HC711 and DSP programs, putting my own menus into the LCD and
getting the DSP to accept serial port commands through the MCU via
SPI) and while I've gotten quite proficient in "fighting through" the
sometimes perplexing lack of documentation on some of its features,
there's STILL a few areas I can't figure out! My current problem is,
every time I feed an analog sine wave into EITHER the on-board ADC or
an external AKM (24-bit) unit via the fiber interface, I get annoying
aliasing or "birdies" - the first "zero beat" I get is usually around
2800 hertz. If it were a faint artifact I might be able to live with
it, but it's LOUD and actually interferes with my ability to take
real measurements. What's even more perplexing is, I incorporated
a "bypass" mode for either the left or right channel (sort of like
the way it comes) and the aliasing is present even on the channel
that's NOT being processed - it WILL go away if I "patch around" the
board and just feed the AKM ADC fiber output into an AKM DAC fiber
input so it's not in EITHER ADC! Could this be an artifact of some
jitter or decode glitch in the clocks generated by the Lattice part
or perhaps the DSP itself? Or would it be related to a problem in the
(ill-documented) I/O timing? I realize this PCB is a
little "developer-obscure", does anyone have any experience tracking
down the source of this problem on OTHER platforms? I'm totally
clueless on this one...thanks in advance for trying!
Jeff Lawton