Xilinx Frequency Generator manual Phase Accumulator Spectrum

Phase Accumulator Spectrum

An alternative way to observe the quality of the waveforms synthesised by the phase accumulator is to look at the frequency spectrum. I was lucky enough to have a 2048-point FFT feature on my oscilloscope which allows some simple observations to be made. Once again I have set the display to infinite persistence in order capture the spectrum over a long period of time (>15 seconds). I suggest that you do not look for exact values, but compare the plots which have been captured using the same scales in each case. On the left are plots that show the spectrum up to 50MHz and on the right the plots show are zoomed in to show ±5MHz centred on 12.5MHz.

3rd

12.5MHz Harmonic Fundamental

12.4125MHz Fundamental

As we know the 12.5MHz signal is
nominally clean in terms of the synthesis
process and this is reflected by a distinct			12.5MHz
12.5MHz component which is >45dB above			Fundamental
the noise floor. The zoomed plot shows how
the fundamental covers only a narrow
bandwidth keeping in mind that the
resolution of the FFT is only ~50KHz.
Note the third harmonic of a square wave is
also very distinct at 37.5MHz and is even
close to being the theoretical 9.54dB down
from the fundamental (1/3 amplitude).
	5MHz/division	1MHz/division
	10dB/division	10dB/division
			12.4125MHz
When we move to the less than perfect
			Fundamental	12.4125±0.74MHz
12.4125MHz waveform the effects of that
5ns of cycle jitter and jumping between

12.5MHz and 11.76MHz waveforms is obviously having an impact on the spectrum. Although the fundamental frequency component is good, if a little wider in bandwidth, there are significant modulation effects leading to a family of spectral components and raised noise floor. Hint – 12.5MHz-11.76MHz = 0.74MHz

Frequency Generator for the Spartan-3E Starter Kit 10