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

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Xilinx Frequency Generator manual Phase Accumulator Spectrum, 12.5MHz Harmonic Fundamental 12.4125MHz Fundamental
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Frequency Generator specifications

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All in all, the Xilinx Frequency Generator exemplifies cutting-edge technology in frequency generation, offering flexible configuration, excellent stability, and user-friendly integration, making it a compelling choice for engineers across various industries seeking a reliable frequency generation solution.
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