Jitter: 100MHz Clock 10x Better Than 80MHz Clock?
I decided to compare the jitter value between the 100 MHz clock vs the 80 MHz clock based on the datasheet from Crystek. This is what I got:
80 MHz: 2.29 ps RMS
100 MHz: 0.24 ps RMS (almost 10x better!)
I used the online jitter calculator to determine the numerical value for the jitter based on the data points shown in the phase noise graph.
For the 80 MHz part: RMS jitter=2.2864 ps (keep in mind that most specifications start at 100 Hz whereas this measurement starts at 10 Hz)
For the 100 MHz part: RMS jitter=0.23586 ps! WOW. This is 10x lower than the 80 MHz part. (but whether you can hear a difference or not, that is another matter)
Notice that most of the jitter in the RMS measurement comes from the close-in part of the frequency spectrum, in particular the region up to 100 Hz.
“Lower grade” clocks conveniently ignore this part of the measurements in their jitter specifications. For example, the specification for the Crystek C33XX, (the typical “small” clock found in many audio products) says:
Jitter RMS: 12kHz~80MHz: 0.5psec Typ., 1psec Max
DOES IT MATTER?
One authoritative paper is an AES paper titled Specifying the Jitter Performance of Audio Components. The concluding remark with respect to “useful” jitter measurement is the 100Hz value and the RMS jitter from 100 Hz to 40 KHz
Jitter spectra, when plotted properly, can be very useful. They can be even more useful when accompanied by numbers for RMS wideband jitter (100Hz corner) and RMS baseband jitter (100Hz-40kHz). The makers of audio clocking chips should consider providing this information in their product datasheets.
If we then compare both clocks starting from 100 Hz, then we get:
- 80MHz clock: 0.00462 ps (100Hz ->)
- 100MHz clock: 0.00123 ps (100 Hz ->)
Now we are talking about a difference of about 3 fs (femto-sec) or 340 atto-sec