Don't believe the hype. In the real world ALL so-called Femto clocks are just marketing BS when applied to audio.
This specifically applies to clocks intended for SONET applications (Read internet backbones), which need to have very low phase noise (jitter) above 12KHz, but care zero about phase noise below 12KHz. Which is why clock jitter for these clocks is specified usually as RMS 12KHz - 1MHz (or higher) and a very low level of phase noise / jitter at these frequencies is essential. This is why this specification recently became important and specified (many vendors simply adjusted their datasheets for what is the part that previously was a "picoclock")
Now here is the killer. it is practically impossible to make a crystal oscillator that is fixed frequency and does not measure at the max. a few 100 Femtoseconds RMS phase noise/jitter under the tests conditions. The cheapest clock you find in China will do that.
For Audio use what matters most is audioband (20Hz-20KHz) jitter. In this case a really basic "Tent-clock" rated at 3pS P-P Jitter RMS IIRC from a commonly available source with a cheapo 3-Pin chip regulator will beat the pants of ANY Femtoclock most designers are aware of.
None of them provide even 50pS peak-Peak jitter 10Hz-20KHz.
The GMT System, which uses a programmable clock that was not optimised for SONET, but for an application that needs low audio band phase noise levels., when measured it "Femto Clock" style, shows around 200 Femto Seconds RMS phase noise.
Most any clock in audio (1 rung above lo-fi) use if measured "Femto Clock Style" is a Femto Clock .
Finally, jitter is NOT the end-all be-all of good sound in digital.
This specifically applies to clocks intended for SONET applications (Read internet backbones), which need to have very low phase noise (jitter) above 12KHz, but care zero about phase noise below 12KHz. Which is why clock jitter for these clocks is specified usually as RMS 12KHz - 1MHz (or higher) and a very low level of phase noise / jitter at these frequencies is essential. This is why this specification recently became important and specified (many vendors simply adjusted their datasheets for what is the part that previously was a "picoclock")
Now here is the killer. it is practically impossible to make a crystal oscillator that is fixed frequency and does not measure at the max. a few 100 Femtoseconds RMS phase noise/jitter under the tests conditions. The cheapest clock you find in China will do that.
For Audio use what matters most is audioband (20Hz-20KHz) jitter. In this case a really basic "Tent-clock" rated at 3pS P-P Jitter RMS IIRC from a commonly available source with a cheapo 3-Pin chip regulator will beat the pants of ANY Femtoclock most designers are aware of.
None of them provide even 50pS peak-Peak jitter 10Hz-20KHz.
The GMT System, which uses a programmable clock that was not optimised for SONET, but for an application that needs low audio band phase noise levels., when measured it "Femto Clock" style, shows around 200 Femto Seconds RMS phase noise.
Most any clock in audio (1 rung above lo-fi) use if measured "Femto Clock Style" is a Femto Clock .
Finally, jitter is NOT the end-all be-all of good sound in digital.