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The clocks in several modern DACs are outstanding for the task. Many external clocks targeted at audiophiles that claim incredible accuracy or ultra-low phase noise [jitter] are mostly snake oil in terms of improving SQ. Rubidium atomic clocks are accurate to a few parts in 10E11 and some audio companies will gladly sell you these "atomic clocks" for several thousand dollars. Antelope Audio's Rubidium based clock is only $6500. dCS will sell you their Vivaldi Master Clock based on an Oven Controlled Crystal Oscillator (OCXO) for only $17,000. Their Rossini Master Clock is only $9000. OCXOs are generally a tier below Rubidium based atomic clocks and OCXOs are MUCH cheaper. So dCS will gladly sell you LESS for much MORE money! Truth be told, you can buy accurate and extremely low phase noise Rubidium clocks and OCXOs "off the shelf" without pretty audiophile cases at huge savings but they don't come with audiophile marketing hype. In fact, none of the audio companies design their own OCXOs or Rubidium atomic clocks. They buy them "off the shelf" just like you can. Audio companies simply do not have the in house expertise. These are highly specialized components. Excellent OCXOs about $300 - $500
 Excellent Rubidium based clock about $1600
 A fancier GPS disciplined clock. $3850 w/OCXO. $4800 w/Rubidium option.
 Stanford Research Systems just happens to be a brand that I am familiar with. There are several others that make OCXOs and a few that make Rubidium based clocks. Efratom of Germany pioneered the commercialization of Rubidium oscillators (clocks), many of which are in satellites orbiting the earth today. I did some work for them in Irvine, CA when they were developing a compact hydrogen MASER. Efratom was later sold to Ball Aerospace.
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