Rubidium Clock

The SRS FS725 is the older, simpler version of the Stanford Research Systems rubidium clock.
The PERF10 has a nicer box and a power switch, but the same internal clock as the SRS FS725

If you get the SRS rubidium clock, you still need to get an Antelope Isochrone OCX master clock which outputs 44.1K. The Rubidium clock acts as an external clock to the Antelope
http://www.antelopeaudio.com/en/products/OCX-master-clock

YMMV but together I have found that these 2 clocks can make a big difference at considerably lower cost than the Esoteric rubidium clock which I have not heard. And power cord and digital IC choices can also make a meaningful difference raising the overall cast.

Thanks for the great info. I will need to try one of those.

I've done a lot more research in this regard and have had a number of conversations with SRS as well as other options. The lead-time on the PERF10 or FS725 is longer than I would like to wait personally and my RFQs out to LibertyTest, AllTest and other certified/used equipment vendors for a PERF10 or FS725 have come up dry at the present time. I stumbled on Cybershaft Japan as an option via listings on various sites. It seems they make OCXO, Rudidium-only and OCXO+Rb clock units (single port each standard) for high-end audio, video and studio purposes at an extremely aggressive price point (the Rb-only unit is about $1300-$1500 USD depending upon JPY->USD conversion rate). They are rated for 100-120V and 50-60Hz use which makes them perfect for US or Japanese usage. The Rb-only unit has a single FIFTY (50)-ohm BNC clock output; they supply all their test and calibration results with each unit plus a 1 year unconditional warranty.  I have taken the plunge and ordered one as of yesterday; this should be a very nice and affordable test.  I plan to use this 10Mhz reference clock sync to drive my Esoteric D-02's clocking baseline while leaving the D-02 to sync itself and the P-02 transport together at 22.5792 Mhz.  This should surpass the already great performance of the D-02 clock circuit by a clear margin.  Unlike the PERF10 or particular FS725, this Cybershaft unit seems to have a better internal PSU but I'm not discounting the potential move to another HDPlex power supply with a retrofit in the future to get an even better bespoke PSU to drive the Rb-clock.  I will report back and let you know my findings between mid-late April after I receive it and break it in.  They state specific compatibility with Esoteric D-02, D-03, D01, G-01, G-02, G-03x, G-02s, K-01, K-01X, K-03 and K-03X.  You may read more here (use Google Chrome and let it translate the page);

www dot cybershaft dot jp

There are many relatively inexpensive scientific instrument versions with 50 ohm outputs. I could find none with a 75 ohm output, as needed for optimal impedance match with 75 ohm digital audio input as used on the Esoterics. Through a web search I found posts and screen shots by an engineer who measured a degraded 10Mhz waveform caused by reflections between mismatched 50 & 75 ohm BNC impedances. That was enough for me to pony up the added $$$ for the SRS.

The 10Mhz inputs on the Esoteric gear is spec'ed at 50-ohm, not 75-ohm. The "WORD CLOCK" In and Out BNC connections on Esoteric gear is spec'ed at 75-ohm.  Put another way, if you use an atomic/other external clock standard to supply a 10Mhz pulse, Eso gear expects that at 50-ohm whereas for Word clock purposes (44.1, 88.2, 176.4, 22.5792Mhz), they expect 75-ohm...