Acam, now ScioSense, has also the 4 channel GPX2 that achieves up to 35Msps over LVDS. It has the same time resolution as the AS6501.
Michael Nowak (http://www.mino-elektronik.de/) build nice simple little counters with the TDC7200 and the AS5601 (the later is not on his website, I think) that do gapless measurements and have the option to use internally linear regression (the AS6501 version with more than 100.000 sps). He kindly provided me with several revisions of prototypes of both counters for testing. In loopback measurements the improvements of linear regression are as expected. The problem is that the performance of linear regression can break down when DUT and reference have the same (stable) frequency with a small unfavorable offset. Unfortunately this is easily the standard situation if you test two 10MHz oscillators against each other. Things get worse the shorter the measurement time is. I think the three cornered hat approach will not necessarily eliminate that problem, it is more for overcomming the limits of the reference oscillator. Achieving a good part of the higher performance is possible if you adjust the frequencies of DUT and reference carefully and if they are as stable as supposed. But that is a messy process where I am always in doubt if the displayed result is the performance of the DUT/reference or a problem with the counter, unless I do several cross checks. Best would be the possibility to use reference with a frequency skew to the DUT. I attached a quick example with Michael Nowaks AS6501 counter: There are three loopbacks one without linear regression, and two with linear regression (with 1s and 10s measurement time). Then I compare a 10MHz OXCO, with good short time stability, to a well performing FE-5680 Without linear regression the Allan deviation below 100s is due to the AS6501 resolution and not due to the oscillators. Next measurements with linear regression. If I match the frequencies to about 1mHz the limits come mostely from the FE-5680. If I adjust to 0.5Hz offset the linear regression breaks down to the raw AS6501 performance and worse. To demonstrate that that OCXO performs well with 0.5Hz offset, I use it as reference to measure a fine 16.384MHz OCXO. What you see in the plot is, more or less, the performance of the 10MHz-OCXO. There is also a plot of the 16.384MHz OCXO against the FE-5680, with and without linear regression. Combining both you see that linear regression gives improvements - you get at least about 10^-12 with 1s measurement time. I have no better 10MHz oscillator to explore the limits with 1s measurement time. But if you decrease the measurement time you fast get in messy measurements that give you headache with attributing the culprit.
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