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The Design of a GPSDO - The Beginning

There is currently a discussion on the time-nuts mailinglis about designing a GPS Disciplined Oscillator (GPSDO). It is by far not the first discussion of this kind and most likely will not be the last. What makes this one stick out, though, is that that Tobias Pl├╝ss has put quite a bit of thought effort into the design and it shows. While a GPSDO is not the most difficult electronics a hobbyist or time-nut can build, it is one of those designs where the details matter a lot. A small short-cut taken here, a non-optimal component choice there and a lot of performance is suddenly gone. And it is not easy to see where the critical points are. As quite often when it comes to precision metrology, each and every step has to be carefully evaluated and judged on its merits and demerits.

Simulating Metastability in Digital Circuits Using Spice

Most people probably regard metastability as a solved issue, or rather an issue that barely surfaces. Common wisdom tells us that if you fear that metastability is an issue, then all you need to do is add a synchronizer or wait long enough until it goes away. But when is a synchronizer necessary? And how long is long enough? Measuring metastability is difficult at best in real circuits, more often than not it is impossible to measure directly. Simulation is an alternative, but it is not without problems.

High-speed FPGA to CPU Interface

From time to time one has the need to transfer lots of data between an FPGA and a CPU. Unfortunately, there are not that many ways to do that, if you want to get more than a couple MByte/s.

GPS Disciplined Oscillator

From time to time I get asked, what to use as accurate time source (better than a few microseconds) or frequency reference (better than one ppm). One affordable way to get there is to use a GPS disciplined oscillator (GPSDO). This post elaborates on some of the choices you have when shopping for a GPSDO and the issues you can expect.

FPGA based Time to Digital Converters

There are many ways to measure time of an event precisely. Probably the most common way is to use a counter, as for example used in the timer units of micro-controllers. Unfortunately, this approach is limited in precision to the clock at which the counter runs. A micro-controller gets usually to 10-20ns, while high performance systems might get down to 1ns. If higher precision is needed, then a method that runs independent of the system clock is required.

Harmonics of Ring Oscillators

Ring oscillators are probably the most prevalent oscillators these days. They are the oscillator of choice for most micro-controllers and are used more and more often in RF systems as well, due to their low space requirements and easy integration. Having such a wide use, you would think, they are pretty well described in literature, but quite the contrary is true.

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