# Clock and Data Recovery/Conclusion
## Many things have changed..

Many things have evolved in the domain of CDRs during the recent decades, and –as a consequence- so has the work of the CDR engineers.
* Unit cost. The CDR is often just a small section of a complex but tiny silicon chip. More than one CDR can be found inside the chip, and it may even happen that a series of almost identical ones, differing slightly in some critical circuit elements, are made inside the same chip. Just one of those CDRs will actually be used. The characteristics of certain critical circuit elements can not be chosen exactly in the first place because of their manufacturing variability. The one in the series that will come out of production better balanced is the one that is chosen. The other are left unused with little concern for their cost that is practically negligible.
* Quantity manufactured of each design. On one side the unit cost declines with the improvement of the manufacturing technology, on the other each design happens to be manufactured in larger and larger quantities, following the trend of modern electronics.
* Circuit complexity. Each circuit element can today be implemented using blocks of really complex circuitry, and there are more and more cases when the (slow speed) functions are made by dedicated software rather than by hardware.
* Frequency. From MHz to GHz. The widespread use of media with THz of useful bandwidth, like the optical fiber, opens every day possibilities of higher speed applications. In parallel, the accuracy of the reference frequency has evolved from hundreds of ppm to ten ppm or less, and consumer electronics with frequency accuracy of the reference oscillator below 1 ppm are becoming common use.
* Application specific CDR. General purpose CDRs have disappeared. In new systems and equipment, the CDR is defined for a very precise application. If the application is subject to change with the operating conditions of the equipment, more CDRs are present and the one best fitting the requirement is activated at any moment.
* CAD tool versatility. For high level description, synthesis, simulations as well as for design for testability and for automatic generation of test software of the resulting circuit.
* Test equipment versatility. The circuit can be stimulated, checked, characterized, troubleshooted to an extent that was unconceivable not many years ago.

## Engineers can become lazy or neglect theory

The CDR engineer has become more and more dependent on predefined solutions of parts of his circuits, on software for simulation and circuit synthesis, on characterization and test equipment. The engineer becomes specialized for just one task in the complex organization that deploys so many CDRs in the world.
The negative side of all this is the possibility to lose the ability to understand the circuit behavior. The need for an engineer to understand the CDR with its own intelligence may be neglected, and just hardware and software tools may be relied upon. Such extremes may be rare, but it is not infrequent to find good technical literature on CDRs that could be improved if the author had a better knowledge and more command of the fundamental mathematical models.

## Just three models are needed for a solid background

       “All models are wrong; some models are useful” (George E. P. Box)
       In the case of CDRs, it might be reworded as: “All CDR models are wrong; there are three models that are useful”.
Many problems can be solved better, or quicker –if not avoided altogether-, if the engineer is familiar with, and uses three simple mathematical models, because the fundamental behavior of every CDR can be referred to one of them.
Three structures (1-1, 2-1, 2-2) out of two architectures (1st order and 2nd order) are the ones used in practice.
They are the only really important models to use (even when some blocks in the actual implementation are non linear).
In fact, a good part of the actual CDR PLLs incorporate one or two non-linear blocks: in those cases the linear model is insufficient. Still, the structure is a good reference, and the linear model -even if incomplete- a very useful tool.
Even if the actual CDR structure incorporates non-linear backs, the 3 fundamental structures are referred to with the two numbers that -in the fully linear case- identify there order and their type.
Obviously, in these cases the engineer must equip himself also with computer simulation tools.
          1 . 1 This structure is found in actual implementations either with linear blocks, or with non-linear phase comparator (and maybe VCO);
          2 . 1 This structure is essentially used with all linear blocks;
          2 . 2 This structure is found in actual implementations always with non-linear phase comparator (and maybe VCO) ( 2 - 2 );
As a result, the three fundamental structures find their best fit as follows:
* 1 . 1 linear or non-linear blocks; preferred when E$_s$ is zero by definition (= for phase aligners). Preferred when the acquisition time shall be minimum (burst-mode transmission). Furthermore, a good choice in many cases where no specific requirements suggest a 2nd order loop.
** 2 . 1 : preferred for regenerator applications where linear blocks are used.
** 2 . 2 : preferred with non-linear (= widely variable gain) blocks in continuous mode applications, especially high performance monolithic implementations.
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