High-Speed Data & Telecommunication Modules

A Digital-to-Analog Converter (DAC) is a device that converts digital data to an analog signal. It takes the binary input bits, weights and adds these together. Electrically SHF DACs do much more because of the on-chip 3R regeneration (re-timing, re-shaping & re-amplification) but logically this is the only operation a DAC does. To have it less abstract, let’s assume a 3 Bit DAC in symmetric configuration (i.e. with equal amplitudes steps between the output levels) and for the sake of getting an overview, let’s normalize the voltage contributions. The output amplitude can be realized by adding the individual levels (basically it is just like counting binary). The table below shows a typical 3-Bit DAC scheme: This scheme holds true for the 3-Bit DAC as above as well as for the 6-Bit DAC. However, for a 6-Bit DAC one has 2⁶=64 instead of the 2³=8 output levels shown above. There is no other difference between a 6 and a 3-Bit DAC. The more bits are used, the more levels the output has (with means greater resolution).

It is possible to generate a PAM-4 signal by using a broadband passive combiner. However, this has many drawbacks compared to an active DAC module. Just a few to be mentioned:   Output Power A DAC usually provides more output signal swing than a passive combiner since the least significant bit into the passive combiner must be attenuated accordingly (usually 6 dB).   Reflection A passive combiner is transparent in all directions. This means, the signal applied to one input will be present not only at the output but also at the other input. This always has an influence on the output signal quality because multiple reflections impact the overall output signal quality accordingly (due to the imperfect output reflection coefficient (s₂₂) of the driving source). Usually this influence is minimized by inserting attenuators at both pattern generator outputs. This however, further reduces the output voltage. By using a DAC this problem can be disregarded. In contrary, even if you have a bad signal from your BPG the DAC will always provide a perfectly shaped output as long as it is good enough for sampling by the input latch of the DAC   Skew The two signals into the passive combiner must be extremely well aligned as even a small skew between the input signals will distort the output signal. In case the used pattern generator does not have an internal fine skew control, an additional external delay line has to be added.  This makes the passive combiner approach rather inconvenient. Compared to the passive combining the DAC is extremely robust against skew between the driving signals.   Placement at the DUT A passive combiner must be placed very close to the pattern generator outputs. This means, even if the combiner provides a reasonable good PAM-4 signal it must probably be transmitted through a long cable to the DUT. This will distort the high speed multi-level signal. The DAC can be placed very close to the DUT. The extension cables can be inserted between the pattern generator’s outputs and DAC. The signal impairments introduced by these cables will not influence the DAC’s outputs as the input DFFs retime and reshape the signal on-chip before the voltage contributions are combined. After all, the DAC is much more robust and can be physically moved on the laboratory bench without any influence on the signal quality.   Additional Levels SHF DACs have at least three input bits. The SHF 614 A even has six. With this additional bits one can do much more than just ordinary PAM-4. Examples are unequal amplitude PAM, PAM-5/8/16/../64, PAM with pre-emphasis. With a passive combiner all this is impossible.