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Grasping Bessel-Thomson scope response

Posted: 13 Jan 2016     Print Version  Bookmark and Share

Keywords:analogue scopes  frequency response  oscilloscope  DSP  Gaussian 

That's why the key standards for optical transmitter testing typically specify that the measurement receiver should have a fourth-order Bessel-Thomson response with a corner frequency of 0.75x the bit rate. This allows for more meaningful and accurate measurement of the extinction ratio. Some oscilloscope manufacturers (e.g., Teledyne LeCroy) incorporate DSP-based group delay compensation to equalise pre-shoot and overshoot, maintaining the symmetry of the histogram so its all-important centre (technically the mean or median) does not drift.

The effect of reducing bandwidth and applying the correct filter using a reference receiver, as opposed to using a wide bandwidth oscilloscope, is brought out quite nicely in figure 1 of Tracking Transmitter Compliance Testing.

The usefulness of the Bessel-Thomson filter is not limited to eye diagrams. This filter's response actually closely approximates a Gaussian response, so it is also used to enable a scope to behave almost like it would have in the good old pre-DSP days. The almost constant group delay also has another beneficial effect—pulse waveforms are distorted symmetrically rather than asymmetrically as with other filter types (in addition to being less distorted due to no overshoot). This makes the Bessel-Thomson response good for many pulse measurement applications.

This is why some oscilloscope manufacturers in fact have this as their default response for pulse and eye diagram measurements. Note, however, that we said its response is "almost" linear phase. With the advent of DSP, we can tweak the traditional Bessel response for exact linear phase. This is what Tektronix does for its Bandwidth Enhancement filters, which then become general-purpose enough for Tek to recommend that you leave it on by default unless you need other response types (e.g., a flat response) for very specific measurements.

Have you been exposed to oscilloscopes with selectable response shapes? How did you react?

About the author
Ram Ramaswamy earned his M.Sc in Physics from Delhi University in 1990 and his PhD in Operations Research from the Indian Institute of Management Calcutta in 1994. He built his first audio amp at the age of 12 using the popular TBA810 which, much to his delight then, instantly became a local MW receiver too. While as a pragmatist he believes that digital technologies have replaced analogue largely for the better, he rues the fact that some of the best principles of analogue design, such as the principle of minimalism which it implicitly embodies, are slowly dying away.

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