loose-electron
Senior Fellow
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Best Design Tool = Capable Designers
Posts: 1638
San Diego California
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I have been involved with DFE (used to be known as QFB or quantized feedback) several times. Once as part of Maxtor, in their advanced channel development group (circa 1989-1992) then as a read channel IC designer doing designs for Seagate and Conner (circa 1994) and then as part of Quantum (around 1995)
I have also used DFE in SerDes design for Cray, back in 2004. (My dates may be off a touch.)
The limited use of DFE in disk drives was an exercise in duct tape being placed on top of duct tape to make the concept work.
The issues of ISI were compensated by using pattern dependent feedback, making for a rather messy system architecture, and multiple signal processing paths. Not easily done and not reliable.
A few disk drives went out with DFE systems in them, but the compensation to deal with amplitude variance head to head, media to meda, and ID to OD on the drive was again, a messy case in tuning the system to work. Huge adaptive data got developed to tune the devices (a unique lookup table for each readback head/media, with a cylinder dependency set of factors was needed, to adjust the feedback, plus a calibration routine to get things set up was needed)
Magnetic tape drives have used DFE with success, because the amplitude changes are not as nasty as with a disk, and compensation for a track can be made readily.
SerDes has used DFE successfully because the channel characteristics remain consistent, and can be compensated for. Pulse to pulse variance is minimal, and the overall compensation feedback can be adjusted for.
What happens is that people fall in love with the simple concept of DFE, and then to make it work reliably, it becomes a very complex system, with lots of adjustments needed.
A few years back a VC talked to me about funding a DFE startup, my advice at the time, was not to do it. About 2 years and $3 (5?) million dollars later, they had something that would work under very limited and controlled situations, and never sold a single one.
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