Slew rate control in high speed LVDS signals

[Marco]

You could also build a hacky thermometer DAC using FPGA output delay lines. Vary the delay for say 16 outputs from 0 to X seconds, then feed the outputs to UHS logic to square them up if necessary, then sum them with a 8+4+2 chain of resistive combiners (is all 50 Ohm matched). Then you have a 4 bit DAC which can only output square waves of varying slew.

You'd need to low pass to clean up the staircasing, and a single low pass wouldn't work across the entire range.

[EEEnthusiast]

I was thinking of a simple RC filter (differential) , with the R being replaced by a NTC thermistor. Using 0201 or 0402 components. Just heat or cool the NTC on the  filter board to vary the F3dB of the filter, and hence the slew rate. But this RC circuit will alter the source impedance as seen by the transmission line. I may not be able to keep it at 100 ohms differential at all times.
Seems like a crazy idea, which will not work in this case.

[EEEnthusiast]

Note that a chip component doesn't have lumped ESL and EPC and whatever, it has transmission line characteristics.

Down in the 50ps regime, even this distinction matters.

Corollary, everything you do must necessarily be done in a transmission line, with respect to its impedance, and accounting for any stub lengths, unterminated reflections, etc.

This is why I suggested a filtering approach over a conventional slew limiting approach.  Yes, as noted elsewhere, it will have a limited range, so needs to be done in multiple sections to cover the requested span.  That's not too terrible, I think; good RF switches exist.  Granted, I'm not sure the stub lengths in the filter itself, or in the switches, can even be low enough to get a good result.

Generating the initial edge rate, and verifying operation with a suitable probe and scope, are separate matters entirely.

It's just not a good problem, and all of this is damn good reason to question how/why it's a problem in the first place.

Tim

The problem is that the signal is differential. Tunable RF filters do exist, but most of them are single ended. Getting a differential tuned RF filter may be tricky task.

[David Hess]

It's just not a good problem, and all of this is damn good reason to question how/why it's a problem in the first place.

*Looks at the nanosecond slew rate control mountain*  Because it is there.

[T3sl4co1l]

It's just not a good problem, and all of this is damn good reason to question how/why it's a problem in the first place.

*Looks at the nanosecond slew rate control mountain*  Because it is there.

[Marco]

The problem is that the signal is differential.

You can make the signal single ended somewhere near the right levels and then use a fast FDA to make it differential.