It's frequently communicated that when working with signals in the Ghz range, routing is extremely important to signal integrity. Pretty much any inclusion of unnecessary stubs is out of the question, traces must be length matched to some fraction of the wavelength, vias backdrilled, etc.
That being said, I've been tasked with an unconventional goal of creating a high speed (1.5Ghz) signal interface along a large circular ring (e.g. a slip ring) of a maximum 3 inches in circumference. The signal will be introduced at one random point in the ring and picked up at a fixed location at another place in the ring.
Specifically, I'm not quite too sure what the implications are / if it is possible to operate at that level of speed given the self induced jitter from the phase shift. Assuming the following, are there any opinions out there on if this is possible or not? If so, what additional mitigation measures can be taken in this particular scenario?
- Annular ring impedance matching to termination
- Gold on gold contact to introduce signal
- EQ/DI/OS SC IC (ReDriver) immediately after for insertion loss
- Clocked Retimer eventually after to hopefully save the signal
Best Answer
I've done a similar system that may help you understand some of the problems. The payload data rate was 53.46 Mbps and the "ring" was about 600 mm in diameter: -
The important thing about the ring is that it was "balanced" and terminated. The receiver made no contact but sat a couple of mm from the ring. The ring rotated at several thousand revs per minute. Having a balanced ring means low EMI to other systems (also important).
The really clever thing about the design is that the rotating ring's coils were wound in such a way that the static receiver coil never saw a signal inversion or a cancellation point.
If you have serious alignment problems this will not work because it is intended to only generate near-field transmissions and the magnetic loop receiver could not be misaligned more than +/- 2 mm axially and radially. This was because the rotating transmit coils were spaced 6 mm apart. A bigger spacing gives more axial leniency but it then starts to be an emitter.
With a 3" diameter you would want to try and scale things down as frequency rises. Anyway, just a few thoughts.
So it boils down to what basic data rate you are using and how good you are with electronics in the GHz range. My design was limited to 54 Mbps because we used conventional fast CMOS and no modulation scheme.