=suggestion =design =explanation =electrical engineering =radio
Coaxial cable is the best way to
send a single electrical signal with minimal losses and noise relative to
the cable size.
Attenuation is generally proportional to sqrt(freq) *
distance / diameter. Potential data rate is proportional to frequency, so
for a given distance, the data capacity of a coaxial cable is proportional
to its area. Adding more wires to a cable does not inherently improve its
bandwidth. In fact, a coax cable (with MoCA 3.0) has a better data rate per
cable area and cable cost than ethernet cable IF you need to move data 100m.
The problem is that for shorter cables, the desired
frequency is too high for power amplifiers. 5 GHz is about the limit for
affordable hardware, and that used to be lower.
And for longer
cables, fiber has lower losses than coaxial cable; the only problem is that
it's more expensive for shorter cables.
Waveguides have lower losses
than coaxial cable, but their minimum diameter is too large at the
frequencies low-cost hardware can handle. If our frequency was unlimited, we
could use a 50+ GHz signal in a waveguide the size of Cat5 cable.
How
about using a bundle of smaller coaxial cables? That approach was used
extensively in the past, but it's expensive and fiber is generally better.
Still, that is done today for a few applications: as whitequark noted,
Thunderbolt 3 uses micro-coaxial cables.
You might ask: if single-mode
optical fiber can transmit 100 GB/s, then doesn't that imply people can
generate 100 GHz signals to feed laser diodes, which is a high enough
frequency to use waveguides?
Yes, that's true, but that's part of why
fiber is more expensive for short connections; if you're going to use that
kind of electronics you sort of might as well use fiber. Also, those use
interleaved
DACs
with low efficiency, which is compensated for by the low losses in optical
fiber. It's not uncommon for data cables to have 50dB attenuation, meaning that
only 0.001% of the transmitted signal is received, and WiFi often has 60+dB
attenuation.
But we can match the attenuation of typical fiber links with a
50+m waveguide.
Now, I said "5 GHz is about the limit for affordable hardware" and that's conventional wisdom that's guided designs, but if we're willing to accept lower output powers (<30 mW) we can use SiGe amplifiers at >50 GHz. [1][2] Net energy usage of 6 pJ/bit should be feasible with current technology, which is less than PCIe typically uses.
How big would waveguides for this
need to be, and how long could they be? That depends on the mode used.
To be practical, cables need to be flexible. So, the TE01 mode (with a
circular waveguide) is one option: it has low losses and makes constructing
flexible waveguides much easier because no axial conductivity is needed. A
TE01 waveguide can be a stack of conductive rings, which can bend relatively
well. But it has a higher minimum waveguide diameter than some other modes.
What if we used corrugated waveguide? Does that work? Actually, circular
corrugated waveguide with the right corrugation spacing and depth is better
than smooth waveguide. The corrugations make the HE11 mode possible, which
usually has even lower losses than TE01.
The HE11 mode can't be made
directly, but that's not really a problem. For a 175 GHz signal, the system
would be something like:
PCB antenna <-> TE11 mode in 2.1mm ID smooth
circular waveguide <-> mode converter <-> HE11 mode in 3mm ID (including
corrugations) corrugated waveguide
Copper, aluminum, and
silver-plated steel are all fine for this type of waveguide. Mode converters
between TE11 and HE11 are relatively simple and have ~99% efficiency.
A 3mm corrugated waveguide should be good enough for 100m links, and
larger ones could support longer distances; a 30mm waveguide should support
6km links. Of course, dispersion is also an issue as lengths increase, but
dispersion is related to losses because both are caused by conduction in the
metal walls.
While dielectric filled waveguides generally have much higher losses, using them for a short distance to connect to the transceiver is better than using a longer wire to the coupler. By filling the waveguide with glass, sintered alumina, loose alumina powder, or alumina filled teflon, it should be possible to reduce the inner diameter to 1mm. That's small enough to place the waveguide ends directly against the top of the Si interposer.
Does this type of system have any
advantages over optical fiber? Potentially, yes.
Fiber couplers are
relatively lossy and expensive, but waveguide cable sections could be easily
connected together with minimal losses.
Because this would require a
relatively small amount of SiGe area, it should be cheaper than fast fiber
transceivers. This might need 1mm^2 of 55nm SiGe per waveguide, if the
transmission line components were on a 130nm Si interposer. The
manufacturing costs for such a transceiver might be something like $3 for a
bidirectional 40 GB/s link.