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- what would I see on a scope?
June 27, 2016, 11:42 am
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Wikipedia and other sources say that for long length of USB cable, one
should use a USB repeater. A hub will do. The use of a powered hub may make
a difference. I have been experimenting with an Alfa USB WiFi adapter. It is
very hard to find convincing benchmark measures of improvement. The only one
I see is a comment for a driver program. With no powered hub it says "poor
signal". But with a powered hub it says, "good/excellent signal". I've look
at the schematics for a typical powered hub and in the simplest, there seems
to be a zener of transistor in line to amplify a signal.
Any comments to improve understanding would be helpful. Thanks in advance.
Re: what would I see on a scope?
I think it is telling you about the Wifi signal
quality, not the USB signal.
First off, I'm not an expert on USB. The specs
are 500+ pages a piece and there are multiple of
them. And I just can't keep up-to-date on stuff
like this. If I worked full time at it, I could
probably memorize the whole thing. But visiting
the spec once a year, it's not going to happen.
The USB forum isn't of much use when it comes
to understanding the technical details.
"Cables and Long-Haul Solutions"
They seem to imply the technical limitation
on cable length, is fixed by time-of-flight
and some timeout counter. So the reach is not
fixed by signal amplitude or group delay or
the barometric pressure. I'm sure that's not
the whole story. The explanation is too short
and facile to be trusted.
Look at the eye on this USB2 signal. It's wide open.
"You could drive a truck through there and
get 25 cents change." Have a look at a closed
eye some time, and marvel at how the data
still gets through the interface. Something
that looks this good, can't be for real. You
can see it passes the template with flying colors.
Note that the digital scope producing that diagram,
uses a software package that automatically adjusts
the vertical gain. It "normalizes" the signal automatically
for you. And as such, looking at those diagrams,
you really don't know the amplitude. But just
looking at how sharp and non-degraded those
signals are, that tells me it's a high amplitude
signal, and not down in the soup.
And the receiving chip can have plenty of gain on
its own. You can easily build diff receivers capable
of picking up a 50mV amplitude signal. The receiver
likely runs single ended for 12Mbit/sec USB, but
operates differential for USB2 protocol at
480Mbit/sec. And as long as the front end of the
chip is "true differential" and not "fake differential",
it will have pretty good gain on its own. With
"no zener". Fake differential, is where two single-ended
receivers are subtracted from one another. True differential,
there is a differential circuit on the front end. The
basic principle is shown in this example long-tail pair.
A constant current source in the lower leg, and push pull
operation in the upper two transistors. And you put that
inside the USB chip.
Sometimes, a USB product may be compromised by the
power quality available on the cable. I put three
USB2 repeaters in a row, and there was still enough
power (without excessive voltage drop) to run a
webcam with an autofocus motor. Impressed the hell out
of me. I was expecting to need to set up a power source
at the remote end. But the webcam... just worked.
As for how many repeaters you can run, that is fixed by
the hub count. Each (regular) 5 meter active repeater
has one hub inside. To digitally regenerate the USB
packets. You are allowed five hubs in a row. However,
when Intel built some of their more recent Southbridge
or PCH chips, they changed the USB implementation such that
the block inside the chip is a hub. Which subtracts one
from the max hub count. That means, on my new PC, I could
put four repeaters in a row. On an older computer, I
could put five repeaters in a row. So I stopped buying
repeaters, at three of them :-)