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what would I see on a scope?
Norm X wrote:
Hi, 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. 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" http://www.usb.org/developers/usbfaq#cab1 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. http://www.eetimes.com/document.asp?doc_id=1275156 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. https://upload.wikimedia.org/wikiped...d_pair.svg.png ******* 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 :-) Paul |
#2
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what would I see on a scope?
Hi,
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. --- news://freenews.netfront.net/ - complaints: --- |
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