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Re: [Xylo-SDR] [Flexradio] Proof-of-concept successful!
Chapter 7 of the USB 2 spec has this information in great
detail. Twited pair cable impedence is nominal 90 ohms. Given the popularity of
USB, surely here must be driver chips id LVDS can't do it
directly.
Chris - AE6VK
LVDS is made for a tightly twisted pair with 100 Ohm impedance, a
pair in a CAT5e cable is exactly what is meant to be used with. Does anyone know
what the characteristic impedance of a USB cable is? If not, then it's best to
stay away from anything unknown.
It's a transmission line from the
transmitter to the receiver and should be treated as such including impedance
matching so you do not get reflections, shortcuts will lead to phase
noise.
As far as a connector, a CAT5e RJ45 connector is meant for that
impedance, and does an excellent job.
At 11:30 AM 1/31/2006, Christopher
T. Day wrote:
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Could you use standard USB cables with a type A connector on the end
where you have room and a type mini-B on the end where you don't? USB is a
single differential twisted pair with additional gnd and Vbuss wires rated to
420 Mbits/second. Be sure it's protected if someone accidentally plugs in a
real USB at either end.
Chris -
AE6VK
From: Phil Harman [
mailto:pvharman@arach.net.au]
Sent: Sunday, January 29, 2006
12:31 AM
To: FlexRadio@flex-radio.biz; Xylo-SDR
Discussion
Subject: [Xylo-SDR] Proof-of-concept
successful!
This weekend
Bill, KD5TFD, completed his integration of our Wolfson A/D + Xylo FPGA +
USB interface into PowerSDR.
This proof-of-concepth has been completely
successful and we are eagerly pushing forward to the next phase of the
project.
The screen
shot below is of our 48k 16 bit full duplex sound card running in
PowerSDR using an SDR1000 on 20m
http://www.hamsdr.com/dnld.aspx?id=126
The noise floor of the Wolfson A/D in this
configuration is -140dBm in a 500z bandwidth. The next steps are to
increase the sampling rate to 192kHz and 24 bits.
Bill is also going to implement his system to
overcome the frequency error and drift of the 200MHz Valpey Fisher oscillator.
He is using a GPS 1Hz or 10kHz clock to measure the frequency of the DDS
output and pass this back over the same USB signal that carries the digital
audio. A software routine in PowerSDR will then compare the actual DDS
frequency to the desired frequency and make the necessary correction in
software. If this works OK then he will look at counting the 200MHz
oscillator directly, although picking this off the SDR1000 hardware is bit
more involved than the DDS output hence the reason for starting with the DDS
output.
We are
looking at deriving an LVDS signal from the 200MHz oscillator and if anyone
has any suggestions as to suitable connectors to use with a single LVDS signal
then we would appreciate the information.
There is still plenty of room for more C++ and
FPGA developers if you are interested in assisting with the project - those
willing to lay out PCB's will be especially welcome!
73's Phil.... Vk6APH
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FlexRadio
Homepage: http://www.flex-radio.com
Cecil Bayona
KD5NWA
www.qrpradio.com
I fail to see why doing the same
thing over and over and getting the same results every time is insanity: I've
almost proved it isn't; only a few more tests now and I'm sure results will
differ this time ...