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#7
November 28th 05, 06:47 PM
posted to rec.radio.shortwave
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"Can twisted wire replace shielded wire?"
David wrote: On 28 Nov 2005 08:59:00 -0800, wrote: David wrote: Up until about 30 years ago, almost every radio station on the air was feeding audio to their transmitters via unshielded wire phone lines. No buzz. Optoisolators as suicide devices. +++++++++++++++++++++++++++++++++++++ Now I get it. I am trying to recieve signals near or at the noise floor which is set first by the local QRN. there are as amny different levels as refference books, so I will use John Doty's figures which are wildly optimistic in my experience. I will post links to my refferences so others may follow them. From:http://anarc.org/naswa/badx/antennas/grounding.html "According to "Reference Data for Radio Engineers" (Sams, 1975), the wintertime level of natural noise in my area at 10 MHz should be about 32 dB above the thermal reference level: this would produce a noise floor of -104 dBm in this bandwidth with a perfectly efficient antenna. " A very handy chart that converts dBm to Volts can be found at: http://www.minicircuits.com/dg03-110.pdf Sherwood Engineeringhttp://www.sherweng.com/table.html gives the R2000 noise floor as -130dB in the SSB Mode. So I am trying to capture signals in the range of -130dB=0.058uV to -119dB= 0.251uV. Your refference to audio being delivered over balanced line is at 0dBm (nominal) for a voltage of 0.775V. (*600Ohm) or 0.225 at 50 Ohms. There is more then a slight voltage level diffference. Can we agree that: Adding 1uV of noise to a 1uV(-107dBm) signal will significantly degrade the SN ratio. Adding 1uV of noise to a 0dBm 50 or 600 ohm line will result in minimal, as in I would like to see it measured, degradation of the overall SN ratio. And I bet you won't try to run the output of a moving coil phono cartridge over ~70' of balanced audio line. In a uV world I will stick to single ended, unbalanced, coaxial/shielded cable interconnects. If you are lucky enough to deal with signals in the 0.1V range then twisted or parallel cable will work just fine. Terry |
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