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Old 30th May 2012, 06:50 PM   #31
RJM1 is offline RJM1  United States
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No, It's not the diameters of the cables but the ratio of the inside diameter to the outside diameter and the dielectric constant of the insulator. This is the problem, if you keep the physical ratio of the inside and outside conductors the same you must change the dielectric constant of the insulator to match the impedance to 75 ohms. What they did to compensate for the change needed in the dielectric constant was to add an air gap which reduced the high frequency response. 75 ohm tnc. Amazon.com: 75 Ohm TNC Female Crimp for RG59 Cable: Electronics

Last edited by RJM1; 30th May 2012 at 07:04 PM.
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Old 30th May 2012, 07:19 PM   #32
DF96 is offline DF96  England
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I'm not saying it is the cables, I was just using that as an example of geometric change which apparently preserves impedance yet still creates reflections. I think it is unlikely that an airgap will reduce HF response as air is a fairly good dielectric, unless it is just the discontinuity which is the problem.
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Old 30th May 2012, 08:12 PM   #33
Pano is offline Pano  United States
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It's the standard way to tell 75 ohm from 50 ohm BNC. Look at the center pin. 50 has insulation around the pin, 75 does not.
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Old 30th May 2012, 08:44 PM   #34
AndrewT is offline AndrewT  Scotland
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Let's take a 50ohm M & F connector and a pair of 50ohm cable ends designed to be compatible with the connector.
They will have a bandwidth that works within a tolerance.
Now replace all those 50ohm components with 75ohm components, again with compatibility between connector and cable.

Why does the 75ohm connection version lose 10GHz of bandwidth?
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Old 30th May 2012, 08:56 PM   #35
RJM1 is offline RJM1  United States
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The thing is they didn't change the size to make it compatible the just fudged it.
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Old 30th May 2012, 09:08 PM   #36
AndrewT is offline AndrewT  Scotland
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Changing the dielectric changes the impedance. That was explained a little earlier.
Introducing a thinner dielectric and using air for the remainder also changes the impedance.
That introduction of the air to replace the missing dielectric does not help explain the missing 10GHz.
There's something else you are not telling me.
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Old 31st May 2012, 01:42 PM   #37
macboy is offline macboy  Canada
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Quote:
Originally Posted by AndrewT View Post
Let's take a 50ohm M & F connector and a pair of 50ohm cable ends designed to be compatible with the connector.
They will have a bandwidth that works within a tolerance.
Now replace all those 50ohm components with 75ohm components, again with compatibility between connector and cable.

Why does the 75ohm connection version lose 10GHz of bandwidth?
Go back and read carefully. RMJ1 explained that all TNC connectors are 50 ohm, even when used with 75 ohm cable in a 75 ohm system. The impedance mismatch causes issues (poor SWR) above about 1 GHz. Below that point, the mismatch occurs over a small enough length of the transmission line that there is little effect. At higher frequencies, the physical length of the impedance-mismatched portion (the connectors) is bigger relative to the wavelength of the signal in the transmission line, so it has more impact.

Getting back to BNC, these are commonly available in 75 ohm, but even then they perform worse than the 50 ohm versions. This is due to an air gap in the dielectric which is required to provide the 75 ohm impedance from a connector whose dimensions were originally chosen to provide 50 ohm impedance. BNC is also generally not used above a few GHz anyway since the slots can start to radiate signal. The TNC is mostly identical but is threaded so it does not have this problem. Regardless, this is SPDIF and the driver circuits simply do not have the slew rate to provide GHz range signal components.
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Old 31st May 2012, 04:40 PM   #38
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So gentlemen, what sort of "bullet" will provide the least signal degradation at spdif frequencies and 75ohm output and input? Could you specify type of connecter, type of insulation, and metal content. Thanks
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Old 31st May 2012, 04:47 PM   #39
SY is offline SY  United States
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Any 75 ohm BNC which isn't defective will not degrade an spdif signal (the GHz discussions were academic, not practical- these are hundreds to thousands of times higher than spdif frequencies). If you stick to a name brand (e.g., Amphenol) purchased from a reputable distributor (e.g., Mouser), the signal transmission will far and away not be limited by the connector.
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Old 31st May 2012, 07:51 PM   #40
macboy is offline macboy  Canada
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I agree with SY, BNC is an excellent interconnect and should work very well for this purpose. Make sure to stick with 75 ohm, and don't bother too much with teflon or such. The 75 ohm ones can be identified visually, so look into that (try here). Don't be too surprised if the BNC input or output jacks on your gear are not 75 ohm!

I also agree with sticking to name brand stuff, I bought some cheap BNC T's off eBay and they are garbage.

Last edited by macboy; 31st May 2012 at 07:55 PM. Reason: URL
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