For some reason which baffles Europeans, North American practice seems to be to use RCA for all sorts of inappropriate purposes such as UHF tuner inputs and VHF transmitter outputs. This does not mean that RCA can be considered to be 75R, just that the mismatch was regarded as being acceptable - perhaps low cost was a consideration?anatech said:
Of course, over here we used the Belling-Lee connector almost everywhere and that might not be too good for UHF.
OK, another way to change the problem in order to make the solution easier. So to achieve 75R they added inductance to the shield and reduced shielding effectiveness i.e. lose full coaxial construction. That seems a poor compromise. It seems that in order to achieve something which can be sold as '75R RCA' (but why bother?) you have to lose something, and so create problems which may be worse than a little connector mismatch.jneutron said:
The connector used does represent a short section of transmission line so it will have a characteristic impedance just as the coax cable attached to it does. Here is what I figure for the precision
RCA plug system we all know and love.Given a fixed center pin diameter of 0.125" (3.18 mm), and outer diameter of .325" (8.26 mm) the metal thickness of the outer jack shell and center female insert needs to be 0.010" minimum even if made from beryllium copper, thinner metal would make the contact easily permanently deformed and the contact pressure too low. This results in a basic geometry for the RCA plug/jack system of 0.145" (3.68 mm) center conductor 0.305" (7.75 mm).
These dimensions would apply best case to the basic RCA plug/jack system, thicker metals would lower the impedance further. Here is a table of the characteristic impedance of this system with different dielectrics I have seen used over the years:
Air - 44.6 Ω
Teflon - 30.8 Ω
PP - 30.0 Ω
PE - 29.7 Ω
Paper - 25.7 Ω
PVC - 25.0 Ω
Nylon - 22.3
Porcelain - 19.0 Ω
Phenolic - <18 Ω
Despite claims by WBT and others like Pasternack that they carry 75 Ω RCA connectors, since they embed no impedance transformation network they cannot be more than ~45 Ω or so, or as JN suggested, a dielectric constant less than 1. Even if they had zero metal thickness and air dielectric they would be ~57 Ω.
Since the RCA plug/jack system represents a very short (1"/25mm) transmission line, issues caused by the mismatch between an RCA plug/jack and 75 Ω cable only apply to signals where this represents 1/8 wavelength or shorter, above 1.5 GHz or thereabouts. In my experience, since the resulting pulse distortion and impedance discontinuity presents is a stable, fixed effect, I have seen zero issues using a decent RCA cable to transmit SPDIF or other low frequency digital audio signals.
Cheers,
Howie
(sorry all, I had nothing better to do this AM, I'm hunkered down for a hurricane, awaiting that call from a station off the air...)
The connector used does represent a short section of transmission line so it will have a characteristic impedance just as the coax cable attached to it does. Here is what I figure for the precision
Given a fixed center pin diameter of 0.125" (3.18 mm), and outer diameter of .325" (8.26 mm) the metal thickness of the outer jack shell and center female insert needs to be 0.010" minimum even if made from beryllium copper, thinner metal would make the contact easily permanently deformed and the contact pressure too low. This results in a basic geometry for the RCA plug/jack system of 0.145" (3.68 mm) center conductor 0.305" (7.75 mm).
These dimensions would apply best case to the basic RCA plug/jack system, thicker metals would lower the impedance further. Here is a table of the characteristic impedance of this system with different dielectrics I have seen used over the years:
Air - 44.6 Ω
Teflon - 30.8 Ω
PP - 30.0 Ω
PE - 29.7 Ω
Paper - 25.7 Ω
PVC - 25.0 Ω
Nylon - 22.3
Porcelain - 19.0 Ω
Phenolic - <18 Ω
Despite claims by WBT and others like Pasternack that they carry 75 Ω RCA connectors, since they embed no impedance transformation network they cannot be more than ~45 Ω or so, or as JN suggested, a dielectric constant less than 1. Even if they had zero metal thickness and air dielectric they would be ~57 Ω.
Since the RCA plug/jack system represents a very short (1"/25mm) transmission line, issues caused by the mismatch between an RCA plug/jack and 75 Ω cable only apply to signals where this represents 1/8 wavelength or shorter, above 1.5 GHz or thereabouts. In my experience, since the resulting pulse distortion and impedance discontinuity presents is a stable, fixed effect, I have seen zero issues using a decent RCA cable to transmit SPDIF or other low frequency digital audio signals.
Cheers,
Howie
(sorry all, I had nothing better to do this AM, I'm hunkered down for a hurricane, awaiting that call from a station off the air...)
Thanks for the work, it's all good.
The nextgen rca PAIR is indeed 75 ohm. I emphasize pair, as if a nextgen male is plugged into a normal rca female, the z will be exactly as you calculate. The RCA female is what dominates the z. If the female shield metal is cut away into two blades, it can be made into a higher z, but requires a suitable male with shield "blades" as well.
But, the nextgens are definitely 75 ohm as a pair.
jn
Hi 20to20,
Since I don't work in consumer television or video, I only have whatever knowledge I picked up over the years. But again, that's why I have been very clear on the fact that I'm only reporting on things I have witnessed and heard from people in those industries. I am not confusing two impedance levels, nor am I confusing two connectors.
On my bench everything is on a 50R standard for RF except for FM tuner inputs which are on either "F" connectors or a European type connector I don't know the name for. But that's it. The only other RF connectors I have are "N", "BNC", "SMA" and "TNC", and I'm good to 500 MHz with some counters exceeding 1 GHz. RCA connectors are only used for audio unless I get a strange device that uses RCA connectors, then adapters are used to bring it onto a 50R system. I also have two FM generators that use 75R BNC connectors, which requires impedance adapters unless they plug directly into an FM tuner.
So that is my environment, and I am happy to say that I'm learning a great deal on this now that it is being discussed. Trusted information comes from the usual suspects. Some others are generating noise.
Shields up Scotty!
-Chris
I was only observing that UHF tuners did use an RCA connector that was soldered to the mating jack in most cases. It was referred to as a 75 ohm connection. These were stripped down versions without a gripping surface, only an area where the twisted shield was looped around and soldered. The centre conductor was soldered in the end of the male part in the normal fashion. The other end was typically soldered directly to a PCB. More recently, I've seen component video delivered on RCA connectors, supposedly a 75R level as well.
Since I don't work in consumer television or video, I only have whatever knowledge I picked up over the years. But again, that's why I have been very clear on the fact that I'm only reporting on things I have witnessed and heard from people in those industries. I am not confusing two impedance levels, nor am I confusing two connectors.
On my bench everything is on a 50R standard for RF except for FM tuner inputs which are on either "F" connectors or a European type connector I don't know the name for. But that's it. The only other RF connectors I have are "N", "BNC", "SMA" and "TNC", and I'm good to 500 MHz with some counters exceeding 1 GHz. RCA connectors are only used for audio unless I get a strange device that uses RCA connectors, then adapters are used to bring it onto a 50R system. I also have two FM generators that use 75R BNC connectors, which requires impedance adapters unless they plug directly into an FM tuner.
So that is my environment, and I am happy to say that I'm learning a great deal on this now that it is being discussed. Trusted information comes from the usual suspects. Some others are generating noise.
Shields up Scotty!
-Chris
When the plug and jack are a specialized construction for the coax to maintain the integrity of the dielectric across the connection then it would have a rating for the cable. So I renounce my previous assertion that there is no real 75R RCA plug to ponder... never say never.
I had to go dig out my my RGB component cables to see if they were marked for specs/impedance but, nope. No printing at all. But at the frequencies they carry I'm not surprised there isn't much need for specs to be prominent.
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What sorts of equipment are using 75R RCA inputs? Close coupled computer/digital stuff?
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It is important to make the distinction between the impedance of the cable that the male is designed to mate with from the impedance of the female rca.
There is an impedance discontinuity even if you use a double female to attach two male cables.
That said, I only worry when the impedance discontinuity get to within two orders of magnitude of the fastest "event" I want to pass. If I'm sending a 2 ampere 250 picosecond rise signal down a 75 ohm cable, it's about 2nSec per foot, or about 1.5 inches of travel in that time.. I would try to keep the break to less than .015 inch, so an rca would be out. (I had a TRR set where I had the circuit under an aluminum panel, with two posts through the panel for putting the diode under test. The eddy currents at the holes fought the slew, so I had to cut the opening to a rectangle and at least an inch from the posts, replaced it with lexan. the eddies introduced a prop delay.
At audio and even video, that necking would not be so important.
To me, the liability of using a nextgen w/r to emc concerns outweighs the z match. If I were really nitpicking (as opposed to my normal everyday nitpicking), and if they were a dollar a pop, I'd buy them..but not for critical apps.
jn
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Unless they pull tricks with the shield like the WBT Nextgen, that is incorrect advertising. It may instead indicate some design that makes terminating coax easier?
Cheers,
Howie
This is interesting, once upon a time Nelson was designing nice amplifiers
Threshold T-200 power amplifier | Stereophile.com
Threshold T-200 power amplifier | Stereophile.com
One of the things I do for the day job is testing / evaluating antennas, usually for cellular / lpr / wifi applications. They often come complete with impressive spec sheets and nice return loss or vswr plots. Mostly - the specs are total fabrication, and many times the plots are exact matches for one I've seen before, easily downloaded from the good old interweb.
So, don't always trust nice looking published specs....
So, don't always trust nice looking published specs....
Digital SPDIF out is about 110 ohms. Video was supposed to be a 75R standard, and baseband goes up to 5 MHz anyhow. So you should be able to see the difference between video cables. I have a nice Monster video cable that I use for the modulation input on a generator. It was one of the better video cables I had used. They made nice audio cables too, but stay away from the turbine cut shield types.
-Chris
-Chris
AES/EBU was 110R while SPDIF was specified with 75R according to IEC 60958.
Specification weren´t that tight, as the cable shall have 75 +- 26,x while the line driver output impedance shall be 75 +- 15 .
Characteristic impedance between 0,1 Mhz and 128 times maximum sampling frequency.
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