Coaxial Impedance :
Z0= (138/ Sqr(Er) )* log(b/a)
Er is relative dielectric constant of isolation material in use
b=dist between inner and outer conductor, - a= dia. of inner conductor.
This is the ONLY parameters that matters.- If you connect all or parts of the screen the distance between the conductors is still the same.
Hence -- a phono can NEVER be 75 ohms - No matter what obscure explanations are constructed!!!
Want constant correct impedance ?? Go BNC, N, TNC or any good quality coaxial plug... and good quality coax cable..BTW- sofar I haven't seen any "snake oil" cable being correctly made for constant impedance.
PS- the 75 ohm comes from being 1/4 of 300 ohms- the average impedance most Yagi-Uda antennas ( - i.e normal TV antennas)
A single foldeed dipole is appx 240 ohms- and the 60 ohm coax used in the 50-60's being 1/4 of 240.....
Don't get it????
Go get a book on transmission line theory - and maybe we can get rid of some of the pointless discussions and threads on cable impedance
( and very hopefully also some of the snake oil....)
EDIT:
Good quality coaxial cables cost just a fraction of the snake oil....
So does BNC bulk heads...........
But it is all of no avail if your drive and receive ends are not properly made to use this impedance... i.e properly terminated!
Z0= (138/ Sqr(Er) )* log(b/a)
Er is relative dielectric constant of isolation material in use
b=dist between inner and outer conductor, - a= dia. of inner conductor.
This is the ONLY parameters that matters.- If you connect all or parts of the screen the distance between the conductors is still the same.
Hence -- a phono can NEVER be 75 ohms - No matter what obscure explanations are constructed!!!
Want constant correct impedance ?? Go BNC, N, TNC or any good quality coaxial plug... and good quality coax cable..BTW- sofar I haven't seen any "snake oil" cable being correctly made for constant impedance.
PS- the 75 ohm comes from being 1/4 of 300 ohms- the average impedance most Yagi-Uda antennas ( - i.e normal TV antennas)
A single foldeed dipole is appx 240 ohms- and the 60 ohm coax used in the 50-60's being 1/4 of 240.....
Don't get it????
Go get a book on transmission line theory - and maybe we can get rid of some of the pointless discussions and threads on cable impedance
( and very hopefully also some of the snake oil....)
EDIT:
Good quality coaxial cables cost just a fraction of the snake oil....
So does BNC bulk heads...........
But it is all of no avail if your drive and receive ends are not properly made to use this impedance... i.e properly terminated!
@ commstech, @ gasho:
well, RCA is used for historical reasons - a pure legacy connector.
and of course it's a matter ofthe price, just as Gasho has mentioned it: Easier to produce, a littel bit cheaper, and easier to work on when you have make a complete cable without professionol tools.
I prefer BNC also because it "locks" on the connector (not so easy slipping of unintentionaly.
My Tek 1730 has BNC connections, but of course adapters to RCA are included too.
hth,
Andreas
well, RCA is used for historical reasons - a pure legacy connector.
and of course it's a matter ofthe price, just as Gasho has mentioned it: Easier to produce, a littel bit cheaper, and easier to work on when you have make a complete cable without professionol tools.
I prefer BNC also because it "locks" on the connector (not so easy slipping of unintentionaly.
My Tek 1730 has BNC connections, but of course adapters to RCA are included too.
hth,
Andreas
Good, at least you seem to understand what the problem is..(that will be my last quip in response to your silly profanity. Perhaps it can remain professional and mature now?? thanks..)Jocko Homo said:
I'll give some explanation for all, I assume you already know this..
A coaxial structure defines it's characteristic impedance by seveal things:
1. The inner pin OD. (outer diameter)
2. The outer shell ID. (inner diameter)
3. The dielectric constant of the material between the two.
Jocko points out (correctly) that for the RCA connector, the pin and shell size conspire to create a characteristic impedance that is LOWER that that required. To raise the impedance to 75 ohms, one must either raise inductance (which is locked by the coaxial structure dimensions), or lower capacitance (which is also defined by the coaxial structure AND by the DC of the insulator.
Jocko is also correct in the relative magnitude that the inductance must be increased {Z = sqr(L/C)}, or the capacitance must be decreased...
The capacitance cannot be decreased the amount needed, even if the insulator is removed leaving vacuum. This is a physical reality, vacuum still has a DC of 1. Again, a constraint of the coaxial design.
So, how to get around it? Simple, break the coaxial design.
The coaxial design works by constraining the magnetic field to the volume between the pin and shell, following the equation:
L = {mu/2 pi} Ln {Ro/Ri}
Note the pin and shell dimensions define the inductance.
inductance is by definition, the relation between the energy stored within the magnetic field as a result of the current.
A coaxial structure constrains the field outside the shell by simple cancellation....the inner wire produces a field strength which falls off as 1/R...the outer one produces an opposite field which also falls off as 1/R, but since the current density of the outer shell is lower due to surface length, its field strength at the shell is exactly the same but opposite to that of the inner wire. I had some pictures showing how this works, but I can't find them on my desktop..windows, sheesh..
Now, the fun part....STOP CONSIDERING THIS AS ONLY A COAXIAL PROBLEM!!! That is where you, Jocko, have not looked..
Once you break the symmetry of the outer shell, you no longer constrain all the field to the volume within the shell..because it is no longer fully cancelled outside the shield volume.
Break the shield at 12:00, break it at 6:00. Then keep backing it off, towards a final destination of a wire at 3:00 and a wire at 9:00. (analog clock, thank you...
)While this "morphing" is occurring, the inner wire field gets farther reaching as the shield current no longer cancels it...and the shield field gets bigger and bigger..THIS IS AN INCREASE IN INDUCTANCE!
AND, as this morphing occurs, less and less of the dielectric is being used, so the capacitance starts going down..
Increase L, decrease C...voila...impedance goes up..when it hits 75 ohms, stop...
That is what the wbt part does..
Downfall to this method? Glad you asked...
1. Breaking the magnetic symmetry this way makes the local area more susceptible to RFI, both radiated and received.
2. Use of a standard rca male or female with a wbt unit negates entirely the 75 ohm impedance, it goes back down to it's design impedance. This forces the use of pairs to achieve 75 ohms, but at least the design is compatible with existing product physically.
3. It's too darn expensive..
Jocko Homo said:
I used electrons, bits, and photons...print it out if you wish..
Jocko Homo said:
Umm, I missed something...who is talking about an XLR?
You are standing in the middle of the road claiming cars don't exist, only to be run over by a tractor trailer..Jocko Homo said:
(i love that smiley)My experience exceeds (by a small tad), a simple exercise in modification of a coaxial structure for magnetic reasons. You need only examine the wbt part to see how others, by both experience and by shipped product, have done what you claim is impossible..
I have tried to keep the explanation as simple as possible without losing content. If you wish more elaboration, just ask..
Interestingly enough, this is the simpler of the two methods I know of to get an RCA up to 75 ohms. The second opton does not suffer points 1,2, or 3 above...
Cheers, John
No, I am not standing in the middle of the road.......
I am standing in front of my TDR, and does not lie.
Unlike the people who make up stupid **** like "75 ohm RCAs" to sell to gullible audiophiles. It is a lot cheaper, and far better, to use a real 75 ohm connector. But no money in that for crooks. So they invent garbage like what you describe.
You obviously missed the point about XLRs. A connector that does measure 110 ohms, but is totally useless.
The "75 ohm RCA" may look like 75 ohms, for one small point in time, but will be just as useless as a 110 ohm XLR.
Go put one of your magic RCAs on a TDR, and then try to tell us that it really works. And while you are at it, try to figure out how high the rho can be before it makes the SPDIF sound even worse than it really is. At that point you may understand why it is a stupid idea.
At least to the rest of us. A great idea to the crooks who peddle such nonsense.
Jocko
I am standing in front of my TDR, and does not lie.
Unlike the people who make up stupid **** like "75 ohm RCAs" to sell to gullible audiophiles. It is a lot cheaper, and far better, to use a real 75 ohm connector. But no money in that for crooks. So they invent garbage like what you describe.
You obviously missed the point about XLRs. A connector that does measure 110 ohms, but is totally useless.
The "75 ohm RCA" may look like 75 ohms, for one small point in time, but will be just as useless as a 110 ohm XLR.
Go put one of your magic RCAs on a TDR, and then try to tell us that it really works. And while you are at it, try to figure out how high the rho can be before it makes the SPDIF sound even worse than it really is. At that point you may understand why it is a stupid idea.
At least to the rest of us. A great idea to the crooks who peddle such nonsense.
Jocko
Re: No, I am not standing in the middle of the road.......
If you have actually taken the time to aquire a pair of wbt's and they do not measure as good to 1 GHZ, as is claimed in their ad copy, let us know...
What I describe is the reality of using maxwells equations to produce a solution you didn't think possible. Unfortunately, the wbt patent was applied for 31 calender days before I posted the solution on AR.
I also agree that a 75 ohm rca is pointless..given the choice between a bnc and an rca, it's a no-brainer..
I didn't see the point of discussing an audio connector in an rf and video discussion..
Figures..
Cheers, John
Um, Jocko...what are you measuring with your TDR? As I said, normal RCA's are not 75 ohm..did you measure one of the wbt's, or are you generalizing.Jocko Homo said:
If you have actually taken the time to aquire a pair of wbt's and they do not measure as good to 1 GHZ, as is claimed in their ad copy, let us know...
Jocko Homo said:
What I describe is the reality of using maxwells equations to produce a solution you didn't think possible. Unfortunately, the wbt patent was applied for 31 calender days before I posted the solution on AR.
I also agree that a 75 ohm rca is pointless..given the choice between a bnc and an rca, it's a no-brainer..
Jocko Homo said:
I didn't see the point of discussing an audio connector in an rf and video discussion..
I do not understand the words"one small point in time"..nobody is discussing a time varying impedance...you will have to explain that. A 110 ohm XLR for audio frequencies is a convienience of form, not reflection coefficient.Jocko Homo said:
Didn't you just tell us you are standing in front of a TDR, and it doesn't lie??? Are you saying you DON'T have one to measure?? Then, how "is it not lying "...you aren't measuring one, are you?Jocko Homo said:
Figures..
Cheers, John
Yes, you will have some discontinuities breaking out the centre conductor from the shield. Maybe they figure that into their increase in inductance.
Of course, all of this nonsense is negated the moment you plug it into a standard female RCA. I wonder if they ever thought of that? You have to measure the connector pair to see if it really does hold its characteristic impedance.
Which, of course, this silly thing will not.
BTW..........let us see a show of hands of folks who think that they can hear the effect of a 50 ohm barrel used to connect two 75 ohm SPDIF cables together. Asssumes that the rho of the rest of the signal path is already low. While experience says that below 300 MHz, it should not matter, it does on SPDIF. Go figure.
Jocko
Of course, all of this nonsense is negated the moment you plug it into a standard female RCA. I wonder if they ever thought of that? You have to measure the connector pair to see if it really does hold its characteristic impedance.
Which, of course, this silly thing will not.
BTW..........let us see a show of hands of folks who think that they can hear the effect of a 50 ohm barrel used to connect two 75 ohm SPDIF cables together. Asssumes that the rho of the rest of the signal path is already low. While experience says that below 300 MHz, it should not matter, it does on SPDIF. Go figure.
Jocko
Please, read the explanation and understand it..AuroraB said:
I already do..I do not consider RCA's as viable. I hate them..AuroraB said:
AuroraB said:
Why is the 1/4 of 300 important? I cant help but recall from '74, some kind of 1/4 wave transformer thing...but when I run the numbers, I get 112 ohms...Zo=sqr(ZrZi),
AuroraB said:
I have three of them 2 feet away, next to Jackson, Becker, Shadowitz, Rojansky, and Ragan. Plus, I can walk 100 feet down the hall to the people who write the books..
AuroraB said:
I currently use a 160 foot run of rg-6 as a video feed, bnc at the receiver, rca at the transmitter..works very well..thank goodness the receiver had a bnc connector, as the rca one woulda reflected all to heck.I don't recommend rca at transmit end, but the dvd player didn't have the option.
Cheers, John
They actually don't break the center conductor out, as they retain symmetry for both the jack and plug..Jocko Homo said:
They don't retain cylindrical symmetry of course, but bi-lateral symmetry. And yes, this is how they increase the inductance, and lower the capacitance too.
I have stated that twice already within this thread.Jocko Homo said:
The fact that they made a jack that functionally is compatable to existing RCA's proves they indeed did think of that.Jocko Homo said:
They actually specify the connector via TDR, out to a ghz..Jocko Homo said:
Haven't you measured a pair?
Jocko Homo said:
Hmmm..it will, just measure a pair. Simple enough with the instrument in front of you..Ask Ray for a freebie pair..
Jocko Homo said:
Why? I'm discussing characteristic impedance, and the fact that it is indeed possible to achieve 75 ohms by alteration of the connector e/m symmetry..
Cheers, John
Guys, look at the pdf on the first post, the datasheet.
On the first page there's an 'exploded view' of the plug.
At the right there's a 'table of contents', where you can see:
3) Head of the clamping barrel. Alluminium-magnesium alloy.
I suppose this is conductive, no?
Well, it's in tight contact with that fancy 3-part shield.
On the first page there's an 'exploded view' of the plug.
At the right there's a 'table of contents', where you can see:
3) Head of the clamping barrel. Alluminium-magnesium alloy.
I suppose this is conductive, no?
Well, it's in tight contact with that fancy 3-part shield.
carlosfm said:Guys, look at the pdf on the first post, the datasheet.
On the first page there's an 'exploded view' of the plug.
At the right there's a 'table of contents', where you can see:
3) Head of the clamping barrel. Alluminium-magnesium alloy.
I suppose this is conductive, no?
Well, it's in tight contact with that fancy 3-part shield.
Carlos, if you look down to the fourth bullet under 'mechanics' they state: "the electrical insulated aluminum clamping barrel..
I assume they know how to insulate..
BTW, in looking at the pic, it kinda looks like they have two conductive contacts, and two non conductive ones at 90 degrees..that'd be for mechanical stability..
And, this connector STILL doesn't have 75 ohms if it goes over any regular rca jacks..if they sell a mate to this design, it has to have some kind of keying mechanism to align the plug in rotation.
Cheers, John
Which one is it? Is it 75 ohms with any RCA, or just their special mating connector? It is either is 75 ohm, or it isn't.
If it is the latter, as Gmarsh points out, what is the point??
The point is to make money selling nonsense to people dumb enough to fall for it. Why go to all the bother when you can buy a true 75 ohm connector for much less. I don't think that the average Joe wants to have to buy the audio world's version of a GR connector. (On second thought, most are willing to spend $$$$$ on over-priced crap just to impress their friends, so maybe.)
Sorry, I don't know Ray, or who he is. If you do, please feel free to supply him with my e-mail, via the forum e-mail. I will test any such samples, and post any findings here.
Jocko
If it is the latter, as Gmarsh points out, what is the point??
The point is to make money selling nonsense to people dumb enough to fall for it. Why go to all the bother when you can buy a true 75 ohm connector for much less. I don't think that the average Joe wants to have to buy the audio world's version of a GR connector. (On second thought, most are willing to spend $$$$$ on over-priced crap just to impress their friends, so maybe.)
Sorry, I don't know Ray, or who he is. If you do, please feel free to supply him with my e-mail, via the forum e-mail. I will test any such samples, and post any findings here.
Jocko
It's 75 ohms ONLY WITH THEIR MATING 75 OHM JACK...geeze, didn't you know that???Jocko Homo said:
Just teasin ya, Jocko...they certainly don't point that out, do they..
To buy one of those connectors and plug it into a regular jack is pointless. But bet the owner feels better..
Last I checked, I don't use any...as you say..why bother..I'll stick to bnc's when I need them.Jocko Homo said:
Ray Kimber. He reps the stuff in the USA..figured you might know him..Jocko Homo said:
Jocko Homo said:
The point is, it is certainly possible to make a mating pair of rca compatable connectors that continue the 75 ohm impedance of the cable..this has been what I have been stating from the jump.
A second point is...who cares? To me, it is a product which provides a solution to a problem that is not a problem..
What's the saying? A difference that makes no difference.....is no difference.
Cheers, John
Oh..........that Ray. Yeah, I know who he is. I never was concerened to know who was carrying it now. Haven't had the need to deal with them in years. I still have some of their stuff in my junque box, from years ago. (If anyone wants to buy a set of binding posts that you can use to arc weld.........make an offer! Never used. And yes, I did pay for them. Don't ask how much or why the guy I ordered them for backed out of the deal.)
Anyway.....the offer stands. Anyone who wants can try to convince Ray to send me some. I am not going to, as I have more important things to do. But I stand by my offer: you send 'em, I'll measure 'em. Results posted here, and not in some place of dubious impartiality.
Like Stereophile.
Jocko
Anyway.....the offer stands. Anyone who wants can try to convince Ray to send me some. I am not going to, as I have more important things to do. But I stand by my offer: you send 'em, I'll measure 'em. Results posted here, and not in some place of dubious impartiality.
Like Stereophile.
Jocko
To get back to basics, I've often noticed that decent BNC connectors actually cost less (in small quantities) than supposedly good RCAs.
I can understand the 'legacy' point, but legacy devices usually eventually become extinct in the light of superior ones.
OK, so they're a bit more difficult to use (manual soldering), but this surely doesn't apply industrially.
BTW, does anyone have the impedence specs of DIN connectors to hand <ducks>?
I can understand the 'legacy' point, but legacy devices usually eventually become extinct in the light of superior ones.
OK, so they're a bit more difficult to use (manual soldering), but this surely doesn't apply industrially.
BTW, does anyone have the impedence specs of DIN connectors to hand <ducks>?
Reina,
A good question. There is no general answer, the actual trace width over a ground plane will depend on a lot of factors, like dielectric constant of the board, the thickness, the actual shape of the trace..
Take a look at this:
Search the link for Si6000
A very useful tool for calculating this.
I have just used it for a general case: 1,58mm [1/16"]70um[2oz] FR4 PCB material, surface microstrip case. For 75 ohm the trace should look like a trapezoid, upper width: 1,377 mm, base: 1,403mm.
The distance from the other traces should be at least the same width.
Ciao, George
A good question. There is no general answer, the actual trace width over a ground plane will depend on a lot of factors, like dielectric constant of the board, the thickness, the actual shape of the trace..
Take a look at this:
Search the link for Si6000
A very useful tool for calculating this.
I have just used it for a general case: 1,58mm [1/16"]70um[2oz] FR4 PCB material, surface microstrip case. For 75 ohm the trace should look like a trapezoid, upper width: 1,377 mm, base: 1,403mm.
The distance from the other traces should be at least the same width.
Ciao, George
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