| Prune |
| I'm using relays for I/O switching in the DAC. I'm concerned of whether that may introduce problems. My internal wiring for the digital signal is also coax, to try to maintan the characteristic impedance as much as possible, and even the jacks and cable plugs are 75 Ohm. However, this is clearly not the case for the relays. My question is how much of a problem that is, when for a couple of inches the signal path is not with 75 Ohm impedance. Also, the relays are latching, so no current flows through the coil except momentarily when switching, and they are sealed, so contacs should last. But I'm worried about reflections and so on. |
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| tiroth |
| It is going to cause problems. You should use 75 ohm relays intended for this purpose, or terminate the line and use logic. |
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| Prune |
| Well I searched for coaxial relays, and it comes up for $125. That's not reasonable for one relay. |
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| andrei |
| Go to DigiKey and search for COTO 9002 series SIP reed relays. They are coaxially shielded (50 ohm) and not expensive. Alternatively, you could use solid state switches (like SN74CBT series). |
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| rmorris |
digital audio signals (spdif / aes-3) are not actually very sensitive to impedance matching. You shouldn't go too far out of course, but relatively minor impedance mismatch will result in 'ripples' at the top and bottom of the waveform but with minimal influence on the signal timing as it passes through the logic transition level.
Video signals are sensitive to this hence the emphasis on Z-matching there.
Nothing is ever perfect of course and i don't know what signal path length you are looking at but if I were you I'd try it then evaluate for real. I reckon cable capacitance / program induced jitter is much more of a problem.
regards |
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| OliFilth |
SPDIF is digital, so why use relays (mechanical or solid-state) that are designed for analogue signals?
Why not use a digital multiplexer, like the MC14512, which is designed for applications like this? (and make sure you terminate the digital line appropriately) |
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| emuman100 |
Prune,
I would use solid state switching. Convert the coax to TTL and use a multiplexor like OliFilth said. It would be much better, with less loss, noise and transient replay "pops".
I'm jealous of you're DAC working. :) Well, now that I have everything but the soldering station (which comes in a few days) I'll be able to do a much better soldering job. After all, I'm certified to operate Pace equipment, so it'll be easy. |
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| rmorris |
Like the last two posts say it's probably better to use logic gates/switching. I don't know your particular application but if you can do the switching at logic level rather than the spdif signal itself then it becomes much simpler and you can terminate the lines correctly. If you need to switch the spdif/iec950 itself then there are probably devices designed for video signal switching that will do the job - maybe look at Elantec semiconductors ?
Having said that there can be cases where relays might be preferable depending on power supply voltages available etc etc... |
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| Jocko Homo |
I once used a relay to switch SPDIF siganls......and a cheap one from NAIS worked better than the expensive one designed for RF.
You can do it, but it does take some work to keep reflections down. CMOS solutions can also contribute to reflections, so it is not a foolproof solution.
Jocko |
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| rmorris |
what i'm suggesting is a properly terminated 75ohm receiver interface which produces a logic level which can then be switched/distributed locally without influence on the 75ohm line.
See Crystal Semiconductor / AKM datasheets / application notes for examples also the 'Digital Audio Interface' (or something like that) book by John Watkins (or similar - this from memory) or if you want a big read on the whole digital audio field - 'The Art of Digital Audio'.
Not that it has to be complicated.
What sort of problem symptoms did you get with the rf relay ?
You can get problems if there is not enough current for the contact design. |
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| Jocko Homo |
The impedance pertubations were actually higher, when viewed on the TDR.
Jocko |
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| Prune |
The problem is that the nice pulse transformers I use for termination on the DAC side aren't cheap, and terminating each of the inputs with them before feeding into a multiplexer is not something I can afford. That's why I only used one transformer after relays. Also someone pointed out where to buy 50 ohm coaxial relays, but S/PDIF is 75 ohm. On top of this, my manual and remote control system is set up for latching relays, and I've never found any 75 ohm latching relays.
I wonder if I just leave the non-coax relays I have in, would there be any improvement if I were to shield them with grounded foil, or would that make it even worse? |
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| Jocko Homo |
I used relays for the same reason.
Foil over them??? I don't see any reason for it.
There are so many things that cause reflections, that you might have more than you realise already. I have seen some DACs where they run several inches of twisted pair wire into some big caps (physically, not electrically), and then some PCB traces....and finally into the RX chip.
(And a chip that I hate, but we won't go into that..........)
But speaking of which.....it causes a boatload of relections all by itself.
Bottome line......
You would be amazed by how much **** you can get away with, and still have it work. The jitter may be off the scale, but it will work.
You probably have around 1 nSec jitter in your setup to begin with, even with everything working at 100%. So you add another nanosecond or so.......
Getting that extra amount out takes lots of work, and fancy test equipment. Trust me.
Jocko |
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| Jocko Homo |
And were concerned about reflections........
And had has much money as The King of Chip Amps.........
(Like any of us do!)
You could build one of these:
Input return loss is >30 dB.
Jocko |
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| tiroth |
I'll take three. Thanks.
;) |
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| Jocko Homo |
Take up a collection, and I'll build more.......
Jocko |
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| Elso Kwak |
I take two!
Looks neat.
Well done Jocko!
How about a ......."groupbuy"?. (hardly dare to speak out the word!):D
Edit: what is that black squarish thingy? |
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| Jocko Homo |
Any and all profits go the the Jocko Homo Avoid Bankruptcy Pizza Fund.
Here is the back view.......big enough to see, but too small for the "vacuum cleaners" to get the component values.
Heh-heh.
Jocko |
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| tiroth |
| quote: | Originally posted by Elso Kwak
Edit: what is that black squarish thingy? |
Transformer I would guess. I think I have this schematic on a napkin somewhere... ;) |
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| fmak |
| quote: | Originally posted by Jocko Homo
Any and all profits go the the Jocko Homo Avoid Bankruptcy Pizza Fund.
Here is the back view.......big enough to see, but too small for the "vacuum cleaners" to get the component values.
Heh-heh.
Jocko |
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Jocko
I'll take 4; pl email.
Fred |
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| Cobra2 |
I want 10 !
Arne K |
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| Calimero |
| and CS..2, and O..ps. Or has he (at last) seen the light? Or .......... |
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| Jocko Homo |
Just because I pick on P-A all the time.....that has absolutely nothing to do with it.
Hey.....I never said anything about putting it into my DAC...... now, did I? [joke]
So just exactly what light is it that I am supposed to have finally seen????
Oh.....I know......the light of plundering DIY for my personal profit!
Yep, that has to be it, right.
In that case.....
OK. I only made a few of these for a customer who begged for some. If it works, I'll make more...complete with solder mask and silk screen. And charge $$$$$$ for it. And then whine that I am only doing it for fun.
Just like some other guys here do!
Only everyone will know whether I am telling the truth or just pulling their leg.
Right, guys????
(BTW.......I have a TX board also..........!)
"Show me the money" Jocko |
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| Calimero |
for the design, only based on the picture of the board, so ....
It would highly surprise me if $$$$$ is what you are aiming for, it would certainly ruine your career here as objective critic-caster.
As a relative newbie, and having read a lot of the very informative threads on this board, I consider you as somebody that focuses on the concepts behind a design. Try to reduce the complexity of a design to its basics.
SMD is another type of packaging with its strenghts and weaknesses, and as such ...... you know.
Keep on pulling their and our legs and at the end the real guys will keep walking (and talking).
What was the subject of this thread?
'relays in digital signal path'. Well I am going to remove my logic chips (DG271) used between the puls-transf. of the SPDIF-input to the CS8412 and expect a lot of improvements.
If not i'll let you know. |
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