Word Clock Distributor design

Hello List:

I am building a Word Clock Distribution box to distribute WC from my RME ADI-88-DD (which has only one WC output) to 11 devices in my studio. If I understand correctly I can route the WC (from a BNC "WC input" connector on my diy chassis) directly to the inputs of 10 HC family gates which will buffer the clock input. Then I connect the outputs of these gates to BNC connectors, with a 75 ohm resistor to ground on each output. Is this correct?

Has anyone posted any schematics of how to do this properly? I searched the archives but could not find any relevant material.

Thanks for any input,

Keith
 

rfbrw

Member
2001-10-26 11:51 pm
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OliverD said:
I suspect you would need to connect the 75 Ohms resistor between the gate's output and the "hot" pin of the BNC connector at the TX end, and from BNC "hot" pin to ground (as you described) at the RX end.

But better wait for some comments from our digital interface experts...


http://www.diyaudio.com/forums/showthread.php?postid=275730#post275730

Post 115 has the schematic of a simple clock distribution scheme. Note the series resistors.
 

Ulas

Banned
2003-03-13 8:37 pm
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Or, you could use a proper clock driver. Take a look at the 49FCT2805. It has a 1:11 fanout and has integrated series resistors. Closely related is the 'FCT807. It is only 1:10 fanout so you will need two of them and supply your own series rersistors. I designed a small PCB that holds two '807s and resistors and plugs into a normal 24-pin DIP socket. It makes a handy 2x10 clock distribution "chip."
 
That is one way, but..........

How long are these cables going to be? I suspect that they would be too long for that simple scheme, which was intended for a few cm at most. WC is 64 x Fs?

First, in order for your idea to be right, you would need to reduce the buildout resistor so that the impedance is 75R. If you are using HC logic, figure around 25 ohms less. (I would of course measure it, as each brand is a bit different.)

But 10 gates............2 packages at a minimum......all operating in unison will add jitter. Picogate time.

Or................maybe better.........for long runs and higher frequnecies.......use video buffers. Terminate the far end in 75R. Would entail using a 2X op-amp to bring it back up to logic levels.

Jocko
 

Ulas

Banned
2003-03-13 8:37 pm
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Jocko Homo said:
WC is 64 x Fs?

WC = Fs.

Picogates don't address the fanout problem nor do they address clock skew. Each picogate will likely have a different prop delay.

What's the difference between 1 package with 10 gates all switching simultaneously or 10 packages with 1 gate each all switching simultaneously?
 
Consider using the Elantec EL2099 - now available form Intersil:
datasheet
It can supply 6* 75ohm terminated lines.
Wordclock distribution is much the same task as video distribution except for the larger signal swing. This is easily accomodated with this chip.
Be careful to keep the feedback path short and avoid parasitic capacitance around the invering input pin.;)

I see you'll need 2!
 

Ulas

Banned
2003-03-13 8:37 pm
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Jocko Homo said:
Each package would have its own Vcc and ground connection, and you would isolate the Vcc pins with ferrites. Less di/dt per connection.

In case you don't get around to looking at the '807 datasheet, it has four VCC and five GND pins per package which allow ample opportunity for bypassing and isolation. With multiple gates within a single package the operational characteristics of each gate are more closely matched. Unlike ordinary logic, clock drivers are specified more tightly and are speed graded. You rant about the prop delay differential between Q and Q-bar in the same flip flop: What do you think the prop delay differential between two picogates is? I’ve never seen a picogate, one-gate, or tiny-logic datasheet that specified package-to-package output skew. Have you?
 
Thanks for pointing out the features of the '807. That would do the trick.

The "rant" about prop delay between Q and Q_bar is because you can not make a truly balanced signal using them. Apples and oranges.

No, I haven't looked at the output skew between a boatload of picogates. Actually, I wouldn't try it. The point was that a lot of guys like to use all 6 inverters in an '04, which isn't a great idea. Just trying to get his head pointed in the right direction. I think between the 2 of us he is.

Jocko
 
Word Clock Distributor Design

Thanks everyone for the input. I think I will start by building up Dhaen's EL2099 idea. Seems the part is out of production but I'm going to try getting a hold of a couple of these, or a similar part.

I am new to the diyaudio board. In a few weeks should I post my results to this same forum, or start another one?
 

Terry Demol

Member
2002-04-07 3:12 pm
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matrixtwelve said:
Hello List:

I am building a Word Clock Distribution box to distribute WC from my RME ADI-88-DD (which has only one WC output) to 11 devices in my studio. If I understand correctly I can route the WC (from a BNC "WC input" connector on my diy chassis) directly to the inputs of 10 HC family gates which will buffer the clock input. Then I connect the outputs of these gates to BNC connectors, with a 75 ohm resistor to ground on each output. Is this correct?

Has anyone posted any schematics of how to do this properly? I searched the archives but could not find any relevant material.

Thanks for any input,

Keith

The 75ohm resistor is in series after the gate or whatever you
are driving th OP's with. The receiving box should have 75 ohm
to gnd.

*However* , believe it or not, there is no actual standard for the
transmission of wordclock WRT 75 ohm transmission line. Most
devices do conform to 75 ohm std but not all. In the bigger
scheme of things this is not such a big deal, as most of your boxes
are D in D out (I assume) so jitter is not a problem.

The best way to configure the whole system is to use the ADC's
internal clock as master clock, which appears to be the way your
setup is. If you are slaving a separate DAC off WC then make
sure it is a proper 75 ohm connection as jitter does count at DAC.

Probably not a bad idea to check some of the tech notes
about dig connection applied to studio at www.digido.com (Bob Katz)

Best,

Terry
 
LVDS?

Hello matrixtwelve,

If you are going to go to the trouble of building a solution, and you want to take your WC signal more then a few cm of transmission distance, lvds (low voltage differential system) http://www.national.com/appinfo/lvds/ could well be worth a look into. no royalties, cheap and perfect for digital audio over <30m distances, and SPECIFICALLY designed for high speed data transmission/reception over short distances up to 30m at up to 1.25 Gbps per stream


Not so easy if you want to be compatible with existing systems, but who wants to be a conformist anyway?!

just my 2 cents worth.

Mark