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Old 7th April 2009, 07:12 PM   #11
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Join Date: Oct 2006
Location: Charlotte, NC
Hello Guido:

Nice to have you join in. Now that you mention it, it does make sense to place the resistor after the transformer. I will move it.

Since the receiving end has a Lundahl transformer, I would think the problem is from a poor power supply. Battery power for the converter works great. I do know the receiving end has the resistor on the D/A side of the transformer, not the cable side. Could you elaborate on the net effect this has?
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Old 7th April 2009, 07:17 PM   #12
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Quote:
Originally posted by zigzagflux
Hello Guido:

Nice to have you join in. Now that you mention it, it does make sense to place the resistor after the transformer. I will move it.

Since the receiving end has a Lundahl transformer, I would think the problem is from a poor power supply. Battery power for the converter works great. I do know the receiving end has the resistor on the D/A side of the transformer, not the cable side. Could you elaborate on the net effect this has?
Hi

It depends how the Luhndahl is connected. ie how the cabling is connected in terms of common mode. Anyhow, when battery-powering the circuit, you could stll induce a common mode effect by connecting the ground of the original wall supply to the ground of the circuit.

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Old 9th April 2009, 03:10 AM   #13
BFNY is offline BFNY  United States
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Quote:
Originally posted by zigzagflux
Yeah, I got the datasheet mixed up there.

Still, 13ns (or even your 2ns measurement with light load) is slower than the performance of the video amp. At 100 ohm loads, they show large signal step responses between 2 and 4ns. I would be shocked if the 74HC family could drive that fast at 150 ohms. Ten LSTTL loads seem pretty small in comparision to a 150 ohm load along with a coax cable and its capacitance. The AD8055 is rated for driving up to 4 video loads.

I'm not saying digital logic wouldn't work; after all, it's what I currently use. I just think the typical circuits can be improved. Whether or not it is audible is of course another story. I suspect the greatest gains for me will be in the improved power supply.
Well, if you have convinced yourself, go ahead and build it, then see how it does.

I'm telling you what I did over 10 years ago. If I was to do it today, I would likely do it different. You know, you need to treat it as the whole system, transmit and receive. Just talking about one side of it, without matching it to the other end, is not the best way to optimize it. But frankly, it sorta typical for the way people tend to think.

If you want to use a high feedback single ended analog op-amp to send digital signals, go for it. But don't condemn digital logic when driven by a $1.99 wall wart transformer, and then thing the digital logic is the problem

Bob
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Old 18th April 2009, 03:58 PM   #14
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Got the circuit built, and have some fun graphs to display. First, the existing circuit, built with 5V HC logic:

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

Two basic observations I had: The output was a little lower than I would have liked, at +/-0.35V, instead of the specified 0.5V. I suspect, since it was functional, the CS8416 was able to read it just fine anyway. Rise times were around 5-6ns, with some ringing on the output (not transformer coupled).

Then, compare to the new design using AD8055 and output transformer:

Click the image to open in full size.
Click the image to open in full size.

Ringing now moved from the rising edge to the falling edge, but drive magnitude was higher at +/- 0.75V. Rise times were slower, too, at maybe 13ns. This bothered me, since it made little sense to me why the slew rate would be so poor. So, I took a look at the output of the torx converter, and saw this (purple is torx output, yellow transformer secondary):

Click the image to open in full size.

So it looks like the AD8055 is doing exactly what it's being told to, and the slow transitions are due to the optical converter. Going back to the datasheet, this is reasonable performance from the optical converter. When motivated to build another circuit, I'll pursue a schmitt input gate just before the AD8055 (I think most SPDIF converters use schmitt gates). With the CMOS gate driving a high impedance, I hope to achieve below 2 ns transition with maybe an AHCT device. Now I'm not too sure how successful this will be, since decreasing transition times alone do not guarantee jitter improvement, but it could help. It does appear that the AD8055 and transformer have the bandwidth to permit the quicker transitions, though.

Next day or two I'll pull out the analog scope and look at the RF noise compared to the existing converter. And, of course, I need to listen to the device. Might be good enough as it currently stands; as long as I get rid of the hiccups, I'll be more than happy.
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