I want to drive a digital audio signal over CAT5. I intend to use the DA103 transformer but need to check impedance matching.
Will 100R each end of the CAT5 be suitable? Should the 100R on the transmitting end be before, or after, the xformer?
I want to drive a digital audio signal over CAT5. I intend to use the DA103 transformer but need to check impedance matching.
Will 100R each end of the CAT5 be suitable? Should the 100R on the transmitting end be before, or after, the xformer?
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
The terminating restance should be at the far end of the cable. I gotta ask though, in there any reasonable way that you could send the signal digitally? like manchester encoded? The whole thing can work much better that way.
Forgot to add... The input signal is standard SPDIF. I'm going to bump it up to TTL level and use that to feed into a am26C31 line driver and then into the transformer. The other end is an SPFIF receiver, so I have to do an impedance match (and voltage reduction to around 1V).
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
OK.
Actually, i think all you really need is the impedance match (both ends and let the voltage scale). Unless you are going a lot farther than SPDIF is rated for, in which case the driver may be called for.
The only worry is that the receiving chip is 3V3 and I do not want to risk damaging it by feeding it some overvoltage. Can bad impedance match bump up the voltage through reflections?
I checked the specs for the am26c31 and the output is around 3V (5V Vcc), so after the xformer and resistors it will certainly be less.
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
Yes it can. With this configuration and guessed line lengths it may show up as increased overshoot / undershoot, more ringing or leading pedistals on the pulses at the receiver.
I still recomment just Z matching both ends and just using Cat 5 / Cat
5E cable. It really is nice cable for fast signals. I know, i done got me a copy of TIA-568 set.
matching.
Given that the SPDIF input is 75R, a couple of 12R resistors in each signal path at the SPDIF end should be sufficient?
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
On a related issue, how important is termination over short runs with low(ish) frequency? Audio is around 10Mhz and I doubt we will ever go beyond 30m in length. In this case the wavelength is around the same lenth as the cable. Of course, this is square wave and obviously there are higher frequency components.
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
1/4 wavelength is an impedance inverter (an open 1/4 stub acts a short circuit, a shorted 1/4 wavelength stub acts as an open circuit).
With a typical velocity factor for CAT5 cables of 0.67, at 10 MHz, the wavelength is about 20 m. When operating below 1/10 wavelengths, the transmission line issues are not very critical. Thus, with 10 MHz signals, termination should be used for cables longer than 2 m.
matching.
Might be. You may also try 15R or 18R to see what works best, then a
1:1 transformer right into the receiver. Resistors at both ends, it will cost some signal amplitude, maybe 40% in voltage. (105/75)^1/2 ~= 1.18 so 1.2:1 transformers are likely to work better, 1.2 side on the Cat5 cable side.
It mostly depends on the edge speeds you need to maintain. They determine the highest frequency components with signifcant (spectral) power. So if you need 10 ns edges you have stuff up in the 100 MHz range involved. Or if you use another approximating rule 35 MHz.
matching.
xformer?
The SPDIF input is already xformer isolated, 1:1 However, there should be plenty of signal amplitude
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
matching.
xformer?
So let us know how it all works out, ok?
Yes it can. Depending on line length and signaling rate it may be in the form of overshoot and undershoot or maybe ringing. Or simple changes in volatge amplitude. Or a mixture of these.
I still recommend Z match and go for it, Cat 5/ Cat 5E is pretty nice cable for fairly fast signals. I done bought me a copy of TIA-568 set.
matching.
xformer?
Well, feeding straight into CAT5e and AES/EBU i/f it all works fine. SPDIF will have to wait until next week.
-- Dirk http://www.transcendence.me.uk/ - Transcendence UK
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