| Ipanema |
Hi,
This might not related to audio.
For an amplifier driving a digital pulse at 1Gbps and I want to measure the amplifier output impedance, at what frequency should I measure the output impedance, since impedance varies with frequency and also pulse are consists of many orders of harmonic.
Pls help.
Thanks. |
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| EWorkshop1708 |
1Gbps = 1 Ghz.
1Giga bits per second............ a bit is just one rise on a square wave, so 1ghz would be the same. You want to test the amp impedance at the frequency you will use it for.
Also 1Gbps = 128MB per second :hot: Sounds like a fast signal! :eek:
What is it for? |
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| Ipanema |
1Hz=2bps, so at 1Gbps should I look at 500MHz. Someone tell me that I should at 0.7*bitrate=0.7*1Gbps=700Mhz. I am not sure which one is more accurate. Can you pls clarify?
Thanks. |
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| Chris Jennings |
Data cable is nomally tested higher than the bit rate to ensure the necessary harmonics are carried to accurately reconstruct the data stream.
At 1GbS the highest transition rate would be 500MHz, but to maintain a waveform approaching square you will need the 3rd harmonic so test to 1500MHz.
HTH
Chris |
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| Ipanema |
For example. if my design is to have 50ohms impedance matching at 1Gbps. From my impedance vs Freq plot, I get 50ohms at 500MHz 30ohms at 1GHz and 10ohms at 1.5GHz. In this case which frequency should I match the output impedance of my amplifier?
Thanks. |
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| darkfenriz |
| quote: | Originally posted by Ipanema
1Hz=2bps, |
I think it depends on modulation:
return-to-zero, non-return-to-zero, freq. shift key,.......
| quote: | | For example. if my design is to have 50ohms impedance matching at 1Gbps. From my impedance vs Freq plot, I get 50ohms at 500MHz 30ohms at 1GHz and 10ohms at 1.5GHz. In this case which frequency should I match the output impedance of my amplifier? |
All of them I'm affraid. Standing waves at any harmonic can be a disaster to digital signal.
What's the amplifier?
Class C power MESFET ? Or small signal? |
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| bscally |
1) I would take a TDR with a pules faster than 1Ghz and use that.
2) Alternately if you can trigger a single ouput pulse ( or a 10000000 bit pattern) you can look for reflections on the line with an osciliscope. You will need a really good probe though. I use a 8Ghz tek differential probe for looking at those kind of things here.
3) Take an eye pattern and look at the crossing points.
I base this on too much experience with USB 2 (480MHz ) and Sata 1 and 2 - 1.5Gbps and 3Gbps.
Brian |
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| bscally |
danm send button....
once I have an impediance plot I would match the amp to the plot.
However depending on line length you really need a good match @ 5 times fundamental to get a nice clean edge.
Brian |
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| Jocko Homo |
You have to be able to pass at least the 3rd and 5th harmonics to get a decent pulse. You have not told us anything about the power levels involved, or how long of a cable this will travel over.
Sometimes you will pre-distort the signal to compensate for anomalies in the output. Such as impedance mismatches.
Jocko |
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