output impedance

Ipanema · 30th September 2005, 03:54 AM

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.

EWorkshop1708 · 30th September 2005, 05:20 AM

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

Sounds like a fast signal!

What is it for?

Ipanema · 30th September 2005, 06:31 AM

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.

Chris Jennings · 30th September 2005, 09:03 AM

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

Ipanema · 30th September 2005, 09:41 AM

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.

darkfenriz · 30th September 2005, 10:10 AM

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?

bscally · 30th September 2005, 06:35 PM

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

bscally · 30th September 2005, 06:49 PM

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

Jocko Homo · 1st October 2005, 02:25 AM

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

30th September 2005, 03:54 AM	#1
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	output impedance 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.

30th September 2005, 06:35 PM	#7
bscally diyAudio Member Join Date: May 2005 Location: USA at the moment	Impedance... 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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
3C24 in a SE output stage - output transformer impedance recommendation !!!	aldovan	Tubes / Valves	15	6th September 2008 10:18 AM
Output impedance	overdrajv	Tubes / Valves	8	5th March 2008 05:54 PM
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms	kelticwizard	Everything Else	11	25th March 2007 05:17 AM
Output Impedance?	ble0t	Parts	4	19th January 2006 05:45 PM
Low output impedance pre	miguel2	Tubes / Valves	13	23rd July 2003 09:51 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

30th September 2005, 05:20 AM	#2
EWorkshop1708 diyAudio Member Join Date: Nov 2004 Location: Florida	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 Sounds like a fast signal! What is it for?

30th September 2005, 06:31 AM	#3
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	1Hz=2bps, so at 1Gbps should I look at 500MHz. Someone tell me that I should at 0.7bitrate=0.71Gbps=700Mhz. I am not sure which one is more accurate. Can you pls clarify? Thanks.

30th September 2005, 09:03 AM	#4
Chris Jennings diyAudio Member Join Date: Jan 2005 Location: Southampton	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

30th September 2005, 09:41 AM	#5
Ipanema diyAudio Member Join Date: Jun 2004 Location: KL	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.

30th September 2005, 06:49 PM	#8
bscally diyAudio Member Join Date: May 2005 Location: USA at the moment	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

1st October 2005, 02:25 AM	#9
Jocko Homo Banned Join Date: Feb 2002 Location: As far from the NOSsers as possible	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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)