termination and reflected power in audio o-scope measures

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decaying.sine
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termination and reflected power in audio o-scope measures

Post by decaying.sine » Mon Feb 15, 2010 10:14 pm

i got a tektronix 453 on craigslist for $30 a year or so ago and got a bnc -- banana adapter. now i'd like to see my waveforms!

when i do RF stuff with sensitive equipment, i have a nice oil, impedance matched dummy load for reflected power on the RF lines.

is there an equivalent in audio measurement? In other words, should i mult the audio signal and terminate into a load. or does the speaker in audio serve the same function as the 50 Ohm load, say in an RF setup? is the reflected power distributed through the system adding noise through common ground?
Brian
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JohnLRice

Post by JohnLRice » Mon Feb 15, 2010 10:47 pm

I don't think it's nessesary for audio but, I'm pretty much a noob when it comes to o-scopes so if there is somethng like that, I'd like to know! :hyper:

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decaying.sine
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Post by decaying.sine » Mon Feb 15, 2010 11:36 pm

I was thinking about your setup too when typing this John. You have the various power at a panel with a banana jack, right? It would be really easy for you to terminate, chain, etc. given the setup.

I'm used to meausuring RF signals from people's brains so any amount of noise that we can eliminate, or reduce reflected power results in a better quality signal in the end.

I guess in audio signals, the SNR is so large that I don't need to worry about these things.
Brian
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Post by paults » Tue Feb 16, 2010 1:23 am


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decaying.sine
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Post by decaying.sine » Tue Feb 16, 2010 4:07 am

It said probe! :oops:

Thanks.
Brian
"I must create a system or be enslaved by another mans; I will not reason and compare: my business is to create." William Blake

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Post by daverj » Tue Feb 16, 2010 1:08 pm

A probe is good because:

- it doesn't need to be terminated
- it reduces the capacitance of the scope input, so it doesn't capacitively load the signal
- it's shielded so it doesn't pick up stray noise in the cable


A probe is bad because:

- it doesn't plug into a jack on a modular

I would definitely get a pair of probes like Paul suggested, to use for probing circuits that you are building. They are important for good measurements. If you want to see waveforms created by patches in your modular than I'd make a special cable to use for that.

A special cable that you can plug into module outputs can be as simple as a BNC cable with a male plug on the other end that matches your system. This works best if your system uses 3.5mm or 1/4" jacks. If you have a banana jack system then the cable needs to get ground from the system somewhere. For a banana system you could get away with the BNC to banana adapter as long as the scope is right next to the system AND you connect the ground jack of the adapter to a good signal ground.

A better cable would be to add a series resistor inside the male plug. If the scope has the typical 1M ohm input (it should be printed on the scope next to the BNC), then put a 9M ohm resistor in series with the hot pin inside the plug. This turns the cable into a 10:1 probe and cuts the capacitive load of the scope, giving more accurate readings. If it's not a 1M ohm scope, then use a resistor that is 9 times the input impedance of the scope.

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decaying.sine
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Post by decaying.sine » Tue Feb 16, 2010 6:38 pm

This is all perfect info. I did order the probes Paul suggested because the ones I had were falling apart, but your ideas were more along the lines of what I was originally wondering about.

Thanks for the info everyone.
Brian
"I must create a system or be enslaved by another mans; I will not reason and compare: my business is to create." William Blake

"Vactrols ringing, Dude." "Thank you Donny"

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Re: termination and reflected power in audio o-scope measure

Post by frijitz » Tue Feb 16, 2010 8:39 pm

decaying.sine wrote:when i do RF stuff with sensitive equipment, i have a nice oil, impedance matched dummy load for reflected power on the RF lines.
...is there an equivalent in audio measurement?
At audio frequencies the wavelength is so long that you don't have to worry about standing waves, reflected power, etc. IOW, as far as your scope is concerned, you are at DC.

:grin:

Ian

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