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Discussion Starter #1
Hi all,

Call me Rusty... I'm trying to set up my Oscope for measuring noise (alt noise specifically)

Well, wouldn't you know it's been 15yrs since I last turned one on and can't for the life of me remember proper set-up...

I'm sure it's like falling off a bike, once I do it once I'll be fine.. it's just a matter of getting it right the first time..

Here's what I'm working with..







Anyone care to lend me a moment...

Thank you..
 

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if all you want is the AC component of the signal, then make sure you set it to AC coupling.(this will de couple the DC off the signal)

start at 1mS. 10-100mV/div (adjust as needed :D )

set probe to x1, unless you expect noise to be in the "volts" range

make sure your trigger source is chA and that you are not using any math on it. I wouldnt even worry about triggering from an external channel.

also make sure all your "cal" knobs are set to "cal" or readings will be garbage.


let me know if you need more than that.
 

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Using Mini's instructions above, I would grab a test cd and find the 1Khz track and set to repeat.

Once this has been set up, just test the outputs from the deck with the engine off. Tweak the o'scope for the highest resolution and get looking clean on the crt. Then start the car and see if the noise is apparent with at the deck.

If no noise, move back in the signal chain. If the noise does originate at the deck, you likely have a differential in ground. I would grab a scrap piece of 14 gauge or better and connect it to the same ground and were you grounded the cap, then strip back the head unit ground and unify them. Just my two cents...
 

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Using Mini's instructions above, I would grab a test cd and find the 1Khz track and set to repeat.

Once this has been set up, just test the outputs from the deck with the engine off. Tweak the o'scope for the highest resolution and get looking clean on the crt. Then start the car and see if the noise is apparent with at the deck.

If no noise, move back in the signal chain. If the noise does originate at the deck, you likely have a differential in ground. I would grab a scrap piece of 14 gauge or better and connect it to the same ground and were you grounded the cap, then strip back the head unit ground and unify them. Just my two cents...

but he wants to probe the output of his alternator.
 

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That would be kind of a moot point. Alternators are inherently noisy. If it is an aftermarket HO alternator, they are worse....the ripple at low rpm tends to be a bit crazy.

The charge state of the battery will have alot to do with the filtering capability. So if the battery is presenting a higher than normal load (alternator being load dependent) it (the battery) will pass more of the noise through the electrical of the car.

Aaron, if you have been doing alot of testing of the system without going out for some nice long drives, you will cause some intermittent noise issues. Granted the cap should help with the ripple, it is not an end all be all.
 

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Use only one input channel. Insert the probe into left “Y” input and set the mode switch in “CH A” . Now the right lowest part of knobs (all within CH B frame) is nonfunctional.
If there is attenuator on the probe, set it to “x1”.
Put the left AC/GND/DC switch in AC position to see only AC component of measured signal.
These red knobs must be turned in clockwise max position .
By left knob “CH A” you can choose the sensitivity in Volts per division (division is the one square).
You can start from 2V/DIV and then increase the sensitivity. 20mV is the highest sensitivity. In other words it is vertical scaling
For example: you have 5.1 divisions between min and max levels of displayed signal, with settings 50mV/Div. Then the measured AC peak-to-peak voltage is: 5.1*50mV=0.255V

Horizontal scaling, or speed of horizontal sweep is being chosen by sweep knob.
1Hz – 1second
50Hz - 20ms
100Hz-10ms
1000Hz-1ms
5kHz-200mks
Let say you have a cyclic signal, 2.8 horizontal divisions between peaks, with the sweep knob been set to 2ms/div. Then frequency can be calculated as :
1(sec) / 2ms / 2.8=178 Hz.
For alternator 400-500Hz you can choose
1sec./400Hz /4= 0.00065sec. or 50mks-1ms.
4 is desirable width on display in squares.


“Source” of horizontal sweep is internal generator as it is set on your picture.
Horizontal sweep may run constantly (pulled “Level” knob) or can be synchronized by trigger.
Trigger can be pulled by positive or negative edge of measured signal (slope switch) .
Sensitivity of the trigger can be adjusted by “Level” knob.
Trigger helps to see repetitive but not stable signals or short pulses.
Sometimes you have to change the speed of horizontal scanning to make trigger to work with particular signal.
Very short pulses can be barely visible as vertical lines or even plots. Signals with too high frequency or random noise will produce just blurred field.

Try it first at home with any signal source: noise induced by body, player output, charging device, etc…
 

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Discussion Starter #8
Thanks everyone.. huge help.. like I said, I've done this a LOT, it's just been a LONG time since I've done it... :laugh:

That would be kind of a moot point. Alternators are inherently noisy. If it is an aftermarket HO alternator, they are worse....the ripple at low rpm tends to be a bit crazy.

The charge state of the battery will have alot to do with the filtering capability. So if the battery is presenting a higher than normal load (alternator being load dependent) it (the battery) will pass more of the noise through the electrical of the car.

Aaron, if you have been doing alot of testing of the system without going out for some nice long drives, you will cause some intermittent noise issues. Granted the cap should help with the ripple, it is not an end all be all.
Yep, it's an aftermarket alt, I'm getting alt whine through the system, with the system OFF...

I recently installed a capasitor under the hood to try and emulate the battery that is now in the trunk...






My train of thought is that the cap would act as the filter that the battery is no longer, because of it's placement...

I didn't get very far with testing, but what I found was the "alt whine" was replaced with straight up static... :(

I also found another GD wheel bearing going out on me, which spells the end of my season...

I will try testing the alts output first...
 

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Yep, it's an aftermarket alt, I'm getting alt whine through the system, with the system OFF...
The only way that is possible is that your speaker wire or crossovers are picking up the noise. Disconnect them from the amp(s). does the sound persist? If so you have a bad amp that isn't turning off properly, which in turn means you need to replace it or repair it.
 

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The only way that is possible is that your speaker wire or crossovers are picking up the noise. Disconnect them from the amp(s). does the sound persist? If so you have a bad amp that isn't turning off properly, which in turn means you need to replace it or repair it.
can disconnect main power from the amplifiers too and see if it quits. if the noise is still there after that, you have something coupling that noise to the speakers.
 
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