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LM317/337 ripple/noise/oscillation
LM317/337 ripple/noise/oscillation
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Old 1st February 2018, 12:28 PM   #21
belyakove is offline belyakove  Netherlands
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Yes, I've just tried 1k resistor disconnected from everything, thus measuring just Johnson noise.
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Old 1st February 2018, 12:46 PM   #22
chip_mk is offline chip_mk  Macedonia
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Quote:
Originally Posted by belyakove View Post
Yes, I've just tried 1k resistor disconnected from everything, thus measuring just Johnson noise.
Noisy scope, it seems. You can try with various resistor values to learn more about the behaviour of the scope. However as it it seems you can't reliably measure noise.

Try to measure AC component with hand multimeter. Put 1uF in series to + supply than 22k to GND (any value 10-47k will do). Use multimeter to measure AC voltage across the resistor. Same procedure for - rail.
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Old 1st February 2018, 12:50 PM   #23
halfgaar is offline halfgaar  Netherlands
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What does the scope show when the probe is directly shorted to ground (clip the ground pin on the probe).
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Old 1st February 2018, 01:02 PM   #24
chermann is online now chermann  Austria
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check if the setting of the probe is set to 10k (normaly a smal switch on the probe) and the setting on the scope at the correct channel with 10x too.
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Old 1st February 2018, 01:40 PM   #25
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by chip_mk View Post
Noisy scope, it seems. You can try with various resistor values to learn more about the behaviour of the scope. However as it it seems you can't reliably measure noise.

Try to measure AC component with hand multimeter. Put 1uF in series to + supply than 22k to GND (any value 10-47k will do). Use multimeter to measure AC voltage across the resistor. Same procedure for - rail.
Isn't it almost the same as putting scope in AC coupling mode?

EDITED: Ah, you mean to measure it with multimeter.

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Originally Posted by chermann View Post
check if the setting of the probe is set to 10k (normaly a smal switch on the probe) and the setting on the scope at the correct channel with 10x too.
This one I've checked. 10x is set correctly both on the probe and on the scope.
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Old 1st February 2018, 06:01 PM   #26
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by halfgaar View Post
What does the scope show when the probe is directly shorted to ground (clip the ground pin on the probe).
Here's image of what it shows when shorted. Peak to peak voltage is fluctuating from 10mV to 15mV
Attached Images
File Type: png DS1Z_QuickPrint3.png (46.9 KB, 294 views)
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Old 1st February 2018, 06:34 PM   #27
tomchr is offline tomchr  Canada
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What's the bandwidth of the scope? >88 MHz?

I the past two places I've lived, I was able to pick up about 20 mVpp of FM radio on a 400 MHz scope with a scope probe shorted to itself. The signal is picked up by the ground lead usually.

You can try a couple of things: Twirl the ground lead around the scope probe as tightly as you can. Connect the ground clip to the probe tip. This minimizes the loop area, thus the inductive pickup of RF. If this lowers the noise level, you're dealing with inductive pickup. Do your best to minimize the loop area when you take your measurements.

Engage the bandwidth limiter on your scope. If your scope has a bandwidth of 100 MHz or higher, you'll probably have a button you can push to limit the scope bandwidth (thereby the noise bandwidth) to something more reasonable - like 20 MHz.

If none of those work, use a shorting BNC plug (or 50 Ω BNC terminator) to short the input on the scope. If this causes the noise to drop, you have an issue with the probe. If shorting the scope input does not lower the noise, the front-end of the scope is noisy.

Tom
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Old 1st February 2018, 07:45 PM   #28
MarcelvdG is offline MarcelvdG  Netherlands
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Originally Posted by belyakove View Post
Tried measuring in on and off state. Not much changes in on and off state and during capacitors discharge. Here are images of these two measurements. First is in on state and second is in off state.
If the plots look practically the same with supply on and off (with capacitors empty), then I wouldn't worry about it. Apparently it's some interference picked up by the supply wiring.
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Old 1st February 2018, 08:37 PM   #29
MarcelvdG is offline MarcelvdG  Netherlands
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Off topic:
@tomchr: Could you put the numbers you put below your posts in this thread: How low can you go? ? The thread starter tries to collect numbers with lots of zeros.
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Old 1st February 2018, 10:01 PM   #30
tomchr is offline tomchr  Canada
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Done. Thanks for the poke.

Now back on topic: I think it would have value to figure out if the probe is bad or if the scope is just noisy. If the scope is noisy, it would be nice to know about it so those of us who care about noise performance get to know which scopes to avoid.

Tom
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