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Old 28th December 2011, 09:10 AM   #111
sbrads is offline sbrads  United Kingdom
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I haven't read the whole thread but I think I've got the gist of it and I would like to add a little advice on scopes and how to NOT pick up stray noise. I have to look for small signals immersed in SMPS noise at work.........

1. You have to have an extremely short scope probe earth lead. The usual few inches acts as a nice aerial. We regularly use tip & ring, i.e. tinned copper wire twisted round the probe shield so the earth lead is about 1cm long max.

2. Wind the scope lead at the scope end a few turns through a ferrite toroid. It greatly reduces common-mode pickup from external sources without affecting the real signal at all (I've tested this!). Same with coax leads if that's what you're using. One toroid each end is even better if there's enough lead length.

Apologies if you're already doing all this.

Last edited by sbrads; 28th December 2011 at 09:13 AM.
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Old 28th December 2011, 09:35 AM   #112
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thanks for the tips sbrads!! one of my scope probes I can probably do the short earth wire thing with but the other one has an approx 25cm earth lead and is moulded so can't be changed without cutting it. I think I have a small torriod core in my junk box too

Tony.
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Old 28th December 2011, 10:33 AM   #113
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Quote:
Originally Posted by wintermute View Post
The fn9244-1-06 seems to fit the bill nicely. I'm guessing that the bleed resistor may improve rejection at certain frequencies.
Yeap, that's the line I've been using.
Also check FN9244E-1-06 - though I bought the last one last night from Mouser.
They'll have more in stock in 10 days or so.
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Old 28th December 2011, 10:54 AM   #114
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hmm that's nice and not too much more expensive either. Thanks! I'll be ordering from a place in aus, and they have 26 in stock though they don't seem to have any 3A ones though (for another project).

Tony.
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Old 28th December 2011, 01:51 PM   #115
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yeah, that was the reality
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Old 1st January 2012, 01:20 PM   #116
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OK so tonight I did some more measurements in the case with the IEC filter connected. The results are much better

I tracked the weird waveform (in post 92) down to the spare PC (or it's monitor) So I know to make sure it is off if I am doing any measurements.

First pic is of the output of the positive rail. 2mV/div 1ms/div

Second pic left is +10V rail right is -10V rail recorded in audacity, 10X gain on the preamp. the dreaded 50Hz anomaly is back though, but can be ignored, its probably an earth loop between the PS and the PC.

Third pic is taken on the 3r3 resistor at the output of the second cap. This is to get a reference point for the pc measurements.

Fourth pic is the spectrum on the pc for the same point as the thrid pic. 10X gain on the preamp. So 3mV p2p is about -48db on the spectrum.

Overall I'm pretty pleased with these results though I will have to take some more with a real load. The built in pre load resistors only draw about 38mA.

Tony.
Attached Images
File Type: jpg DSC_0128.jpg (78.5 KB, 328 views)
File Type: png yarps_in_box.png (20.3 KB, 319 views)
File Type: jpg DSC_0136.jpg (91.7 KB, 315 views)
File Type: png yarps_2nd_resistor.png (20.2 KB, 315 views)
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Last edited by wintermute; 1st January 2012 at 01:24 PM. Reason: upload 2nd two attachments.
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Old 1st January 2012, 09:32 PM   #117
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Tony, would you mind taking some more measurements? ARTA Download - this software should help you. It would be also intersting to see loopback measurement of your soundcard.
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Old 2nd January 2012, 12:19 AM   #118
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Hi Dunwenbleidd which measurements would you like? Attached are loopback measurements done in ARTA (I have 1.7.0 latest is 1.7.1)

1st is spectrum analyser mode.
2nd is an fr plot.

I set the levels using holm impulse to the highest reading I could get without it telling me it detected digital clipping.

I'm not sure how I can use ARTA for doing measurements of the PS though

Tony.
Attached Images
File Type: png audigy_loopback.png (34.6 KB, 278 views)
File Type: png audigy2_fr.png (21.6 KB, 75 views)
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Old 2nd January 2012, 12:48 AM   #119
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It's just the same as measuring in loopback. A little example follows - LM317 with Vbe multiplier. Maybe I've made a mistake somewhere, but I have calculated that it shows 5,6uV (at 50Hz)... That doesn't seem quite possible with such a simple design...
Tomáš
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Old 2nd January 2012, 01:01 AM   #120
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I'm not familiar with this... what is the function of these Arta?
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