# quantifying noise on a DC signal

J

#### Jamie

Jan 1, 1970
0
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ
in 100 sample chunks and would like to display a calculation of the real
time noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

cheers,
Jamie

T

#### tm

Jan 1, 1970
0
Jamie said:
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ in
100 sample chunks and would like to display a calculation of the real time
noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

cheers,
Jamie

What's wrong with you? This is a group that discusses global whining,
propane
prices, water heaters, politics, religion, etc. Please try to stay on topic.

tm

J

#### John S

Jan 1, 1970
0
and don't forget the price of tea in china!

Jamie

Ignore him, Jamie. Your post was on-topic and very appropriate.

John

J

#### Jamie

Jan 1, 1970
0
tm said:
What's wrong with you? This is a group that discusses global whining,
propane
prices, water heaters, politics, religion, etc. Please try to stay on
topic.

tm
and don't forget the price of tea in china!

Jamie

D

#### DonMack

Jan 1, 1970
0
"tm" wrote in message

Jamie said:
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ in
100 sample chunks and would like to display a calculation of the real time
noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

cheers,
Jamie

What's wrong with you? This is a group that discusses global whining,
propane
prices, water heaters, politics, religion, etc. Please try to stay on topic.

D

#### DonMack

Jan 1, 1970
0
"Jamie" wrote in message
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ
in 100 sample chunks and would like to display a calculation of the real
time noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

----

Assuming you know the exact representation of the voltage signal then you
can characterize the noise as you wish. RMS, dbV, dbM, THD, or whatever is
appropriate for your context. It may be more appropriate to give several of
these measurements as each one offers a slightly different characterization.

It ultimately will depend on what you want to describe about the noise.
Which is more important? Peak levels? RMS? a logarithmic characterization?
The harmonic relationship?

T

#### tm

Jan 1, 1970
0
John S said:
Ignore him, Jamie. Your post was on-topic and very appropriate.

John

My,my. Humor impaired are we?

Of course he was on topic. It stands out like a sore thumb.

tm

P

#### Phil Allison

Jan 1, 1970
0
"John Smug"
Jamie said:
Ignore him, Jamie. Your post was on-topic and very appropriate.

** It may be on topic - but is it worded in a meaningless way.

What is "good" is both subjective and depends on the OP's secret
application.

" 100 sample chunks" is meaningless without knowing the sample rate and
nature of the noise.

..... Phil

P

#### Phil Allison

Jan 1, 1970
0
"John Larkin"
But check that the DAQ board noise is lower than what you're trying to
measure.

** FFS read the post - wanker.

..... Phil

J

#### Jamie

Jan 1, 1970
0
What's wrong with you? This is a group that discusses global whining,
propane
prices, water heaters, politics, religion, etc. Please try to stay on
topic.

tm

Signal to noise ratio in this group is good compared to some other
groups! I have learned most of what I know about electronics from the
people in this group.

cheers,
Jamie

P

#### Phil Allison

Jan 1, 1970
0
"Jamie"
Signal to noise ratio in this group is good compared to some other groups!
I have learned most of what I know about electronics from the people in
this group.

** Oh dear .....

.... Phil

M

#### Martin Brown

Jan 1, 1970
0
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ
in 100 sample chunks and would like to display a calculation of the real
time noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

Boxcar moving average and sum of squares then divide by the number of
samples and using the identity < (x-<x>)^2 > = <x^2>-<x>^2
That give the variance and sqrt give you an rms value.

NB the latter is potentially numerically unstable so its better to
subtract an x0 from all readings and add it back at the end. There is a
more correct way but for engineering purposes x0 can be your first ever
sample (or better the mean of the first throw away block).

It won't really matter for 5-10% noise on a baseline but it would matter
for 1ppm noise on a baseline where (1+e)^2 = 1 + 2e + e^2 gets bad
rounding problems on the "e^2" term as e becomes small.

Possibly also worth monitoring it for long term linear drift which is
common with thermal effects.

If you intend to look for harmonic content you might find it helpful to
use a power of two length. 2^N FFTs are common 10^n are less so.

Daqarta will actually do a realtime Fourier analysis for you with zero
effort provided that the signal is in the audio band and you don't mind
plugging it into your PCs sound card. Usually a bit of mains hum too.

http://www.daqarta.com/

Actually a very cute little program. It might even do what you asked for
out of the box in it's latest incarnation. It's a couple of years since
I last did anything with it.

Regards,
Martin Brown

J

Jan 1, 1970
0
P

#### Phil Allison

Jan 1, 1970
0
"Phil Hobbs"
Win Hill used to be a regular, and a lot of folks have learned a lot of
electronics from him.

** Another insightless, pseudo academic twit with a giant opinion of
himself.

Just like you.

..... Phil

D

#### DonMack

Jan 1, 1970
0
"Richard Henry" wrote in message

"Jamie" wrote in message
Hi,

I have a voltage signal (average 1VDC) with some noise on it, ie.
50mV-100mV peak to peak. I am reading this signal into a PC from a DAQ
in 100 sample chunks and would like to display a calculation of the real
time noise for each chunk. What would be a good way to do this? I was
thinking something like THD or RMS voltage noise but I am not sure..

----

Assuming you know the exact representation of the voltage signal then you
can characterize the noise as you wish. RMS, dbV, dbM, THD, or whatever is
appropriate for your context. It may be more appropriate to give several
of
these measurements as each one offers a slightly different
characterization.

It ultimately will depend on what you want to describe about the noise.
Which is more important? Peak levels? RMS? a logarithmic characterization?
The harmonic relationship?

What's the second harmonic of DC?

K

#### [email protected]

Jan 1, 1970
0
"Richard Henry" wrote in message

What's the second harmonic of DC?

That's easy. The second harmonic is twice the fundamental. Let DimBulb do
the math.
um... what's DC noise? Maybe you should think about what you wrote before
you click the send button?

Never heard of offset? ;-)

J

#### John S

Jan 1, 1970
0
My,my. Humor impaired are we?

Of course he was on topic. It stands out like a sore thumb.

tm

Ah! Okey-dokey. I guess I'm not the only one who forgets to use the
smiley face thing

N

#### Nico Coesel

Jan 1, 1970
0
Jamie said:
and don't forget the price of tea in china!

And my favorite: DIY projects!

N

#### Nico Coesel

Jan 1, 1970
0
Jan Panteltje said:
You mean you have somebody else do the designs for you?

DIY = fix c.q. 'improve' your home

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