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Products => Test Equipment => Topic started by: JanJansen on June 06, 2017, 03:28:56 pm

Title: How to get 130mv rms with old scope & meter ?
Post by: JanJansen on June 06, 2017, 03:28:56 pm

Hi, i need to adjust noise level to 130 milli volt r.m.s.
I only have old Phillips scope & old multimeter without r.m.s. funtion.
How do i measure that ?, do i need that expensive DMM with RMS function ?

thanks in advance

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: barry14 on June 06, 2017, 11:04:06 pm

If you need good accuracy (say 5% or better), then an RMS meter would be required.  If you just want to be in the ballpark (25% or so), then you can use your oscilloscope if the bandwidth of the oscilloscope is at least the bandwidth of the noise (higher bandwidth would be better).  Adjust your noise so that the peak-to-peak level of the noise is about 5 times your desired RMS value (this is somewhat of a judgement call because some of the noise spikes could exceed that value), and your display will depend on the brightness level (you said an old scope so I assume it uses a CRT).

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: JanJansen on June 07, 2017, 03:47:40 pm

Hi, thank you, yes i use analog scope.

In a while i will buy a nice multimeter with RMS, so i just need to level it about right, then later i make it perfect.
Cant i go measure AC voltage with my multimeter ?

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: barry14 on June 07, 2017, 11:20:19 pm

You did not specify the bandwidth of your noise.  Most ordinary DMMs have very limited bandwidth (some as little as 500 Hz) and they do  not measure true RMS.  Thus they are not very useful for measuring noise type signals.

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: egonotto on June 08, 2017, 12:22:49 am

Hi,

this video shows you how to measure noise  rms with an analog scope:

https://youtu.be/zHU9xdU51FY

Best regards
egonotto

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: David Hess on June 08, 2017, 09:55:42 am

If egonotto had not linked that video then I would have.  You need a dual trace oscilloscope to do tangential noise measurement but it works great.

Offhand I do not know the correction factor for measuring RMS noise on an average reading voltmeter but there is another way which may be easier for you.  Measure the absolute value of the RMS noise using your oscilloscope once as shown in that video, and then measure the noise using your average reading AC voltmeter.  Now you have a calibration for your AC voltmeter and you can fiddle with the noise level and read the output on your voltmeter without repeatedly doing the steps necessary on your oscilloscope.

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: P90 on June 08, 2017, 10:24:03 am

throw in the factor that most True RMS multimeters aren't AC+DC, but rather AC coupled.

Title: Re: How to get 130mv rms with old scope & meter ?
Post by: JanJansen on June 08, 2017, 01:18:12 pm

Nice video,
i tryd all switches on my scope, i cant find any dual traces.

I keep finding more reasons to buy a good DMM.