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Old 24th July 2014, 02:13 PM   #1
udok is offline udok  Austria
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Default Any RMS Millivoltmeter recommendations?

Hi all,

I am looking for a decent True RMS Millivoltmter for measuring low level audio and noise signals. New preferred.

I want to measure:
- Amplifier output noise
( need adjustable high- and lowpass filters, A-weighting, CCIR)
- Flicker Noise of FETs (should be able to measure down to uV)
- PSRR of amplifiers
- exact RMS value of low level signals
- filter transfer function measurements with a signal generator (up to 100 Volt)
- balanced or floating input
- better than 0.1%stability and quality housing
- bandwidth 10Hz - 100 kHz at least
- low noise input (< 2uV rms)

I looked at Agilent.com but did not find anything at the first glance...

What do you use for low level Audio/Noise measurements?

Cheers,
Udo
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Old 24th July 2014, 02:38 PM   #2
AndrewT is offline AndrewT  Scotland
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One of our Members is selling PCBs for a very comprehensive very low noise, uVac meter that is RMS scaled and does much else in the way of filtering etc.

There are a few others that do a 40dB or 60dB wideband amplifier (LNA) to boost the signals prior to measurement with a less sensitive rms scaled instrument.
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Old 24th July 2014, 03:08 PM   #3
udok is offline udok  Austria
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Quote:
Originally Posted by AndrewT View Post
One of our Members is selling PCBs for a very comprehensive very low noise, uVac meter that is RMS scaled and does much else in the way of filtering etc.

There are a few others that do a 40dB or 60dB wideband amplifier (LNA) to boost the signals prior to measurement with a less sensitive rms scaled instrument.
Can you give me a link?
Thanks,
Udo
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Old 24th July 2014, 03:22 PM   #4
AndrewT is offline AndrewT  Scotland
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just enter lna
Quote:
Simple 60dB discrete low noise amplifier (lna) (Multi-page thread 1 2 3 ... Last Page)
iko

19th June 2014 07:04 AM
by EUVL
You have 4 post(s) in this thread, last 20th February 2011 Arrow
New DIY Project for measuring noise : 10Hz..100kHz LNA // 1MHz RMS-DC converter (Multi-page thread 1 2 3 ... Last Page)
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24th May 2014 02:46 PM
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Old 25th July 2014, 06:15 AM   #5
udok is offline udok  Austria
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Thanks Andrew!

Any other recommendations? Or has anbody else an audio precision two on the desk?

Cheers,
Udo
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Old 25th July 2014, 06:36 AM   #6
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Quote:
Originally Posted by udok View Post
Thanks Andrew!

Any other recommendations? Or has anbody else an audio precision two on the desk?

Cheers,
Udo
You can built this.Digital audio millivoltmeter by Silicon chip.
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Old 25th July 2014, 10:05 AM   #7
udok is offline udok  Austria
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Thanks Thimios!

I want something which is tested and specified...

Something similar to MN-447A 2ch Noise Meter / MEGURO ELECTRONICS CORPORATION

or HP3410a
HP 3410A AC Microvoltmeter Restoration ** In operation added** - Page 1

There is even a description in HP journal 1967-05 how it works :-)
Measure Magazine

EDIT: The Link is wrong - correct one is: http://www.hpl.hp.com/hpjournal/pdfs...ndex.html#1967 (click on May-1967).

It should have a decent modern display and an true RMS cirucit based on modern 16 bit ADC chip :-)

Cheers,
Udo

Last edited by udok; 25th July 2014 at 10:27 AM.
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Old 25th July 2014, 04:51 PM   #8
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Most hand-held DMM's RMS detectors are only accurate to a few kHZ -- you will get much better results with a legacy instrument like the HP 3478, a bench top unit which now sells for less than $100. The HP3478 uses a true-rms detector from Analog Devices.

I have an HP403B which wasn't working so I fixed it up with a very low noise front-end using a THAT1510 (like the SSM2019) -- noise is less than 1nV/Rt Hz. The amplifier feeds a Linear Tech LTC1968 true rms converter and another op-amp to drive the meter.

If you're going to measure noise, remember that you will need a filter -- A-Weight, CCIR etc so that you can compare your results to another persons. The Audio Precision has narrow bandpass filtering in both digital and analog modes.

If you're interested in measuring semiconductor noise -- take a look at this paper on measuring below 1Hz -- it uses the TI OPA4140 quad JFET amplifier:
http://jci2012.sciencesconf.org/conf...port/12266.pdf
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Old 26th July 2014, 03:44 AM   #9
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Quote:
Originally Posted by udok View Post
Gotta love a Robrenz restoration. That man is a GOD at restoration, he's a master machinist and as such has some amazing precision manufacturing skills. Check out his youtube videos sometimes to see some amazing work.

Quote:
Originally Posted by jackinnj View Post
Most hand-held DMM's RMS detectors are only accurate to a few kHZ -- you will get much better results with a legacy instrument like the HP 3478, a bench top unit which now sells for less than $100. The HP3478 uses a true-rms detector from Analog Devices.
The 3478A uses the AD536 which has a -3dB bandwidth of 450kHz. Most decent handheld meters now use AT LEAST the AD636 or AD637 which are 600kHz and 900kHz respectively. Some budget meters use the AD736 or AD737, which at worst have over 100kHz of bandwidth. Not to hate on the 3478A which is a solid meter for a variety of reasons. Just that its not for this reason.
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Old 26th July 2014, 05:54 AM   #10
Frex is offline Frex  France
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Arrow ERMSDCV2 LNA+RMS/LOG converter box projects

Hello udok,

I'm one of designers that AndrewT speak about in it's first answer(hello to him !). I have designed for noise measurements purpose a small box that include :
- A 10-100kHz bandwidth very low noise amplifier (En<500nV).
- A DC-1MHz RMS/DC converter with DC and LOG output (direct in dBv).
(I sell the bare PCB, some are available).

I recommend you to read the first pages of this thread, you will see how it looks. As you want a fully tested/specified device, you will see in page 2 of the thread a very comprehensive measurements results.

Note that many low cost RMS voltmeter are not able to measure precisely noise, because crest factor of the measured signal significantly affect the precision.
(For the MN447A, crest factor <2, precision is +/-10% for 10Hz-50kHz bandwidth in scale above 30V !)
So, you must look many parameter to be sure that the intrument will be suitable for your need.
Regards.

Frex
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