Measuring harmonic distortion - Page 1
Forum section: electronics / projects

author: grouchobyte
author_rank: Regular Contributor
timestamp: June 16, 2017, 01:31:52 am
content: I dont have a distortion analyzer but my metrology client has a distortion spec or measure of signal purity that they want me to test on a prototype I am currently designing.Question: what is the best way to measure THD? I can probably derive it from an FFT or make a measurement on a spectrum analyzer. The signals of interest range from tens of hertz to over 1 MHz. Signal levels range from -20dbm to +30 dbm

author: Keicar
author_rank: Regular Contributor
timestamp: June 16, 2017, 02:13:56 am
content: Might be a difficult frequency range to span with a single instrument - Dynamic Signal Analyzers typically top out at around 100kHz, while Spectrum Analyzers don't do the low frequency stuff.

author: Keicar
author_rank: Regular Contributor
timestamp: June 16, 2017, 02:37:35 am
content: What level of distortion are you looking to measure? If it's not too low, the 8-bit FFT of an oscilloscope might suffice.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 16, 2017, 02:49:55 am
content: Maybe eres on a scope with a decent FFT?There should be a number of 14+ bit ADCs available that would cover your bandwidth, there are many high resolution choices up to tens of MHz direct sampling, so there should be standalone ADC units that should do the job... I just don't know of any as standalone standard test equipment. Then you just need the software, and while I'm sure it already exists, it probably wouldn't be hard to rig up in matlab or somesuch.

author: cat87
author_rank: Regular Contributor
timestamp: June 16, 2017, 06:16:59 am
content: You'd basically need a THD analyzer that can do that sort of bandwidth. There aren't many out there, and the ones that are, are expensive beastsAs an alternative, you can do a quick-ish build, to having ADCs in you system, you can do this sort of thing with an RMS converter. There's a nice one from LT, the LTC1968. You can have some filters ahead of the RMS converter so that you can modify the total bandwidth, some notch filters, etc.This allows you ho have everything in mostly analog parts.But, if analog isn't what you want, then there's a niceprojecton the DIY Audio forum, detailing how to build an audio analyzer. There are also 1 or 2 similar projects there.

author: macboy
author_rank: Super Contributor
timestamp: June 16, 2017, 04:21:26 pm
content: What is the customer's spec that you need to meet? 10%? 1%? 0.001%? This is not a small detail, and will be a primary factor in the choice of instrument.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 16, 2017, 07:20:01 pm
content: maybehttp://store.digilentinc.com/analog-discovery-2-100msps-usb-oscilloscope-logic-analyzer-and-variable-power-supply/- for $280quick scan of the datasheet seems to show SFDR of ~ -80 dB which could be 'gaming the spec' since THD could be worseI'm failing so far at finding THD or hd2, hd3 plots in loopback - maybe the default sw and the 16kpt memory makes it difficult - would need averaging, stitching together many acquisitions to get actual distortion components instead of noiseolder version:

author: G0HZU
author_rank: Super Contributor
timestamp: June 16, 2017, 09:57:34 pm
content: Quote from: Keicar on June 16, 2017, 02:13:56 amQuite a few classic old school swept spectrum analysers can operate down to 50Hz with a <= 10Hz RBW. However, the sweep speed will be slow and the 2HI performance can be poor at such low frequencies.My Tek RSA3408A RTSA would do a reasonable job here but probably the most realistic suggestion so far is the Analog Discovery. Some of the high end 16bit Pico Scopes would work well here too.

author: grouchobyte
author_rank: Regular Contributor
timestamp: June 17, 2017, 04:31:16 pm
content: Quote from: macboy on June 16, 2017, 04:21:26 pmIt depends on frequency but generally no lower than 0.1% or -60db. I was thinking Picoscope. My associate has a model 4824 that I can borrow and as I recall in some joint testing we did in 2014, it had that capability@grouchobyte

author: grouchobyte
author_rank: Regular Contributor
timestamp: June 17, 2017, 04:46:36 pm
content: Quote from: DaJMasta on June 16, 2017, 02:49:55 amThanks for your advice...saw your webpage. Small world....you probably know my brother-in-lawhttp://kanatasymphony.ca/specialguests.htmlCheers@grouuchobyte

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2017, 04:53:07 pm
content: Quote from: grouchobyte on June 17, 2017, 04:31:16 pmDid I overlook it, or have you not yet mentioned the frequency range you are interested in? Is this about audio, or something else entirely?

author: grouchobyte
author_rank: Regular Contributor
timestamp: June 17, 2017, 04:58:26 pm
content: Quote from: ebastler on June 17, 2017, 04:53:07 pmQuestion: what is the best way to measure THD? I can probably derive it from an FFT or make a measurement on a spectrum analyzer. The signals of interest range from tens of hertz to over 1 MHz. Signal levels range from -20dbm to +30 dbm

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2017, 05:20:20 pm
content: Quote from: grouchobyte on June 17, 2017, 04:58:26 pmIndeed...Thanks!

author: DaJMasta
author_rank: Super Contributor
timestamp: June 18, 2017, 04:47:59 am
content: Quote from: grouchobyte on June 17, 2017, 04:46:36 pmUnfortunately not, but I'm sure it's only once removed and it's still likely to happen eventually - been to Canada a few times to play, but never met him.