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How to get a cleaner sine signal from a waveform generator?

Hi all,

I'm using a 2-channel waveform generator (YOKOGAWA FG120) to generate low frequency(~ 10Hz), low amplitude (40mVpp) sine signals, which served as differential inputs for measuring a high precision sigma delta ADC (~100dB SNR).

Now, My test result is not very well. I think it's maybe due to the impurity of the input signal, whose harmonics and noise will deteriorate the performance.

Besides buying a more precision generator..., is there way to get a cleaner sine signal from the generator, say, harmonics decreasing from

-55dBc to -85dBc or lower noise?

Does adding bandpass filter work? or using lock-in amplifer?

I need your opinions. Thanks a lot...

Regards,

Kanjun Shi

Reply to
kanjun.shi
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On a sunny day (Tue, 3 Feb 2009 08:26:18 -0800 (PST)) it happened snipped-for-privacy@gmail.com wrote in :

Lowpass

Reply to
Jan Panteltje

Of course... why wouldn't.

Reply to
Jon Slaughter

Any measure you use to improve this is going to be subject to difficulties due to that extreme SNR that you're measuring against. So if you go looking into bandpass filters or lock-in amplifiers you need to make sure that your follow-on block doesn't generate at least as much distortion as it is eliminating. So you can't just sling a solution in the general direction of your workbench and expect it to improve things.

Have you obtained a model with their "Low Distortion Output" option?

Depending on the nature of their output amplifiers, you may be able to improve things by using a resistive attenuator and setting the generator to a higher voltage output -- if they go straight from their amplifier to their output, and they have some crossover distortion, then the attenuator may help. If they _do_ switch in an attenuator, using your own 'fine tune' attenuator (I'd try steps of 1dB) in conjunction with increased voltage output may find a sweet spot in their output amplitude that has lower distortion.

I have to say, if all I ever had to do was this one measurement I'd be tempted to build a Wien bridge oscillator followed by a long bandpass chain (and probably powered by batteries at first); it's a rocks-and-sticks solution, but sometimes those are the solutions that get you the most bang for your buck when distortion and/or speed are the highest priority.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
Reply to
Tim Wescott

.

d text -

Very thanks for your reply!

I will try the attenuator way...

Reply to
kanjun.shi

text -

TTE Inc used to make really low frequency all-passive LP filters. They'll probably distort at some level due to the onset of magnetic saturation, but they worked _great_ back in the day. I used a couple of linear-phase 35 Hz lowpass filters on the same piezo stage I mentioned a few minutes ago. IIRC all their low frequency types are now active. A pity.

Still, 'twere it me, I'd try using a single-section LC lowpass made from the HV secondary winding of a small tube power transformer and an appropriately chosen cap. I agree with Tim about the attenuator, but do put the filter after the attenuator--it'll help the distortion a lot.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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