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DIY Audio Analyzer with AK5397/AK5394A and AK4490
DIY Audio Analyzer with AK5397/AK5394A and AK4490
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Old 14th November 2019, 05:00 AM   #2791
1audio is offline 1audio  United States
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DIY Audio Analyzer with AK5397/AK5394A and AK4490
There are so many issues comparing noise and distortion with FFT's.
1) Make sure the input level is the same.
2) With a digital source the distortion goes down as you reduce the level (dBFS). Typically the best results are at -12dB to -18 dB. However the SNR degrades by the same amount.
3) Looking at the noise floor on an FFT is really misleading without knowing a whole lot of details- FFT resolution, windowing function etc. Use the software's calculator to get a number since it will add the amount in each bin in the listed bandwidth. Still you must compare like for like, level for level and bandwidth for bandwidth.
4) Typically a well designed analog oscillator (HP339, KH4400, Victor) will have significantly lower distortion and noise than any DAC but its limited to one or maybe 2 waveforms/signals.
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Old 14th November 2019, 06:52 AM   #2792
mbrennwa is online now mbrennwa  Switzerland
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@bozidarms: take look here. Scroll down and you'll find a loopback spectrum that was measured with your device (i.e., the one you bought from me). The noise floor is at about 10 nV, or about -160 dB below the fundamental. Maybe your (not-so) "bad" noise floor is just a simple matter of how you set up your software, or maybe the loopback wiring.
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Old 14th November 2019, 08:25 AM   #2793
chris719 is online now chris719  United States
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Demian is correct as usual. It’s also not that surprising that a specialized oscillator is better performing than an AK4490.
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Old 14th November 2019, 08:37 AM   #2794
zfe is offline zfe
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In my measurements with the RTX 6001 the noise floor of the generator was more or less the noise floor of the analyzer (with shorted or open inputs).

Your loopback should be "better", but I have no idea what is going wrong for the moment.
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Old 14th November 2019, 09:23 AM   #2795
phofman is online now phofman  Czech Republic
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I dare to say that a good DAC with properly digitally compensated harmonic distortions could compete with a good analog oscillator in plain THD (not in THD+N, of course).
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Old 14th November 2019, 09:56 AM   #2796
bozidarms is offline bozidarms  Europe
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Hi Demian
Sure, all signals was the same and measured under same circumstances (levels, scales...).For signals over 1V the difference is not so evident, but for low level signals the difference is enormous - look at curves for 6mV input signal.
THD+N 71:53, THD 111:89
There is one difference over 20dB in noise floor - conditions in measurements was the same!
This low noise floor and ability to measure low levels is that what wonder me at most, not a competition between the two oscillators.
Or to say that other way - the noise floor and SNR of 339A is always better, specially on signals under 1V. Simply i haven't expect that, have tough undoubtedly that RTX oscillator is superb in every way.


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Old 14th November 2019, 10:01 AM   #2797
phofman is online now phofman  Czech Republic
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Digital attenuation does nothing to the noise floor, unlike the analog one.
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Old 14th November 2019, 10:04 AM   #2798
bozidarms is offline bozidarms  Europe
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Can someone please, for comparison purposes, make loopback measurements on own RTX with signals under 1V (till few mV)?
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Old 14th November 2019, 10:11 AM   #2799
bozidarms is offline bozidarms  Europe
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I only ask myself why is noise floor and SNR sometimes less, sometimes more but always better on 339A oscillator, and i speak here in difference over 20dB?

Last edited by bozidarms; 14th November 2019 at 10:20 AM.
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Old 14th November 2019, 10:53 AM   #2800
phofman is online now phofman  Czech Republic
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On RTX - is there analog attenuation, or digital, or combination? Analog attenuation at the output reduces noise floor (both the signal and noise are attenuated), while the digital one does not (only the signal is attenuated, in digital domain). I wonder if that mV output signal was analog-attenuated on RTX (like it was on the analog oscillator device).
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