Is this supply modulated jitter ? - diyAudio
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Old 29th April 2012, 04:09 PM   #1
percy is offline percy  United States
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Default Is this supply modulated jitter ?

Attached below are a couple of screen shots of a FFT of a 1Khz signal coming out of the analog outputs of a consumer major brand stereo receiver. The receiver has a stereo dac and the signal was played on a bluray player and fed to the receiver via optical spdif.
There are ~60hz sidebands around the fundamental and those are what I am asking about. Could this be from the source ? or the dac section ?

Note that they are well below 16bit noise floor, so are they worth worrying about ?
Attached Images
File Type: png jitter.PNG (60.1 KB, 104 views)
File Type: png jitterzoom.PNG (55.5 KB, 103 views)
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Old 29th April 2012, 04:19 PM   #2
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I'd suggest they're power supply related - probably common ground impedance coupling or perhaps magnetic coupling direct from the transformer. They are visible as 60Hz plus harmonics on your first plot and at a higher level there (above -100dB).
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Old 29th April 2012, 04:49 PM   #3
jcx is online now jcx  United States
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if the test disk signal is clean and you have a digital interface then the side bands can only be introduced in the receiver - they require a multiplying/mixing process which can only happen in DAC stage or subsequent analog stages - you can't "mix in" 60 Hz error into the spidf bit stream

could be the player's clock has PS jitter - did you look at the player's analog out?

at the DAC input PS correleated "interface" jitter is possible but I'd guess the errors are most likely due to Vref contamination or PSRR in the DAC chip

the 60 Hz harmonics could be test setup issues or a clue to the receiver's problems

while not impressive I doubt the sidebands are readily audible - they are way below the masking threshold, expected to scale with signal amplitude - I'd work to get rid of the 60 Hz and harmonics 1st if the room were quiet enough to care

the -80 dB 2nd harmonic is evidence of nonlinearity that will give IMD products with complex signals that would be expected to be larger, not masked

Last edited by jcx; 29th April 2012 at 05:08 PM.
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Old 29th April 2012, 05:09 PM   #4
percy is offline percy  United States
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yes original signal is very clean and its a digital (optical) interface.
I recollect noticing that the receiver was very quiet with no-signal and switched to analog inputs. Just a barely noticeable 60hz spur by itself and none of those lower order harmonics. I think its something to do with just the supply to the dac.

On the flip side - there is no sign of other jitter - periodic or random(noise floor doesn't increase). So if I could take care of this 60hz contamination then this is just spectacular wouldn't you say ?

Last edited by percy; 29th April 2012 at 05:12 PM.
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Old 29th April 2012, 06:24 PM   #5
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Jitter should be bands close to the signal. Something like below (an extreme example).
The mains frequency should be around 120Hz (bridge double rectifying).
Attached Images
File Type: png jitter.PNG (52.3 KB, 83 views)

Last edited by SoNic_real_one; 29th April 2012 at 06:27 PM.
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Old 29th April 2012, 09:16 PM   #6
percy is offline percy  United States
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the first image below is of the noise floor of the receiver with no signal in. I would imagine it has full wave rectification so I'd suspect the two 60hz and 180hz spurs are probably due to interference.

JCX, below are two additional images for the player's analog out. Clearly ~3.85Khz jitter, haven't calculated the rms value yet. One is a full scale image and the other is zoomed in to show the fundamental and jitter sidebands.
Attached Images
File Type: png noisefloor.PNG (54.7 KB, 70 views)
File Type: png playerjitter2.PNG (57.3 KB, 69 views)
File Type: png playerjitter.PNG (47.8 KB, 33 views)

Last edited by percy; 29th April 2012 at 09:19 PM.
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