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Building a phase noise measurement system for digital audio
Building a phase noise measurement system for digital audio
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Old 20th November 2017, 08:41 AM   #51
TNT is offline TNT  Sweden
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All these 1ps specs and measurements are in a silent environment (headphones). What happens to these figures when the oscillators are exposed to sound pressure and/or vibrations (loudspeakers)?

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Old 15th February 2018, 12:32 AM   #52
zenelectro is offline zenelectro  Australia
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Quote:
Originally Posted by 1audio View Post
Biggest challenge is the lack of mass of crystals or crystal oscillators. Maybe imbedding in a cast iron block on even rubber would be enough. Wenzel has stuff on active vibration stabilization. The crystals are more sensitive in one axis so its less of a challenge than it first seems.

Sent from my LG-H811 using Tapatalk
Yes, the oscillator needs mass and most likely a significant amount. However as soon as a spring and weight interact = resonance. So IMV some form of damping is also required.

Possibly some form of lossy, damped spring material in conjunction with a lead/steel weighted oscillator can could get desired results.

T
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Old 15th February 2018, 01:58 AM   #53
gerhard is offline gerhard  Germany
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A presentation of Mr. Driscoll himself, also covering mechanical noise.
Note that his intended application is fighter airplanes and 'copters.

< https://www.google.de/url?sa=t&rct=j...jBJe0drYR2MHlY >

I hope that this monster link works.

To get a feeling what 1ps means: I have a scope plugin that has 9 ps risetime.
This requires 50 GHz of bandwidth to achieve. Ask yourself if your homebrew stuff
has a stability of 1/9 that value.

A good & cheap entry into the phase noise arena could be the measurement of added
phase noise of buffers or sine/CMOS converters. That can be made for less than $50.
All you need is a power splitter, a ring mixer, a lambda\4 delay line, aka 10 meter
of coax cable, a sensitive preamplifier and something that can do FFTs. For gawd's
sake, a sound card.

You don't need more than 1 oscillator, no PLLs to lock oscillators, no cross correlation.
Just optimizing that sine to cmos converter can easily keep you busy for a year.
Look at the pace of that oscillator thread.

BTW. Using a spectrum analyzer to measure phase noise of a good crystal oscillator
is hopeless, unless it's a R&S FSUP that has dual receivers and cross correlation.
You will measure the noise of the spectrum analyzer's sweeper. That might be good
enough for measuring a free running VHF VCO.

I would like to draw your attention on the Red Pitaya. It has 2 14 Bit / 125 MSPS ADCs
and an external clock input. (and DACs, and Linux, and network, and is cheap, ...)

On that hardware, you could do most things that a Timepod can do. It's only software :-)
Use the clock input for your oscillator, the 2 ADC inputs as references. They don't
even need to be on the same frequency. Do cross correlation as much as you want.

< Red Pitaya >

cheers, Gerhard
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Last edited by gerhard; 15th February 2018 at 02:26 AM.
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Old 16th February 2018, 05:51 PM   #54
jdlvfr is offline jdlvfr  France
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Gerhard thanks for the link. The red Pitaya looks very interesting!

Today I made some phase noise measurements with 2 oscillators and a pll while listening to music, without music and last one dropping the mouse (the computer one!) from 5 cm height.
Vibrations are clearly visible as added phase noise.
To have the phase noise in dBc one should remove 60dB from the yellow curve.
Nothing have been made against vibration, oscillators just sits on the table.

JoŽl
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File Type: png with_music.png (37.1 KB, 231 views)
File Type: png no_music.png (38.6 KB, 228 views)
File Type: png falling_mouse.png (38.2 KB, 226 views)
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Old 16th February 2018, 07:25 PM   #55
TNT is offline TNT  Sweden
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Hmm, cant see a yellow curve... 8-o

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Old 16th February 2018, 07:46 PM   #56
jdlvfr is offline jdlvfr  France
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yellow would it be green ???

JoŽl
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Old 16th February 2018, 08:33 PM   #57
TNT is offline TNT  Sweden
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You tell me I see a Green, blue and grey(-blue...ish).

Do you play spinning discs och ripped files?

I sometimes has a bit a of a green/red problem

Green? -> (the arrow is red!)
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Old 16th February 2018, 09:11 PM   #58
jdlvfr is offline jdlvfr  France
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Thank you, now I can name the colours! I already knew the sky is yellow and the sun is blue. Or something like that, not really sure, maybe it depends on days...?

So green curve is noise within 1 Hz bandwidth, dB/sqr(Hz), blue is RMS voltage in dB and grey-blueish is average when display resolution is lower than frequency resolution.

JoŽl
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Old 22nd February 2018, 08:32 AM   #59
gentlevoice is offline gentlevoice  Denmark
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Building a phase noise measurement system for digital audio
Hi all,

Hmmm ... I realize that JoŽl is underways with sharing/explaining his system setup (and I am personally interested in what he can say here), however, I would like to add that I have built Herbert Rutger's DC-Receiver with some additional features that I reckon could be interesting in the context of a flexible phase noise measurement system.

My aim with these additions to Herbert's design has been to be able to evaluate most all of the individual circuit parts of an oscillator so as to test and optimize each individual part. FYI I have attached a picture of the DC-Receiver PCB.

As it is I have not yet had the chance to really try out the DC-Receiver (I a.o.t. need a good-enough ADC to input the DC-Receiver's output to my PC) - yet it is my impression that the PCB is correct. It is rather big, though - so as to provide some spacing between the various circuit sections, and also because it fits into a steel enclosure I already have of this size.


That said the ideas with this PCB have been:

- The cut-outs in the PCB are for mounting an oscillator PCB, a squarer PCB, and a divider PCB (right to left - top right corner). Traces are kept so as to be 50 ohm impedances. On the picture is shown one of Andrea's Driscoll oscillators - which I had already and have used to do preliminary tests of the DC-Receiver.

- To supply the DC-Receiver & DUT oscillators/dividers from batteries. Additionally, to also allow for supply voltage adjustment - as a part of assessing phase noise/jitter relative to supply voltages - there are four very simple, low-noise & adjustable PSUs on the PCB. Basically emitter followers with decoupling and an MPSA18. It is possible to switch between pure battery supply (individually for each circuit part) and these adjustable PSUs. Nothing fancy but low noise, no ringing, and reasonably low output impedance PSUs.

- Close to the right side of the ring mixer (metal part in the center) there are two 50 ohm attenuator resistor networks so that precise adjustment of the DUT oscillator output level is possible. I chose to not place a "jumper-connector" (don't know what the English word is) at this place so here a small piece of wire has to be soldered to connect with the attenuator network used.

- Herbert's "listening circuitry" is also on the PCB (bottom right corner with the potmeter).

- Courtesy a suggestion by 1audio the PLL input to the VCO (Herbert's Clapp oscillator - to have a known reference) can also be switched to a fixed voltage set by a filtered trimpot so that any very low frequency influences from the PLL may be avoided.

- There may also be included a divider circuitry in series with the reference oscillator (/2, /4 - above the ring mixer) so as to test frequencies e.g. where two identical crystals are not available.

Well, this is FYI and possibly for inspiration ... Also, should any of you like a copy of the PCB layout you are welcome (e.g. pdfs of top & bottom).

Cheers,

Jesper
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Old 28th February 2018, 05:57 PM   #60
jdlvfr is offline jdlvfr  France
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Hi,

here is the schematic of a phase noise meter.
Actually I use a different and much more complex low noise 60dB amplifier with FET input for high input impedance but finally high input impedance is not necessary.
50Ω loading at DC just change the change the phase to voltage conversion form 0.6 volt/radian to 0.3 volt/radian.

Mixer is SRA-1H form minicircuits, 1.9MHz filter is from minicircuits too.
OPA 1611 is low noise bipolar opamp, OPA 1641 is low noise JFET.

The M-off-M switch is useful to set the integrator output to manually help the PLL locking.

Attached is the Wenzel paper "Low cost phase noise measurement" valuable source of inspiration.

Actually I use a DIY DC coupled ADC. Output is optical SPDIF for full galvanic isolation. Schematic is attached.

JoŽl
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File Type: png phasenoise.png (53.2 KB, 55 views)
Attached Files
File Type: pdf Low_cost_phase_noise_measurement_Wenzel.pdf (256.9 KB, 19 views)
File Type: pdf DC-coupled-ADC.pdf (110.7 KB, 21 views)
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