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|3rd March 2016, 05:38 PM||#1|
EOSC10Kv3 - LT AN67 10kHz oscillator : new updated version !
Some years ago now, i showed to DIYers in this forum my own build of the
AN67 LinearTech application note high performance ultra-pure sine oscillator.
I used it to check the performance of my EADCAKMv1 ADC project (based on AK5394A).
Many DIYers were interested to get the PC to build it, and so i decided to organize
some group buy to sale this PCB.
Now, i continue to receive request to provide the bare board of this oscillator regularly.
The concern is that the LT1230 become hard to find, or, at prohibitive price.
Thus, i decided to update the design of the oscillator, replacing LT1230
by the easy to find LT1229 and using SMD parts.
Some improvements listed below has been done :
I have received the prototype PCB last week, it look like this :
The PCB dimensions are 92x75mm, intended to fit in the small
aluminium enclosure (1455J1201 from Hammond).
After some painstaking soldering work (i like that!), it give a fully populated PCB :
In the Hammond enclosure ..
Of course, now the important thing to do is to test it's real performance !
Measuring a THD below ppm level with some confidence is an hard task.
Many parameters can corrupt results.
So, much care has to be done before choosing measurements method.
The measurement below, show the FFT result of the oscillator output
connected to my EADCAKMv1 ADC through a passive stepped attenuator.
The attenuator was set to -10dB attenuation to find the lowest thd point
of the ADC. (the measured THD come mainly from the ADC itself).
The EOSC10KV3 board is set to 1Vrms at output (single ended).
It is supplied by a +/-16dc linear PSU (Metrix lab PSU).
As you can see the spectrum plot is very clean, with all spurious much below -120dBV
and noise floor below -140dBV in the full measured bandwidth.
Here, and without notch filter, the measured THD is 0.00006% (-124.4dBc) !
So, we can assume with confidence that oscillator THD level is then much low this level,
because we reach here probably the lower thd available with any ADC IC !
That confirm the fact (confirmed by JensH and others here) that the AK5394A
is still the best audio ADC available...
(The full test setup is described in spectrum plot)
To finish it, i have to mount the circuit board in it's enclosure.
I will drill it these days.
(all test has been done at free air without enclosure).
I will add pictures with finished design soon.
As my others project, i have done some documents for this updated design,
in order to simplify as much as possible the building task and get a
working oscillator "fingers in the nose".
You can download below the schematics and full bill of material in pdf form :
A good new, now with it's reduced size, the bare PCB price is seriously falling.
Overall BOM cost has been also reduced and all parts are easy to find and
can be ordered using only Mouser and Farnell (or Digikey).
The new PCB price is 18€ outside EU and 17€ inside EU.
The price include shipping, Paypal fees and the complete design folder.
Of course, any DIYers interested to get the PCB can send me PM.
DIY CS5381 24bits 192kHz instrumentation ADC / EOSC10Kv3 - LT AN67 Reference oscillator / DIY DC-1MHz RMS-DC converter & 60dB LNA / EPSUX3V2 160W Triple outputs High performance Lab PSU.
Last edited by Frex; 16th March 2016 at 08:37 PM.
|3rd March 2016, 06:05 PM||#2|
Join Date: Feb 2009
Pretty nice performance. I do have a few questions, though:
What's the idea with the opening in the solder mask in the middle of the board? Are you concerned about leakage?
Why the use of MELF resistors rather than, say, 1206? MELF is a royal pain to solder as the resistors tend to roll off the board.
|3rd March 2016, 07:23 PM||#4|
The solder mask has been partially removed to prevent any parasitic capacitance on the high gain/bandwidth block.
These opamps are current-feedback type, and even a low feedback capacitance can cause instability issue and cause HF oscillations.
I've used 0204 type melf resistors because they have the best cost/performance value for thin film resistors.
Of course they can be replaced by 1206 flat type if you are more comfortable with.
Hello Turbon, thanks !
|3rd March 2016, 07:33 PM||#5|
Join Date: Feb 2009
Ah! Current feedback. Yeah that makes sense. You really don't want those amps to oscillate at any frequency other than the desired 10 kHz. Thanks for clearing that up.
Nice work. It's good to see the interest in high-performance circuits.
|4th March 2016, 01:41 PM||#6|
I agree that the distortion measured is probably determined by the ADC. Using a loopback test I measured distortion of around -124dB to -125dB and somewhat lower at a certain level.
Is the capacitance of the solder mask really relevant? It is very thin compared to the PCB substrate.
It would be interesting to see the distortion measured with a good notch filter.
|5th March 2016, 07:51 PM||#8|
I have a notch filter box, but after doing many measurements
I noticed that I show higher H3 level as expected regarding the spectrum plot without notch.
With the notch filter, H3 is slightly less than -120dBV, and H2 inside noise floor (<-145dBV). No others harmonics are visible.
(Surprisingly, if i compare with the EOSC10KV2, the harmonics level are
inverted, H3 is in noise floor and H2 about -126dBV).
I don't understand how H3 can be -126dBV regarding the measurement
where i can obtain 130dB between fundamental and H3 (Fund=-10dBV,H=-140dBV).
I've done some modification on my notch filter to ensure that it doesn't introduce THD itself,
and it seem to work fine.
It's a good idea, would you be ok if i send you a bare PCB to build it and make some tests ?
(I will of course send it for free).
I need to keep the finished unit to investigate and tried some others things.
|6th March 2016, 09:41 AM||#10|
I am always weary of PPS caps. I used them once and they are easy to damage with too much heat. I prefer C0G, I don't know that PPS is any lower in distortion either.
If assembled with reflow, then sure.
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