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Low-distortion Audio-range Oscillator
Low-distortion Audio-range Oscillator
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Old 18th January 2019, 07:11 PM   #7291
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by EUVL View Post
How did you measure down to -160dB.
You notch first, and then amplifier the residual 60dB ?


Patrick
I did not do this measurement, I wish I could! This was done by Vic, and I understand he uses a 40dB passive notch.
My dream is to get my 40dB tracking notch get down to the same performance.
OK, OK, I can dream right ;-)

The 2722 uses the tunable notch in the analyzer, followed by 60dB gain, then into the hi res ADC then the FFT.

Jan
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Last edited by jan.didden; 18th January 2019 at 07:13 PM.
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Old 18th January 2019, 07:28 PM   #7292
EUVL is offline EUVL  Europe
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You are at least having a lot of fun.


Patrick
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Old 18th January 2019, 07:50 PM   #7293
jan.didden is offline jan.didden  Europe
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As have you!
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Old 19th January 2019, 03:26 AM   #7294
vicnic is offline vicnic  Latvia
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My measurement tools look that:
1. The main thing is Asus Xonar D2 with one of the best ADC chips CS5381 onboard. The symmetrical cable goes direct to the ADC chip input. The buffer is on the other side of the cable.
2. Else one buffer is used, if needs to measure via twin T.
3. Main twin T notch for 1kHz. Soldered point to point.
4. The buffers schematic.
5. 1kHz Twin T schematic.
6. Inside the boxes.
7. All is connected for the 1kHz oscillator test.
8. The divider for the direct to ADC measurements.
9. Connection for the direct measurements.
10. Other twin T notches on parade.

Vic
Attached Images
File Type: jpg Asus.jpg (614.0 KB, 262 views)
File Type: jpg BuffN.jpg (401.6 KB, 256 views)
File Type: jpg MainN1kHz.jpg (556.5 KB, 255 views)
File Type: jpg BuffersSch.jpg (37.5 KB, 316 views)
File Type: jpg TwinT1kHz.jpg (36.9 KB, 304 views)
File Type: jpg InsideB.jpg (619.3 KB, 141 views)
File Type: jpg NotchHeadC.jpg (524.7 KB, 117 views)
File Type: jpg Att.jpg (513.1 KB, 94 views)
File Type: jpg DirectM.jpg (567.3 KB, 85 views)
File Type: jpg ONotches.jpg (779.3 KB, 135 views)
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Old 19th January 2019, 10:59 AM   #7295
jan.didden is offline jan.didden  Europe
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Viktors thanks for giving this insight. As I said before, a heroic effort, and makes us realize that everything matters at this level!

BTW, I measured the 5kHz oscillator at the regular output as well as from the inverted output (direct into the 100k of the 2722, no buffer R).

The two look pretty much the same, and what I see in both cases is the 2722 residual. Would this be a good way to create a balanced test signal? What do you think?

Jan
Attached Images
File Type: png vicnic 5k regular output.PNG (39.3 KB, 135 views)
File Type: png vicnic 5k inverted output.PNG (37.1 KB, 65 views)
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Last edited by jan.didden; 19th January 2019 at 11:02 AM.
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Old 19th January 2019, 11:15 AM   #7296
jan.didden is offline jan.didden  Europe
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Here are measurements using a balanced signal from the 5k osc.

Jan
Attached Images
File Type: png vicnic 5k balanced 2V output.PNG (36.9 KB, 80 views)
File Type: png vicnic 5k balanced 3V output.PNG (37.2 KB, 77 views)
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Old 19th January 2019, 11:46 AM   #7297
Bob Cordell is offline Bob Cordell  United States
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Quote:
Originally Posted by vicnic View Post
My measurement tools look that:
1. The main thing is Asus Xonar D2 with one of the best ADC chips CS5381 onboard. The symmetrical cable goes direct to the ADC chip input. The buffer is on the other side of the cable.
2. Else one buffer is used, if needs to measure via twin T.
3. Main twin T notch for 1kHz. Soldered point to point.
4. The buffers schematic.
5. 1kHz Twin T schematic.
6. Inside the boxes.
7. All is connected for the 1kHz oscillator test.
8. The divider for the direct to ADC measurements.
9. Connection for the direct measurements.
10. Other twin T notches on parade.

Vic
Nice work, Vic.

When using the passive twin-T, how are you accounting for the notch loss at the second and third harmonics? Are you just compensating for that manually, with the known loss numbers?

If the op amps in your oscillator are good enough to deliver such remarkably low distortion, why would you not use those same op amps to implement an active twin T so as to avoid the loss issues at the second and third harmonics?

It sounded like Jan was saying that you used a passive twin T with a known 40 dB loss at the notch, but the twin T you show has a deep notch of indeterminate loss. In your distortion measurement, do you need a known loss at the fundamental to establish a fundamental reference level for the THD calculation?

Regarding that same question, do you deliberately off-tune the oscillator from the twin-T notch to establish a known 40-dB loss at the fundamental?

How do you tune so that the oscillator frequency is exactly at the center of the twin T notch? Do you have a fine tune control on the oscillator?

Cheers,
Bob
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Old 19th January 2019, 03:39 PM   #7298
vicnic is offline vicnic  Latvia
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Quote:
Originally Posted by jan.didden View Post
BTW, I measured the 5kHz oscillator at the regular output as well as from the inverted output (direct into the 100k of the 2722, no buffer R).
The two look pretty much the same, and what I see in both cases is the 2722 residual. Would this be a good way to create a balanced test signal? What do you think?
Jan
Thank you, Jan.
Yes, the balanced output can be get from the oscillator board. This was be done in some cases. Opposite signal is at the 7th pin of the LME49720. Only one important thing - needs to use at least 200 ohm (150 ohm is the critical value) resistor between the opamp output and the cable. Otherwise the unstability of the opamp can degrade the performance.

Vic.
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Old 19th January 2019, 04:24 PM   #7299
vicnic is offline vicnic  Latvia
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Quote:
Originally Posted by Bob Cordell View Post
Nice work, Vic.

When using the passive twin-T, how are you accounting for the notch loss at the second and third harmonics? Are you just compensating for that manually, with the known loss numbers?

If the op amps in your oscillator are good enough to deliver such remarkably low distortion, why would you not use those same op amps to implement an active twin T so as to avoid the loss issues at the second and third harmonics?

It sounded like Jan was saying that you used a passive twin T with a known 40 dB loss at the notch, but the twin T you show has a deep notch of indeterminate loss. In your distortion measurement, do you need a known loss at the fundamental to establish a fundamental reference level for the THD calculation?

Regarding that same question, do you deliberately off-tune the oscillator from the twin-T notch to establish a known 40-dB loss at the fundamental?

How do you tune so that the oscillator frequency is exactly at the center of the twin T notch? Do you have a fine tune control on the oscillator?

Cheers,
Bob
Thank you, Bob.
The real levels of the harmonics can be easy calculated. There is app. 4dB level difference between 2nd and 3rd when this type of notch is used. Every time we know the fundamental level at the notch input. Then we can adjust the proper scale (calibrated injection from other source) at 3rd or 2nd and then calculate the other harmonic level. In my picture, which Jan previously post, needs to add +3dB for the second and -1dB for the third. And it is not so important how deep is notch rejection, only needs to know that the signal after the notch is in the linear region of the ADC. The notch fine tuning practically not affects the second and the third harmonic levels. When the oscillator is adjusted with +/-0,5% precision via the manufacturing process, then in practice usage not needs to tune the notch, and I am using fixed ones.
I don't like active notches. The distortions will depend from the opamp. This is not so good when extremely low harmonics needs to measure.

Vic.
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Old 19th January 2019, 06:37 PM   #7300
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by vicnic View Post
Thank you, Jan.
Yes, the balanced output can be get from the oscillator board. This was be done in some cases. Opposite signal is at the 7th pin of the LME49720. Only one important thing - needs to use at least 200 ohm (150 ohm is the critical value) resistor between the opamp output and the cable. Otherwise the unstability of the opamp can degrade the performance.

Vic.
Yes I took it from pin 7. I see that the main output has 2 x 1.2k in parallel as output R. Could that also be reduced to a few 100 ohms, in the interest of not loosing too much signal in lower loads?
Can you reveal the type of resistors you used for the series Rs?

Jan
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