Low-distortion Audio-range Oscillator

I assume you mean this by bottom.

Jan
This version seems five-six years old. Not so bad. Main improvements was done. The harmonics must be under -140dB. The main problem is the harmonics from the AGC detector output via the parasitic capacitance. Can be lowered when the small shield is used over the AGC detector region as you can see in the attached picture.
 

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This is my take at the Victors's as a part of a measurement chain.

I needed a clean 1kHz signal at an output impedance of 50 ohms, so I've built a sort of a massive parallel buffer with eight LM4562 to follow the Victor's oscillator. The buffer output goes to a passive notch filter by S. Groner, followed by a Scott Wurcer's 60dB LNA (battery inside, double shielding). The ADC is MOTU Audio Express interface (modified).
The spectrum enclosed was obtained after 32 averages (Flat Top window, sidelobes below -144dB), at the osc. output of 1Vrms, and shows only the second harmonic at -97dB. Taking into account the notch filter attenuation of 10dB for the second harmonic, and the nominal ADC FS level of 5Vrms, the THD of the entire chain is -133dB (-97 +10 - 60 + 14), referred to 1Vrms.
BTW the buffer's power supply is Jung/Didden superreg in a separate casing, cable length is 100 cm.


Regrads,
Braca
 

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Received the updated oscillators back from Vic and tested them on the 2722. Here are two shots. One is with Vics oscillator, the other shows the 2722 analog generator measured the same as Vics oscillator.

It seems safe to assume that the 2nd and 3rd shown with Vics oscillator are the residual on the 2722 analyzer.

Jan
 

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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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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
 

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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
 

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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

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
 
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.
 
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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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