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Old 22nd March 2013, 03:59 PM   #1901
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Another issue is that of resultion. I think that quantization noise would introduce significant sidebands/amplitude modulation. A rough calculation lead me to believe that > 16 bit would be required to keep quantization noise below the inherent amplitude noise of a (very) well designed SV oscillator. That's of course again a function of the multiplier decoupling/required settling time. The above number is based on < 10 s to 0.1% for a 100 Hz to 10 Hz range change and a multiplier authority similar to what's found in the AP System One. But that's all plain theory at the moment, I haven't implemented anything.

Samuel
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Old 22nd March 2013, 04:37 PM   #1902
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Quote:
Originally Posted by davada View Post
If we are talking about the SYS1 then the OSC has four 5534 including the multiplier.
Can't accommodate the two pole comp if going to 1468.

The output amplifier is discrete. It looks more like a power amp.

Is that 6-10dB of disto, noise or both?

Cheers,
Distortion.
The noise is very close to the same depending on exact impedance.
But the 1468 has more open loop gain more bandwidth and more linearity so lower distortion.

On the other hand the OPA 827 is a better choice for high impedance because it is FET input and so lower noise current.

And the AD 797 is best for low impedance.
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Last edited by myhrrhleine; 22nd March 2013 at 04:47 PM.
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Old 22nd March 2013, 04:46 PM   #1903
RNMarsh is offline RNMarsh  United States
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Can you show me the Sys1 oscillator schematic - anyone?

Ill try the opa827 to see what happens compared to others tried.

Thx-RNMarsh
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Old 22nd March 2013, 05:16 PM   #1904
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You can download it here: AP High Performance Audio Analyzer & Audio Test Instruments : Home Starts Page 152. (I should have read it before assuming how the box works.) You need to register first. Its an OK scan of the service manual with all the details. You might have been better off servicing it yourself but you never would have gone down this learning path.
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Old 22nd March 2013, 06:19 PM   #1905
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Can you show me the Sys1 oscillator schematic - anyone?

Ill try the opa827 to see what happens compared to others tried.

Thx-RNMarsh
Just for a quick reference.
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File Type: gif APsys1 osc.GIF (95.9 KB, 167 views)
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Old 22nd March 2013, 08:26 PM   #1906
RNMarsh is offline RNMarsh  United States
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Default Lowest THD+N at 1-10KHz - for the KH4402B

Ok Thanks both for the schematic info. looks like a lot of 5532/34 opamps which can get upgraded.

Meanwhile, back at the bench -->The KH-4402B has now found an opamp combo that does better than either AD797 or LT1037 or LT1468 alone can do.

Replacing U125 with the AD797 and replacing U141 with the LT1037 now gives the same THD+N level at 1KHz and at 10KHz.
Namely,just under .0001% at .00008-9% at either 1KHz or at 10KHz.

This was done on the X1K scale.... just switching from 1 to 10 on the freq dial. But when using the X100 scale and dialed to 10 (1KHz) the THD+N is even lower -- .00005% :-)

Before, this combo the THD+N rose at 10KHz. Now it is steady at a low level at 10KHz.

[trim R145 AVC control at 10KHz]

Thx-RNMarsh

Last edited by RNMarsh; 22nd March 2013 at 08:48 PM. Reason: Happy days !
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Old 23rd March 2013, 04:52 AM   #1907
RNMarsh is offline RNMarsh  United States
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Here is where we are now with finding and modifying an existing versatile low distortion generator -->

The 2H and 3H are 2.0dB above and below -140dB. No other harmonics show above the noise of -155dB.

797 1037 4402B.jpg

797 1037.jpg

Thx-RNMarsh

Last edited by RNMarsh; 23rd March 2013 at 04:58 AM.
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Old 23rd March 2013, 04:14 PM   #1908
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The ad797 has much higher noise current and much lower noise voltage than a opa827.
Therefore if the ad797 shows noise improvement then the circuit is likely low impedance rather than high impedance.
So, the opa827 is likely to make things worse.
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Old 23rd March 2013, 04:46 PM   #1909
davada is offline davada  Canada
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Hi Rick,

Look for charts like this on the data sheets.
The one for the OPA1641 compares to the ultra low noise OPA1611.
The 1611 doesn't do nearly as well at higher impedance.

The 1468 covers a wide range of input R without the noise curve taking off which makes it ideal for wide range OSC tuning.

It would be pointless to use an OPA1611 or the like above a source R of 1K

Not all data sheets include this chart.
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File Type: jpg Current N1.JPG (94.7 KB, 123 views)
File Type: jpg Current N2.JPG (73.0 KB, 119 views)
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Last edited by davada; 23rd March 2013 at 04:51 PM.
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Old 23rd March 2013, 04:58 PM   #1910
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Samuel Groner View Post
Another issue is that of resultion. I think that quantization noise would introduce significant sidebands/amplitude modulation. A rough calculation lead me to believe that > 16 bit would be required to keep quantization noise below the inherent amplitude noise of a (very) well designed SV oscillator. That's of course again a function of the multiplier decoupling/required settling time. The above number is based on < 10 s to 0.1% for a 100 Hz to 10 Hz range change and a multiplier authority similar to what's found in the AP System One. But that's all plain theory at the moment, I haven't implemented anything.

Samuel
This is exactly what is shown... a lot of spurious freqs other than random noise and harmonics of the test signal. It makes it pretty hard to find the DUT harmonic signals/levels from the system's spurious frqs. at low distortion levels.

A serious pcb layout R&D effort will be needed.

Thx-RNMarsh
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