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Old 21st March 2013, 10:07 PM   #1881
1audio is online now 1audio  United States
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There are two limits on distortion on these oscillators today: noise and AGC. Noise is best addresses through impedance of the tuning network. However too low and the network takes power to drive with attendant distortion but 600 Ohms at even 7 Volts it a real option with today's opamps.

The AGC seems to be the real limit on distortion. AP had a patent on a digital multiplying DAC for AGC. The patent has expired. I think it would be possible with a fairly simple processor to steer the gain with a digital pot like either the Maxim or the JRC and have the AGC issue become history. It may be possible to use a very simple logic with some of these parts that can be toggled up and down with single inputs. E.G. DS1802 Dual Audio Taper Potentiometer with Pushbutton Control - Overview
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Old 22nd March 2013, 12:26 AM   #1882
davada is offline davada  Canada
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Demian you have your email turned off. I can even get your message.

Just contact me at davidlbarber@shaw.ca
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Old 22nd March 2013, 12:40 AM   #1883
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
There are two limits on distortion on these oscillators today: noise and AGC. Noise is best addresses through impedance of the tuning network. However too low and the network takes power to drive with attendant distortion but 600 Ohms at even 7 Volts it a real option with today's opamps.

The AGC seems to be the real limit on distortion. AP had a patent on a digital multiplying DAC for AGC. The patent has expired. I think it would be possible with a fairly simple processor to steer the gain with a digital pot like either the Maxim or the JRC and have the AGC issue become history. It may be possible to use a very simple logic with some of these parts that can be toggled up and down with single inputs. E.G. DS1802 Dual Audio Taper Potentiometer with Pushbutton Control - Overview
I'm planning on trying this with an Mdac as a multiplier.

I also plan to try a high res Dac and sine look up table to provide a stimulus. Capture the osc peak and adjust the amplitude of the stimulus accordingly. The osc filter operates as a resonator - tuning fork. It also takes care of the frequency pulling at the same time.
I'll need some DSP for this. Too many of these are in a BGA. Any recommendations.

A low distortion buffer on the output of an op amp takes care of the loading issue.

Lots to play with here.


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Old 22nd March 2013, 12:58 AM   #1884
davada is offline davada  Canada
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Hi Rick,

A non symmetrical Twin T is possible where it's leaning toward a higher order response on the high pass side. this would give us more negative feedback where we want it. It might result in lower disto in an oscillator at the 2nd and 3rd H.
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Old 22nd March 2013, 01:39 AM   #1885
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today's op amps aren't that much of a limit

50 Ohms, 10 Vrms is quite practical with DSL driver op amp in multiloop composite - getting low Z passives to handle the power with low distortion becomes a problem
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Old 22nd March 2013, 02:24 AM   #1886
RNMarsh is online now RNMarsh  United States
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Default My conclusions are:

I pick up 6dB on AGC topology changes and at least another 6db from oscillator opamp changes. Another few dB's from polar cap changes to bipolar; Say about 15+dB's for simple mods. But you have to start with an Ulra-low thd oscillator to begin with. Gets down to four zeros and a one with care and tweeking a few values. To go better in noise and distortion with either LT1468 or AD797, the circuit Z values have to be lowered with possible add-on output stage or buffer. After that level of change, the composites would have to be used to advance performance.

Thx-RNMarsh

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Old 22nd March 2013, 02:31 AM   #1887
RNMarsh is online now RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Hi Rick,

A non symmetrical Twin T is possible where it's leaning toward a higher order response on the high pass side. this would give us more negative feedback where we want it. It might result in lower disto in an oscillator at the 2nd and 3rd H.
A sysmetrical-T is more stable in the notch and then use composite to lower thd.

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Old 22nd March 2013, 03:57 AM   #1888
davada is offline davada  Canada
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Quote:
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A sysmetrical-T is more stable in the notch and then use composite to lower thd.

-RNM
I meant as a band pass for an oscillator. Not a notch.

TT in the feedback loop of an op amp.
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Old 22nd March 2013, 04:01 AM   #1889
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Bell Labs Twin-T paper <http://www3.alcatel-lucent.com/bstj/vol49-1970/articles/bstj49-6-1105.pdf>
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Old 22nd March 2013, 04:12 AM   #1890
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Good job. I couldn't find link today.
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