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Old 14th July 2013, 05:34 AM   #2941
davada is offline davada  Canada
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Rick you were asking for some numbers.
The SVO is running at 3Vrms. ARTA FS is 1Vrms. THD reads 0.00001%.

Noise floor is limited by the Twin T.

Cheers,
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Old 14th July 2013, 07:28 AM   #2942
RNMarsh is offline RNMarsh  United States
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4 zeroes and a 1. That's certainly in the ball park !!
Cant wait to measure it here and get pcb built and stuffed into something.
Thinking about writing/publishing an article?



--Richard Marsh

Last edited by RNMarsh; 14th July 2013 at 07:33 AM. Reason: Impressive results -
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Old 14th July 2013, 07:39 AM   #2943
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
4 zeroes and a 1. That's certainly in the ball park !!
Cant wait to measure it here and get pcb built and stuffed into something.
Thinking about writing/publishing an article?



--Richard Marsh
Hi Rick,

I not much of technical writer. But if some else wants to write it up I'll do the footwork.

To get distortion this low the output must be unloaded as much as possible. A good low distortion buffer is required. Op amps just don't have the drive current available. I know the LME's go to 600 ohm but I just don't like them.

Most SVO's I've seen run well below 3Vrms. The distortion drops in more than linear way below this threshold. I need to run it higher for now so I can see the distortion.

Cheers,
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Last edited by davada; 14th July 2013 at 07:42 AM.
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Old 14th July 2013, 03:11 PM   #2944
RNMarsh is offline RNMarsh  United States
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I would not worry about 600 Ohms..... a legacy relic of a bygone era. And opamp buffer will be fine for those 600 Ohms equipment. Or do as Krone-Hite does.... have an output directly off the osc and another output thru a buffer for low Z drive.

The K-H 4402B does 4 zeros and a 5 (with a few changes). Victor's is about half that. At 1KHz.

What does your design do at 10KHz?

Thx-RNMarsh
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Old 14th July 2013, 03:30 PM   #2945
1audio is offline 1audio  United States
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Originally Posted by davada View Post
Hi Demian,

To answer these questions one of us has to build these circuits and test them. I'm kind of busy with my current project.

Cheers,
I'm ready to build and test some filter options. I would like some direction if anyone has explored this earlier. Which types and implementation details would be very helpful.
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Old 14th July 2013, 04:25 PM   #2946
davada is offline davada  Canada
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I'm ready to build and test some filter options. I would like some direction if anyone has explored this earlier. Which types and implementation details would be very helpful.
Hi Demian,

A tracking SV is what I would play with, A while back Samuel posted a 4th order SV oscillator. The design equation are available from Rane's website. It has a high sensitivity to matching elements of the tuning but that was with an un damped filter. There are four sections to tune.
It gives a nice 24dB LP. It provides quadrature outputs in all 4 phases. So a quadrature rectifier is probably the way to go with a mass of filtering or you can try the idea I suggested. If you want to go that route. Otherwise it's more sophisticated control like what Bob used in his analyzer.

What we need to know is how well the filter does with various tuning elements. For that a control system is not required, No point in developing one if the basic idea is flawed.

With that said maybe try some different filter types, SV, Wien etc. VCVA's like Sallen-key generate a lot of noise from the positive feedback. I wouldn't consider them. Passive LCR is a possibility but outside RF it's kind of a lost art.

Aside from your own suggestions I would try Mdac tuning. It's very promising and Mdacs can be paralleled to reduce noise and distortion. El beit a rather expensive route.

Cheers,
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Old 14th July 2013, 04:39 PM   #2947
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
I would not worry about 600 Ohms..... a legacy relic of a bygone era. And opamp buffer will be fine for those 600 Ohms equipment. Or do as Krone-Hite does.... have an output directly off the osc and another output thru a buffer for low Z drive.

The K-H 4402B does 4 zeros and a 5 (with a few changes). Victor's is about half that. At 1KHz.

What does your design do at 10KHz?

Thx-RNMarsh
Hi Rick,

I'm not concerned about 600 ohm loading. Distortion rises below 5k loading. Diving coax cable can be an issue as well. I don't want the distortion to rise because of external loading. So a buffer of some sort is a must. I don't think this will be a problem. One of the ultra low noise low distortion op amps will do. The loading on the output of the SVO needs to be controlled not necessarily high Z.

cheers,
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Old 14th July 2013, 05:09 PM   #2948
davada is offline davada  Canada
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I came across this circuit a couple of years ago.
It's a droopless peak detector using a Maxum digipot and comparator.
With a little modification it could be converted to a tracking peak detector Using a window comparator and a proportional amplifier to detect a zero and control the up/dn pin
It's basically a digital integrator.

If the input signal is used to generate the clock the integration TC is adjusted with input frequency.

http://pdfserv.maximintegrated.com/en/an/AN1163.pdf
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Last edited by davada; 14th July 2013 at 05:25 PM.
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Old 14th July 2013, 06:36 PM   #2949
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Originally Posted by davada View Post
If you use an R2R or log network you can get a lot more tuning out of the relays.
Drive the relays from a successive approximation register.

Cheers,
I prefer to have a minimum number of contacts. So I'll have to live with just 144 frequencies.
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Old 14th July 2013, 07:18 PM   #2950
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Quote:
Originally Posted by RNMarsh View Post
I would not worry about 600 Ohms..... a legacy relic of a bygone era. And opamp buffer will be fine for those 600 Ohms equipment. Or do as Krone-Hite does.... have an output directly off the osc and another output thru a buffer for low Z drive.

The K-H 4402B does 4 zeros and a 5 (with a few changes). Victor's is about half that. At 1KHz.

What does your design do at 10KHz?

Thx-RNMarsh
600 ohms is nice when you get into an output attenuator, which every audio generator really needs. The usual ladder attenuator produces a precision 600 ohms output impedance on each step. for the top (max out) step, you put in a 600 ohm series resistor. Now you have a precision 600 ohm output at all attenuator settings, and the generator is flat into any load in terms of what it is putting out at an impedance of 600 ohms. Note that the ladder does not actually load the output buffer with 600 ohms. For an example, see the output attenuator I use in my distortion analyzer's signal source.

Cheers,
Bob
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