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Old 12th October 2012, 04:35 AM   #681
klewis is offline klewis  United States
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Quote:
Originally Posted by 1audio View Post
I think one aspect of this thread that is causing friction are the different unstated goals of various participants. Rather than trying to figure out those goals we should describe the target use of the device, REMEMBER ITS JUST A TOOL!. While there is intellectual satisfaction in going further past the edge of the art if you have a wrench adequate to tightening or loosening a nut a better wrench won't do a better job of the task.

Ultra low distortion oscillators are relevant to measuring distortion. Typically these devices are less well suited to general purpose oscillator functions, like finding the resonance of a driver and because of frequency limits, the overall response of a circuit. For me the utility today of one of these is evaluating ADC circuits and to a lesser degree analog amplifiers. I have not encountered (even in ultra high end) amps and preamps that are within 30 dB of these distortion goals. Not even close. So my Boonton with its -100 dB THD+Noise is adequate to those tasks. For measurements of really low distortion chains you don't need to have continuous tuning (I can't think of a mechanism in a linear system that would have the distortion change a lot with small frequency changes). This suggests to me that a few cardinal frequencies with a matched twin t filter would be really useful. Reducing the complexity makes the device much more accessible. Making the source floating would make it useful for balanced or unbalanced applications.

That's my use case for these devices. Who has others? more input would help narrow the actual requirements, which in turn can lead to something that really can be made.
I agree with you recommendation. I have found that using one or two cardinal frequencies with a twin-t make for consistent results that can be compared over time. Lately I've been using 10khz with Dick's twin-t set at the same frequency. If I had to do it over I would have built fixed frequency versions of Dick's twin-t, fewer parts and no touchy tuning when changing frequencies.
Ken
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Old 12th October 2012, 09:03 AM   #682
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It is not correct that one can rely on every circuit having well-behaved frequency dependence of distortion. This applies in particular to filters (e.g. EQs, tone controls, RIAA stuff, crossovers), but also to a lesser extent to plain amplifiers and mixed signal designs (ADCs and DACs). More subtle distortion mechanisms such as power supply related issues and layout effects can take on surprising frequency shapes (e.g. a resonance in the power supply distribution may cause ripple to peak around 5 kHz, with little effect at 1 kHz and 20 kHz). The availability of high resolution distortion measurements with good frequency resolution (say, 6-12 steps per decade) is an incredible help to trace these issues.

Having said that I fully agree that a few discrete frequencies are already very insightful in most cases and surely incredibly more informative than no reasonable distortion measurement at all! And I second that a low distortion oscillator does not replace a function generator with its wide frequency range, essentially instantaneous settling and various output signal shapes.

BTW, I have designed a little PCB for a fixed frequency passive notch filter for very high resolution distortion measurements. The exact notch frequency is tuned to a fixed value during construction by iteratively adding selected resistor values. The easily implemented frequency range is about 10 Hz to 20 kHz (I've gone up to 100 kHz, but the notch depth degraded presumably because of strays) and set by chosing suitable capacitor values. As an additional feature it includes a selectable 10/20 dB input attenuator. This allows measurement verification at lower notch filter operating levels to gain further confidence in the measurement result.

This design is not as easily used as implementations which include notch filter bootstrapping (to avoid significant attenuation of the 2nd and 3rd harmonic; for my design you'll need to correct for frequency response) and center frequency trimming (for my design you'll need to adjust the oscillator frequency if it doesn't match the notch frequency). However, it avoids the subtle and difficult to quantify distortion contributions from the bootstrapping opamps and potentiometers and is thus more suitable when working below -120 dB.

I'm still evaluating this design with respect to its distortion contribution, but it looks like it is below -130 dB. As time permits I'll put together a short documentation and then make the Gerbers available at my website. If someone want's to go ahead without the detailed construction info drop me an e-mail for the Gerbers.

Samuel

Last edited by Samuel Groner; 12th October 2012 at 09:11 AM.
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Old 12th October 2012, 02:17 PM   #683
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Originally Posted by dirkwright View Post
Bob's oscillator is NOT easy to build unless you know where to get the rotary switches. I do NOT want to buy an old Heath oscillator just to gut it for Bob's oscillator. Bob's circuit is not easy to modify for pc mounted relays.

You are one of the highly skilled designers here that could easily offer a ready to go PCB and BOM that anyone could buy, but you don't. You refuse to take a leadership role here and force people like me to try to learn and understand all of the details of oscillators when I don't want to learn that.
Hi dirkwright,

The rotary switches are indeed a pain. Frequency selection is always a difficult issue, especially for high-performance oscillators. My oscillator is really quite easy to build apart from one's own decision about how to implement frequency selection. I probably would not do it these days with rotary switches - relays are not that bad, especially if one goes for some kind of PIC microcontroller to manage things. A compromise is to use a 2-gang, 11-position rotary switch for the frequency and a small group of relays for range selection.

Cheers,
Bob
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Old 12th October 2012, 03:12 PM   #684
klewis is offline klewis  United States
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I wanted to add two more items to the list for the cardinal fixed frequency design:
An attenuator at the output, not stepped, rather smoothly variable.
Frequuency fine tuning ability, so that you can lock it into you desired cardinal frequency to a fairly high accuracy - to match up with your fixed frequency notch filters, and to be centered on the steps of your fft analyzer. ( this probably goes without saying )
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Old 12th October 2012, 03:58 PM   #685
1audio is offline 1audio  United States
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I have been researching locking a low distortion oscillator to an external clock. Its not too difficult to do but it can add additional distortion. I have done it successfully with the KH4400. My next goal is to make a divider/dds from a digital audio clock to lock to. It must be very distortion free or it will add distortion compromising the overall measurement.

My other To Do would be to make an IM equivalent. Digital audio in particular and many analog systems have more trouble with two tone HF than THD measurements and the IM measurement process is less sensitive to the waveforms. I got two of Victor's oscillators for this- one at 12 KHz and the other at 11.025 KHz. This is where the locking becomes important since these all drift and enough so to degrade a high res FFT. Now to build a suitable low pass filter without distortion.
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Old 12th October 2012, 08:14 PM   #686
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post



This design is not as easily used as implementations which include notch filter bootstrapping (to avoid significant attenuation of the 2nd and 3rd harmonic; for my design you'll need to correct for frequency response) and center frequency trimming (for my design you'll need to adjust the oscillator frequency if it doesn't match the notch frequency). However, it avoids the subtle and difficult to quantify distortion contributions from the bootstrapping opamps and potentiometers and is thus more suitable when working below -120 dB.


Samuel
Hi Samuel,

I take it this would be a calculated correction for 2nd and 3rd harmonic. Is this correct?
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Old 12th October 2012, 09:10 PM   #687
benb is offline benb  United States
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Originally Posted by 1audio View Post
I have been researching locking a low distortion oscillator to an external clock. Its not too difficult to do but it can add additional distortion. I have done it successfully with the KH4400. My next goal is to make a divider/dds from a digital audio clock to lock to. It must be very distortion free or it will add distortion compromising the overall measurement.
I'm wondering if it would be easier and/or better to use the oscillator at just below oscillation as a high-Q filter for a digitally-generated sine wave. Drift in frequency-determining filter components would only change phase, which could be detected and corrected for, similar to (and in addition to) the amplitude determination circuitry. Setting the filter's phase shift to zero would center the passband around the generated sine wave.
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Old 12th October 2012, 09:33 PM   #688
1audio is offline 1audio  United States
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That's pretty close in a sense to what is happening. On the KH4400 it locks with about 5-10 mV. I think a filter won't get to as low a distortion as an injection locked oscillator. And you have the digital system coming along for the ride with its junk to deal with. I am transformer isolation on the KH4400 to reduce this issue.

This is only useful if you can access the clock of the digital system to lock to.
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Old 12th October 2012, 09:35 PM   #689
1audio is offline 1audio  United States
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The B&K 1604 (?) notch filter has a correction that keeps the harmonics within 1 dB of the ideal value and its all passive. There is a 20 dB insertion loss in that mode.

I need to open and reverse engineer it since they did not publish schematics.
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Old 12th October 2012, 11:54 PM   #690
davada is offline davada  Canada
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The B&K 1604 (?) notch filter has a correction that keeps the harmonics within 1 dB of the ideal value and its all passive. There is a 20 dB insertion loss in that mode.

I need to open and reverse engineer it since they did not publish schematics.
Hi Demian,

I can't get this model number to come with anything in google.
There is a B&K 1604A but this is a isolation transformer for power supplies.
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