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25th October 2012, 11:20 AM  #791  
diyAudio Member
Join Date: Jun 2005
Location: Zürich

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I have a THA2181based design which I could post if interested. Samuel 

25th October 2012, 02:43 PM  #792 
diyAudio Member

I wish I had seen this. A nice overview of the thermally stabilized oscillators, including tungsten filaments. It explains my hours of time spent fiddling.
Amplitude stability and distortion in thermistorcontrolled oscillators This paper appears in: Electrical Engineers, Proceedings of the Institution of Date of Publication: August 1973 Author(s): Taylor, P.L. University of Salford, Department of Electrical Engineering, Salford, UK Volume: 120 , Issue: 8 Page(s): 921  926
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Silence is so accurate. Last edited by scott wurcer; 25th October 2012 at 02:47 PM. 
25th October 2012, 02:46 PM  #793 
diyAudio Member

Yes, I would like to compare. We were only interested in a 1kHz reference oscillator.
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Silence is so accurate. 
25th October 2012, 06:12 PM  #794 
diyAudio Member

Has anyone heard of this before?
Pre AP cleverness.
4.1 Method of measuring distortion To provide the necessary sensitivity and accuracy, a null method was employed. The oscillator output voltage was amplified and clipped, to produce a squarewave voltage of known harmonic content, phase locked to the oscillator. The oscillator voltage was also separately phaseshifted by 90° in an integrator before amplifying and clipping, to yield another square wave in quadrature with the first. Measured fractions of the two square waves were then subtracted from the oscillator voltage, balance being detected by a selective amplifier tuned to the harmonic of interest. By this means the inphase and quadrature fractionalharmonic content of the oscillator voltage could be measured to a discrimination of about 2 parts per million and with a relative accuracy of 1%.
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Silence is so accurate. 
25th October 2012, 06:48 PM  #795 
diyAudio Member
Join Date: Jul 2004
Location: Fort St John, BC Canada

These might be an interesting read.
Particularly this one 'Hysteresis in a light bulb'.
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David. 
26th October 2012, 02:37 AM  #796 
diyAudio Member
Join Date: Apr 2010

So it's an analog way of measuring "real" and "imaginary" components of a harmonic, and of course from these the magnitude (what we're usually most interested in) and phase angle can be calculated. With the square wave sources, it obviously only works on odd harmonics.
I haven't heard of that method specifically, but there was this guy named Fourier ... 
26th October 2012, 06:42 PM  #797  
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Join Date: Jun 2005
Location: Zürich

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http://www.sgacoustics.ch/forum/THA...tiplier_r1.pdf As the VCA has inverting response, fourquadrant operation is simply derived by paralleling it with a resistor (R1). R3/R5 divide the control voltage to a useful range. R7 sets the VCA core bias, at almost twice the recommended datasheet value to reduce noise another few dB. R9 trims 2nd harmonic distortion. A FET opamp is chosen as transimpedance stage, as the feedback network has relatively high impedance. A cheaper opamp would probably do, e.g. OPA1641. The following graph shows the measured signal gain (multiplier input to output) vs. control voltage: The mildly exponential control law characteristics is visible in this graph. To design the leveling loop gain and to estimate the effect of control voltage ripple we need to know the scaling factor of the multiplier, i.e. in how much output voltage change a given control voltage change results. This is straightforward to calculate from the above graph by dividing the derivative of the output voltage by the derivative of the control voltage (for the derivatives simple differences between two data points suffice). This gives the following graph: An ideal multiplier would show a constant value, but the 3x range should be acceptable for most oscillator designs. The next graph shows the measured distortion of the multiplier at 1 kHz: Note that there are numerous ways to misinterpret this data. To calculate the effective distortion contribution at the oscillator output we first have to decide what oscillator topology is used. I presume a statevariable filter as the most promising one for low distortion. It has the significant benefit of attenuating the 2nd harmonic of the multiplier by almost 10 dB, and the 3rd harmonic by 18 dB because of the cascade of two lowpass filtering integrators. As a next step, the nominal oscillator level (which appears at the input of the multiplier) should be fixed. For these measurements I've used +10 dBu, which is a typical value. Then we consider how much the multiplier is decoupled, i.e. how much attenuation of its output is applied when summing it with the main oscillator signal. Basically as much as possible is desired, however there are largesignal limits as the multiplier needs sufficient authority to level the oscillator and to provide fast settling. For a variable frequency oscillator, a decoupling of 4x (12 dB) should be about right for this multiplier. A single frequency oscillator might use more. Now we need to appreciate that, once the oscillator amplitude has settled, the control voltage is (for a welldesigned oscillator) probably in the range of +2 V. The wider range is only used during amplitude settling, and hence of no relevance for oscillator distortion performance. So the worstcase 2nd harmonic within this narrow center range is about 102 dBu (note that distortion is measured in absolute quantities, not relative to the fundamental!). The multiplier decoupling attenuates this to 114 dBu; the statevariable filter pushes this even further to 124 dBu. Relative to the oscillator level, distortion is thus at 134 dB. Doing the same calculation for the 3rd harmonic gives 138 dB. Distortion is to a good extent independent of frequency within the audio frequency range. Also important is output noise: Output noise is of course also attenuated by multiplier decoupling and the lowpass action of the statevariable filter. The latter is not easily calculated, but 3 dB is a conservative estimate for most conditions. So within the +2 V control voltage range, multiplier noise contribution is below 123 dB relative to the fundamental of +10 dBu, and within a 22 kHz measurement bandwidth. As shown, the multiplier has been optimized for best THD+N performance (i.e. distortion and noise). If low distortion is all you're after, the internal VCA operating level can be further reduced by scaling R1, R2 and R4 in proportion upwards. As a first order estimate, noise increases and 2nd harmonic decreases proportionally to these resistor values. If you're working on your own oscillator/multiplier, I can only recommend that you run such measurements too. For the control law only basic equipment is needed, and the noise and distortion stuff can be done with a soundcard etc. Without this information, multiplier design remains plain guesswork. Get to know your multiplier before you even think to let your oscillator oscillate! Samuel 

26th October 2012, 06:56 PM  #798 
diyAudio Member
Join Date: Jun 2005
Location: Zürich

I think I've messed the noise graph up, the control voltage should be inverted (higher noise towards positive control voltages). I'll correct this in a few days.
Samuel 
26th October 2012, 09:43 PM  #799  
diyAudio Member
Join Date: Sep 2006

Quote:
This is an excellent explanation of the tradeoffs involved in the choice and design of the AGC element, especially in regard to distortion and noise. This is largely the process I went through for the oscillator in my THD analyzer, which used a JFET element in an arrangement that resulted in a 4quadrant multiplier characteristic with a similar approach. The tradeoff was pretty much the same. Use the least AGC authority as possible to keep down the injection of noise and distortion from the AGC circuit, then choose the operating voltage level of the AGC element to make the best tradeoff between distortion and injected noise. The tradeoff result is limited by a type of figure of merit for the AGC element. It is somewhat like the product of its distortion and noise, although this is a bit of a simplification because the order of the distortion being created by the element may complicate the attempt to define a more accurate FOM. If you try to reduce the distortion by reducing the operating level of the AGC element, the noise injected will tend to increase because you have to reduce the injection loss to maintain the same necessary level of authority. Similarly, if you try to reduce the noise injected by operating the AGC element at a higher level and graeter injection loss, the distortion in the element goes up. The interesting question here is how the FOM for the JFET arrangement compares with that of other more active multiplier elements like the VCA. For example, a long time ago I convinced myself that the 4quadrant multipliers available at the time had too much distortion or too much noise, and could not beat the JFET. Cheers, Bob 

27th October 2012, 10:19 AM  #800  
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Join Date: Jun 2005
Location: Zürich

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R1 = R2 = 110k and R4 = 30k would propably be a very sensible change to my circuit, and enable well below 140 dB multiplier distortion contribution, with still excellent noise. If I find time I'll try this out. Quote:
Samuel Last edited by Samuel Groner; 27th October 2012 at 10:28 AM. 

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