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Old 12th April 2013, 04:36 PM   #2191
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Quote:
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That was the first suggestion from the gathered elites.
That does make me feel good!

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Old 12th April 2013, 04:38 PM   #2192
RNMarsh is offline RNMarsh  United States
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There are several heat sink mfr which offer combinations of alum and copper in their products. Some blend copper into the Alum to make a composite metal. Other mold Alum over copper. for example, plate copper embedded nearest the device (Xistor) and Alum fins further away.... all in one packaged heat-sink. The best of both worlds.

Thx-RNMarsh

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Old 12th April 2013, 05:26 PM   #2193
RNMarsh is offline RNMarsh  United States
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Is there any advantage to using Injection-Locked prescalers.... running osc at higher freq and dividing down to audio freqs?

What is the jitter in the RC osc related to? characteristics of the C's perhaps?


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Old 12th April 2013, 05:38 PM   #2194
1audio is offline 1audio  United States
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The "close in phase noise" is normally related to the amplifier noise and the Q of the frequency network. Injection locking brings the noise of the locking source to the oscillator and that can be better. Typically the close in noise is from the external source and the further out (and harmonics) are from the local generator.

Its pretty impressive how good the tuning of the Boonton is since its done by a micro and reciprocal counter and a dac into a multiplier. The tuning of the loop must be really good. A divided down crystal should be better but I have had hum sidebands limiting my efforts so far. I need to build a dedicated circuit for this.
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Old 12th April 2013, 05:41 PM   #2195
RNMarsh is offline RNMarsh  United States
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perhaps when I got the noise down the thd was lower also in part due to reduced jitter... nice combination to have.

I seem to recall or imagine that as low as 100Khz Xtlas exist - used to.

-RM

PS -- yes and there are some on eBay at this time (att). Even a 100Khz xtal with oven in octal base for $25.

If there is a choice - best to get used xtals as they are more stable.

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Old 12th April 2013, 07:50 PM   #2196
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I just want to be the first to say this. How about punt the oscillator all together? Put a small pot into the Wien bridge circuit and use it like a "tickler" in a super-regen making a super high Q amplifier. 40-50 dB rejection on a good sound card sine wave is pretty good. Simulation looks fine, as Samuel points out the noise issues are always there since you have an essentially "infinite" noise gain at the oscillation frequency and the skirts seem to vary only a little.
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Old 12th April 2013, 08:05 PM   #2197
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Quote:
Originally Posted by scott wurcer View Post
I just want to be the first to say this. How about punt the oscillator all together? Put a small pot into the Wien bridge circuit and use it like a "tickler" in a super-regen making a super high Q amplifier. 40-50 dB rejection on a good sound card sine wave is pretty good. Simulation looks fine, as Samuel points out the noise issues are always there since you have an essentially "infinite" noise gain at the oscillation frequency and the skirts seem to vary only a little.
I mentioned I did this many posts back! You also want to parallel the just below oscillators to lower the noise.

When I started getting serious mid 70's I made my low distortion oscillators and analyzer. I used a biquad filter made of socketed IC's for the distortion analyzer. I then ran through my stock of then state of the art 5534's and found the NEC version had the lowest noise. By using 18 & 20 khz test signals I was able to measure to 10 PPB. (ish maybe 20)

I found I could make circuits with distortions of under 50 PPB until you actually connected them to a load then they shot up to 1000's PPB.

One interesting tidbit was I originally thought that if an amplifier was set up as inverting you would get lower distortion. Turns out the best I could get inverting was 300 PPB.

Even though I had built a DFT analyzer using the Reticon chip, that test system was my mainstay until around 95. Then I went to a sound card and PC. Now that my AP isn't cutting it I have been doing work arounds and looking for something new.
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Old 12th April 2013, 08:23 PM   #2198
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When I started getting serious mid 70's I made my low distortion oscillators and analyzer. I used a biquad filter made of socketed IC's for the distortion analyzer. I then ran through my stock of then state of the art 5534's and found the NEC version had the lowest noise. By using 18 & 20 khz test signals I was able to measure to 10 PPB. (ish maybe 20)

.
There is no 5534 that supports even ppm THD. Separate oscillators and IMD is a very different problem, PSRR and crosstalk could be factors there. These days the noise is not an issue, why shoot yourself in the foot use FFT's get the noise BW down to an insignificant number. Doubling the FFT length is the same as having two oscillators, the bits are way less pain.
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Old 12th April 2013, 08:39 PM   #2199
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Originally Posted by scott wurcer View Post
There is no 5534 that supports even ppm THD. Separate oscillators and IMD is a very different problem, PSRR and crosstalk could be factors there. These days the noise is not an issue, why shoot yourself in the foot use FFT's get the noise BW down to an insignificant number. Doubling the FFT length is the same as having two oscillators, the bits are way less pain.
That is why I said oscillators and have brought up using IMD measurements instead of THD. IMD works much better and is easier to isolate using analog techniques.

The problem with FFT is A/D limits. If you dither and average you certainly can see distortion a bit lower, but I really don't care about most distortion type of measurements once they go below the other issues.

Noise is more of an issue in design limits from the sandbox I play in.

For example as mentioned I have been letting my guys play with mic preamps. My issue is do I want to actually design and build a new one. If I do then from what I have learned it will be two times two stages with an output buffer. There would be a transformer input split into first paralleled amps noninverting and then single amp (x2) inverted signal paths as Langevin did in the 40's. The gain trim would affect all four stages equally. The output would have a follower buffer inside the second stage global feedback loop.

That addresses EMI, noise, inverted distortion products being equal would series cancel, some would balance cancel and loading issues would be significantly reduced.

Now using a chip like the AD797 with a step up transformer would give me a noise figure better than the real impedance of most dynamic microphones. But I have diverted too much from the topic...
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Old 12th April 2013, 09:00 PM   #2200
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Whether one chooses FFT or other means to determine thd/harmonics.... or some other application for the purest of pure oscillator -- it is clear thd +n is the metric to work with. Get them both down. It doesnt matter that there is noise at the osc freq, either. Or noise gain at the osc freq... except in that it creates jitter induced freqs. But that can be dealt with after the osc itself is optimized. High Q helps, too. So how to combine all into one topology.
Can we get a block diagram from all (Scott, Samuel and Simon) on proposals? Start to fill in some blanks and blocks with circuits... both can be done and compared if they are very different in approach.

Thx-RNmarsh




I like the concept of no osc or one which can include cancellation as opposed to more and more linear -only.
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