DDS Selection

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Oh yes the 22cd number is for 20mA of current... 50mA will give 2.5 times more. so 0.5mW of light power. (Of course I could have made a factor of 2 or 4 math mistake too.)

George H.

The first setup we built about 2 years ago got 0.00001 AU peak-to-peak noise over 10 minute period with 1s sampling, about 400Hz simple LED source.

Our first try was a DC measurement and 1:10^5 sounds very good for DC!

Now we're doing a digital lock-in, but that is more due to low light available. We get 0.0002 AU peak-to-peak noise over 100s with 800Hz LED, 200ms measurements.

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There's no quantisation noise. It's just filtering a square wave, and the number of resistors/length of shift register determines where the higher harmonics begin to come through. You normally shape the frequency response of the summing amplifier to attenuate them.

-- Bill Sloman, Nijmegen

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"Widen the lowpass"? You can certainly complicate the post- demodulation low pass filter by adding a notch at the ripple frequency

- and an easily tunable notch like a bridge differentiator would be the way to go - but a higher order low pass filter is usually a more attractive option, since you don't need to tune it to match your frequency source or worry about temperature drifts moving the notch away from the ripple frequency.

-- Bill Sloman, Nijmegen

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Nonsense. Higher order filters make the phase shift worse. You're moving the goal posts.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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Actually, you are. Phase shift after demodulation doesn't usually matter - the exception is when you are closing the loop around the whole modulator-demodulator set up, and even then you've got to have some kind of phase-lag budget to make a practicable system.

I know you feel the need to pose as infallible expert, in much the same way the John Larkin feels the need to insist that all his products represent "insanely good design", but trying too hard defeats the purpose.

-- Bill Sloman, Nijmegen

Guess if you're not worried about phase error, then go for it.

Jamie

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Moron. I never said anything of the kind.

The real difference between the stuff I design, and the stuff you used to design, is that most of my stuff sells. And, of course, I'm still designing.

John

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Well there is a 'bit of DC drift.

I've got pictures! Source is a Rubidium lamp, through a Rb reference cell*, a little less than 10uA of photocurrent. DC level is near 10V, a gain of 1000 and a 100ms lowpass filter. This is a sweep through zero magnetic field.

There's a little W shaped thing....About 50mV of signal in 50mV of noise. The drift is due to the temperature drift of the Rb cell. Here's a similar signal AC coupled and averaged. I 'claim' the broad dip is an optical pumping absorption and the goes uppy part in the center is CPT. Weird quantum stuff done without a laser. George H.

*there's also an interference filter (picks out one spectral line) and a linear polarizer before the cell... optics details...

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You were the one who said that notches were a problem because of their phase shift, and then you try fixing that by bringing in a higher order filter, which makes the problem worse. Now all you have left is name calling, and I'll leave that to you.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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I am going to build so the light coming out the sides is in a small cavity with white walls for an integrating sphere effect. The photodiode would be at the cavity wall. I expect this would make the monitoring photodiode's current more representative of emissions from the entire emitting surface. The size of the cavity is going to be limited by available space.

Another idea I will look into is to have a surface mount led mounted over a hole in the PCB, and on the other side of the hole have a surface mount monitoring photodiode. What I am not sure about is how much light comes out the underside of a surface mount LED. This also means there will be no lens in the LED package, and I am not sure how accepting the optics designer will be of that.

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Hmm, in a control application you may want the wider bandwidth.

I've got a piezo + (spring and mass) that sings around 1kHz.... What happens if I notch it out of the control loop?

George H.

Hopefully with new applications and not fixing exiting ones! :)

Jamie

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Sounds like you need to cobble together some prototypes.

George H.

Well, I spent a bunch of today changing some code that I wrote in

1996. Some 20-bit serial DACs went obsolete, so we replaced them with a 16-bit DAC and a CPLD to shuffle the bits. But there's a glitch, and the easiest fix is to modify the 68K assembly code that drives the DACs. Having a product line is like having kids or pets.

John

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I always feel like I'm wheeling out a new baby, when some piece of equipment, passes testing and goes to shipping.

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The sawtooth waveform with its even harmonics is a component of a staircase waveform:

This is very much like what the sine wave looks like on its way up and down. The output of the DDS will have even harmonics.

I was wrong about the white noise approximation applying to DDS quantization noise. Because the quantization noise is synchronous with the sine wave there is no psuedo randomization and so no white noise. The harmonics will be clearly defined. My decision not to use the DDS and instead input a square wave into a switched capacitor filter still stands because I think I will still get a good result and it will be easy to generate the square wave from the switching frequency I must generate anyway. I think the DDS will be an additional part with little benefit.

here:

Switched-cap filters are noisy and nasty. They also alias everything in sight, including their own power supply noise. And they make big output spikes.

All you need is a modest analog lowpass filter ahead of the ADC, just to keep the noise down.

John