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Old 25th May 2002, 11:50 PM   #11
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Default Noise in Voltage references

Good article at:

http://www.analog.com/library/techAr...tion_V-Ref.pdf
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Old 26th May 2002, 12:44 PM   #12
CG is offline CG
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Good article, thanks for reminding me.

I was just curious about the LED in specific. Some guys I know have abandoned LEDs in this type of application because they were too noisy.

These same guys also found that you really needed to cascode the current source (hope I didn't just step on sensistive toes) and add a series resistor from the drain of the cascode FET to the rail in order to get the isolation they were looking for.

They were looking mostly in the audio band, and that may not be so important here.

Thanks for the guidance. I've been too lazy to measure these things myself...
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Old 26th May 2002, 04:34 PM   #13
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FET current sources do not have the high impednce BJT types do. Both work fine, but you have to decide what you are trying to accomplish in order to decide which (if any) you should use.

Used to have a lot of data about current sources using LEDs. Can't find it though. Yes, there is a noise issue there. Wish I could remember what exactly the results were.......Oh well.

Jocko
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Old 27th May 2002, 12:39 AM   #14
BrianL is offline BrianL  United States
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I investigated the oscillator/crystal thing recently. I finally
settled on a canned crystal oscillator sold by a guy named
Guido Tent in the Netherlands (I think). A friend and I ordered
one for each. His is installed and he's very happy with it. He
finds it much better than a discete design (LC Audio?) that he
had previously purchased and used.

My oscillator, like other projects, is on my desk awaiting time.

I supposedly has about 3ps (2 sigma limit) jitter. One reason
it's still on my desk is I've got to round up the stuff to attempt
to measure it.

About $25 ea. with shipping. A bargain.
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Old 27th May 2002, 10:28 AM   #15
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Default Re: Time for a good old fashioned arms race!

Quote:
Originally posted by Jocko Homo
Try this one out.

It also uses a series resonsant crystal, has low distortion, and comes with a low-noise regulator. Puts out 1.6 V p-p.
Jocko, your oscillator is a colpitts (you use the big "L" in crystal = parallel resonance)

What is the advantage to take out the signal via a resistor? You lower the Q-factor by that.

Quote:
It is easy to measure if a crystal is series or parallel resonant. If you need to order one, and the supplier doesn't know which one it is, find a different supplier.
Jocko
Most 99.99% (or even more) are parallel mode crystals. You may have to order a special cut crystal. Not so expensive! Check http://www.kve.se Kvartselektronik in Sweden.

BTW: My oscillator is a "Butler"-type did I read in the document below. Didn't know that.

The advantage of a Butler oscillator is that the crystal is very hard connected between the low input impedance of the CB stage and the output of the emitter follower. When I tested the circuit I noticed that it works better with very low output impedance of the emitter follower.

I must add though I'm not an expert so take my advice "with a pinch of salt" as we say in Sweden. Crystal oscillators is a very wide subject if you want to do it totally right.

http://www.euroquartz.co.uk/pdf/tech-notes.pdf
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Old 27th May 2002, 10:41 AM   #16
Electrons are yellow and more is better!
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Forgot to upload the file. Äsch, too big!

Check http://www.euroquartz.co.uk/pdf/tech-notes.pdf
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Old 27th May 2002, 12:24 PM   #17
ALW is offline ALW  United Kingdom
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Default Re: LED noise

I've done lots of measurements on regulators, discrete and otherwise.

I recently did some measurements comparing LED's to my favoured LM329 buried zener reference.

The noise levels aren't that different, but obviously temp. stab. is orders of magnitude worse.

Personally I think LED's are great for low voltage supplies, particularly 5V reg's for digital. For higher o/p voltages the ratio of noise to o/p voltage for the LM329 makes it lower noise in these applications, since less gain is required in the regulator circuit.

The latter point is less of an issue with regulator topologies that allow bandwidth tailoring to reduce noise amplification (Jung, Sulzer et al).

Also remember an LED needs a lot more current than a zener to work.

Andy.
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Old 27th May 2002, 12:33 PM   #18
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Default Also remember an LED needs a lot more current than a zener to work

Not for a voltage reference it doesn't. LEDs should be quieter than a LM329 which is a very good voltage reference by the way.
How are you biasing the LEDs/LM329 for you measurement, current source or resistor? How clean is your supply voltage?

H.H.
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Old 27th May 2002, 02:34 PM   #19
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I have started a new thread about voltage references since it's of topic here.
Low noise voltage references
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Old 27th May 2002, 04:17 PM   #20
ALW is offline ALW  United Kingdom
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Default Harry

Are you implying you bias the LED below it's normal operating current?

The shape of the V/I curve in this out-of-spec region of the diode's operating range means greater voltage variation for a given current change, i.e. higher dynamic impedance. If one is relying on it as an accurate voltage reference I'd suspect temperature stability to be worse at lower currents.

This means that the current source temperature stability (if biasing this way) becomes a factor in voltage stability. I've only used LED's in discrete reg's where the LED power is bootstrapped from the reg. o/p, and most of these have necessitated resistor biasing.

Personally I only use the LED within it's 'normal' bounds which means higher operating currents than LM329.

W.r.t. the 329, output noise as a proportion of output voltage is lower for the 329 than an LED. Whilst LED's (or most forward-biased semiconductor junctions) are very quiet they do not compare with the LM329, which offers the lowest self-generated noise of almost any device.

A.
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