Definition of amp noise

[Ken Smith]

How much money would it free up for social programs, space exploration,
and international relations, if we dismantled the nuke subs and used the
material to make stuff like power plants and other peaceful stuff? Y'know,
the old "beat your swords into plowshares" thing?

I'd bet it would cost money. The reactors will cost a lot to get rid of
and we just know they are all painted with lead paint.

 

[Ken Smith]

But clearly you didn't read the 74hc4053 datasheet. All early lock-ins
used balanced signals with reversing switches operated by a synchronous
square wave. Tens of millions of these are still made, and work well.
This approach has the advantage of being highly linear as well as being
very inexpensive.

You can also use the 4051 and some resistors to supress harmonics a bit if
your frequency comes from a counter.

 

[[email protected]]

As somone else has said, 37Hz is audio not RF.

I just looked it up. It is ELF in the radio band frequencies. Audio
is reference to the vibrations of entire atoms. RF is a range EM
frequencies in the Radio spectrum which include ELF, SLF, ULF/VF, VLF,
LF/LW, MW, HF/SW, VHF, UHF, SHF, EHF

Check it out on wiki pedia
http://en.wikipedia.org/wiki/Extremely_low_frequency

Paul

 

[Robert Latest]

I have access to the 37Hz signal only by EM, but I also know
the phase.

It's sentences like this that make your problem a bit hard to
understand.

It's the amplitude that I need to detect-- it could from zero on up.
Here's a great lock-in amplifier site:
http://www.cpm.uncc.edu/lock_in_1.htm

Thank Robert! That will make a huge difference. I'm surprised to not
find any lock-in amplifier chips!

Any DSP is a "lock-in amplifier chip."

Sure, you can get as complex as one
wants, but it really isn't that complex. So I'll use Ken's circuit as
a preamp and then feed it to an analog mixer and then to a low pass
filter. Do you have any mixers in mind. The first one I came to was
MC12002. Not sure if it works at 37Hz since some mixers have built in
caps and generally used for VHF. It would be nice to have a built in
oscillator, but problem no at such low frequencies. I could use a
simple audio oscillator.

Given that your grasp on the subject is somewhat shaky I'd
abstain from trying to build a lock-in amp from scratch. When I
suggested to use a "commercial" lock-in amp, I meant just that:
the ready-made unit as can be bought or borrowed or stolen,
whatever you prefer. None of us have the slightest clue of what
you're actually trying to accomplish, but if you are associated
with any kind of research facility you'll have access to such a
device.

Lock-in measurements are a bit finicky at times, which makes it
nice to have a commercial amp with its host of tuneable
parameters, even if what you actually need is a lot simpler.

robert

 

[[email protected]]

It's sentences like this that make your problem a bit hard to
understand.

Well now you have my curiosity. What's wrong with the sentence?

Any DSP is a "lock-in amplifier chip."

Thanks, but I thought there would be a demand for a completed lock-in
amplifier chip. DSP chips are nice but that seems a little over kill
for a lock-in amplifier since the DSP has to be programmed. I mean,
it's an entire cpu system, right? A lock-in amplifier just doesn't
seem that complicated to me. Anyway, no big deal!

When I suggested to use a "commercial" lock-in amp, I meant just that:
the ready-made unit as can be bought or borrowed or stolen,
whatever you prefer.
Lock-in measurements are a bit finicky at times, which makes it
nice to have a commercial amp with its host of tuneable
parameters, even if what you actually need is a lot simpler.

You mean box unit, not a chip right? I understand that anything can be
made as complex as you want. Take a function generator. There are
plenty of chips that have onboard adjustable sine wave oscillators,
even square and triangle waves. Yet you also purchase an entire
generator that does the same thing. I'm looking at a Heathkit Function
Generator IG-1271. Hey, it's old so don't laugh. It has a frequency
knob from 1 - 10. It has a frequency multiplier fro 1 to 100K. It has
an attenuation switch (dB)-- 50, 40, 30, 20, 10, and 0. And an output
connector. That's it! Although I can buy a single chip that's
probably 10 times better as far as frequency range, but it doesn't have
the fancy knobs, and perhaps the 6 attenuation levels. Now isn't there
a demand for lock-in phase amplifiers? Please tell me what I'm
missing. Don't I need a mixer, sine wave signal, and a low pass
filter? I am curious what you mean by finicky. Maybe I need a really
good stable sine wave source.

Thanks for the info,
Paul

 

[Robert Latest]

On 14 Aug 2005 07:54:50 -0700,

Thanks, but I thought there would be a demand for a completed lock-in
amplifier chip.

Like others have said, a 4053 can be used as a synchronous demodulator.

DSP chips are nice but that seems a little over kill
for a lock-in amplifier since the DSP has to be programmed. I mean,
it's an entire cpu system, right? A lock-in amplifier just doesn't
seem that complicated to me. Anyway, no big deal!

The principle is simple. The implementation may not be, depending on the
demands of your application.

You mean box unit, not a chip right?

That's what I'd start with, but then I have plenty of those kicking
around in the University lab where I work.

Now isn't there
a demand for lock-in phase amplifiers?

Of course there is, but they come in so many kinds and shapes that it
doesn't make sense to have a one-size-fits-all chip.

Don't I need a mixer, sine wave signal, and a low pass
filter?

Yes. The mixer can be an analog switch, or a four-quadrant multiplier.
For the former, you don't need a sine wave reference, for the latter you
do.

I am curious what you mean by finicky.

Sometimes your signal is buried so deeply in noise that you have to
twirl a lot of knobs to find it, even after pushing the "set everything
automatically" on a modern DSP unit. This applies mostly to meddling
with the reference frequency though (to move it away from harmonics of
unwanted signals) which in your case doesn't apply.

Maybe I need a really good stable sine wave source.

It doesn't need to be stable as much as it has to be phase-locked to
your reference signal (to which you have access "by EM", whatever that
means), and it needn't be sine if you use the analog- switch mixer
approach.

robert

 

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Thanks for all posts by everyone. I appreciate it *very* much! Now
it's time to stop talking and start buying & build.

Electronics are amazing. Engineers are ingenious.
Paul