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Lock in amplifier theory and justification help

S

Steve Sands

Jan 1, 1970
0
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I should point out that chopping also permits blocking of DC artifacts
from heaters and other sources by using Capacitively-coupled gain
stages.

I know there is a Google optics NG but they are 99% optics 1%
equipment.
 
G

Genome

Jan 1, 1970
0
Steve Sands said:
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I should point out that chopping also permits blocking of DC artifacts
from heaters and other sources by using Capacitively-coupled gain
stages.

I know there is a Google optics NG but they are 99% optics 1%
equipment.

I think I need to nail my head to a bucket

DNA
 
R

Roy McCammon

Jan 1, 1970
0
Steve said:
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I suspect that if you look carefully, you will find that
instead of just looking when the light is on, that you
are actually looking all the time and subtracting the
the signal when the light is off from the signal when
the light is on. This eliminates dc offsets and low
frequency noise.
 
T

Tim Wescott

Jan 1, 1970
0
The problem that they're solving is that there's a lot of noise at the AC
power line frequency and it's harmonics (60, 120, 180 etc. Hz in the US).
Worse, most light-measuring devices drift all over the map, and you need to
pick out a little bitty "real" signal from all of the DC drift.

The idea is that you add signal when there's light and subtract it when
there's dark. What comes out of your multiplier is the wanted signal at DC,
with previously DC componants and AC power line componants at higher
frequencies. Then you low-pass filter this result. This gets rid of all of
the noise at DC and at the AC powerline frequency, leaving you with
(hopefully) nice clean data.
 
J

John Popelish

Jan 1, 1970
0
Steve said:
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I should point out that chopping also permits blocking of DC artifacts
from heaters and other sources by using Capacitively-coupled gain
stages.

I know there is a Google optics NG but they are 99% optics 1%
equipment.

The point of the lock in amplifier is that it inverts the noise half
the time, but the signal is never inverted. So when you average the
result, any noise that occurs at a frequency lower than the signal,
and not phase locked to it, will eventually average out to zero. High
frequency noise is also averaged out to zero regardless of the
synchronous detection, as long as it does not have a fixed phase
relationship to the desired signal.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Steve said:
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I should point out that chopping also permits blocking of DC artifacts
from heaters and other sources by using Capacitively-coupled gain
stages.

I know there is a Google optics NG but they are 99% optics 1%
equipment.

Beside what has been told already, a lock-in amplifier lets you detect
very small signal levels, by integrating the noise away.
Have a look at the datasheet of the AD630 switch. They claim, that it
can pull out signals which are burried in 100dB noise.
Important is a narrow filter around the chopper frequency.
A Q of 10 to 20 should at least be implemented.
Yes, DC offsets are gone. That is one point of it.

Rene
 
G

Genome

Jan 1, 1970
0
Tim Wescott said:
The problem that they're solving is that there's a lot of noise at the AC
power line frequency and it's harmonics (60, 120, 180 etc. Hz in the US).
Worse, most light-measuring devices drift all over the map, and you need to
pick out a little bitty "real" signal from all of the DC drift.

The idea is that you add signal when there's light and subtract it when
there's dark. What comes out of your multiplier is the wanted signal at DC,
with previously DC componants and AC power line componants at higher
frequencies. Then you low-pass filter this result. This gets rid of all of
the noise at DC and at the AC powerline frequency, leaving you with
(hopefully) nice clean data.

I have serious problems with your existance.

Obviously you have no problems..... so it must be my fault.

Buduh?

DNA
 
S

Steve Sands

Jan 1, 1970
0
John Popelish said:
The point of the lock in amplifier is that it inverts the noise half
the time, but the signal is never inverted. So when you average the
result, any noise that occurs at a frequency lower than the signal,
and not phase locked to it, will eventually average out to zero. High
frequency noise is also averaged out to zero regardless of the
synchronous detection, as long as it does not have a fixed phase
relationship to the desired signal.

Thanks John for clear explanation. All non-sychronous signals, IE
artifacts average to zero where the sychronous signal remains intact.
I take it this approach also addresses the inherent 1/f noise coming
from the detector.

Thanks again.
 
J

John Popelish

Jan 1, 1970
0
Steve said:
Thanks John for clear explanation. All non-sychronous signals, IE
artifacts average to zero where the sychronous signal remains intact.
I take it this approach also addresses the inherent 1/f noise coming
from the detector.

Thanks again.

If the rectified signal is averaged with a slow enough low pass
filter, yes.
 
J

James Meyer

Jan 1, 1970
0
I think I need to nail my head to a bucket

DNA

Nonsense. There are plenty of folks in the group that will do it for
you.

Jim
 
W

Winfield Hill

Jan 1, 1970
0
James Meyer wrote...
"Genome" posted this:


Nonsense. There are plenty of folks in the group that
will do it for you.

Never.

Thanks,
- Win

whill_at_picovolt-dot-com
 
B

Bill Sloman

Jan 1, 1970
0
Rene Tschaggelar said:
Beside what has been told already, a lock-in amplifier lets you detect
very small signal levels, by integrating the noise away.
Have a look at the datasheet of the AD630 switch. They claim, that it
can pull out signals which are burried in 100dB noise.
Important is a narrow filter around the chopper frequency.
A Q of 10 to 20 should at least be implemented.
Yes, DC offsets are gone. That is one point of it.

You do have to design your amplifer chain so that the noise and
offsets don't drive it into saturation before you do the subtraction -
which can lead you to put a band-pass filter somewhere in the
amplifier chain, which is fine if you know what you are doing, but can
insert a large and none-too predictable phase shift if you aren't
careful, a trap I fell into when I was very young and inexperienced
......
 
F

Fred Bloggs

Jan 1, 1970
0
Steve said:
A question for the experts here.

I understand the theory of a tuned amplifier or synchronous detector
but I have difficulty understanding the benefits. The concept, as I
understand it, is that by using a synchronous reference signal that is
in phase with a chopper or modulator that modulates a signal of
interest to gate a multiplier at some point in the gain stage, you can
realize significant improvements in signal to noise ratio.

Case in point.

In the coating industry we work with lock in amplifiers to recover
small incident reflectivity signals inside a vacuum chamber where
there are several spurious artifacts both DC and AC, that reside in
the same wavelengths as the incident or targeted reflection signals
do. The source illumination is modulated via a chopper wheel and a
reference signal is locally generated at the chopper to provide a
synchronous signal to recover the incident or reflectance signal at a
multiplier or gated amplifier.

The question is what difference does it make to only look at the
signal during the period which it is present (light enabled) versus
when it isn't (dark)?

All of the unwanted spurious reflections and artifacts are present
during this synchronous "light on period" as the "dark period"
therefore what is the advantage?

I should point out that chopping also permits blocking of DC artifacts
from heaters and other sources by using Capacitively-coupled gain
stages.

I know there is a Google optics NG but they are 99% optics 1%
equipment.

Generally speaking, synchronous detection is a means of translating a
narrowband filtering requirement to DC and therefore a simple lowpass.
In the case of a lock-in the objective is amplify a very small signal
level while maintaining adequate S/N. Since S is small, maintaining S/N
mandates that N be small too, and the only way to do that is narrow the
bandwidth of frequencies about S that undergo amplification. Now you
could achieve the same result by using an extremely narrowband receiver
filter centered on S, but the problem you run into is that the S signal
frequency has to be right on the filter center frequency with extreme
precision across all operating conditions. Synchronous detection
overcomes this burdensome requirement by producing sum and difference
frequencies of its inputs which, if at the same frequency, means the
difference frequency is DC. So a simple lowpass filter about DC captures
the S energy in a bandwidth about S center frequency equal to the
lowpass cutoff- more or less. Since the detection frequencies are the
"same" there is no more problem with S frequency tracking a fixed filter
center. All of your various interfering noise will continue to interfere
but their effect is reduced in proportion to the amplification bandwidth
reduction set by the lowpass cutoff to only that interference energy
centered narrowly on S. Since your detector 1/f noise is vastly removed
from the S frequency, it has no effect on the output whatsoever- for any
reasonable lowpass selection.
 
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