What is a lock-in amplifier with a forcing grid ?

[zheng89120]

What is a "lock-in amplifier with a forcing grid"?

The context of the title is in an undergrad paper, concerning plasma waves in an glow discharge column.

The paragraph which mentioned this is in the section of the paper, Suggestions for Further Research:

The most important step remaining to be done is to get a conclusive estimate
of the wavelength. A rough estimate of k was obtained, but a steel
cylinder attached to the anode to shorten the length of the discharge column
would allow to measure the variation in frequencies with tube length.
Experiments using a lock-in amplifier with a forcing grid and a capacitive
antenna have been used to measure the wavelength.​

The abstract of the paper, "Discrete Modes in an Glow Discharge Column" if needed:

The frequency landscape of waves in the ion acoustic range of frequencies
(IARF) was studied as a function of voltage and pressure in a constricted DC
glow discharge plasma. Capacitive antennae were used to measure density
and temperature oscillations in the plasma. Four different gases (helium,
neon, argon and air) were examined. Discrete modes, with harmonics up to
n = 26, were observed at frequencies in the ion acoustic range, 1-100 KHz.
The optical spectra emitted by the discharges show that air contamination
is a possible cause of the wave. For some pressures and voltages, the normal
modes transition to chaos though a period doubling route. The possibility
of an ionization wave parametrically decaying into other ionization waves or
into an ion acoustic wave was explored by adding a small audio frequency
(AF) component to the high voltage. Sideband frequencies around each naturally
occurring discrete harmonic were observed. The turbulence caused
by parametric excitation is compared with the period-doubling phenomena
leading to chaos. The AF component succeeded in stabilizing a semi-chaotic
signal for a small window of the driving frequency. Thresholds for driving
the plasma with the AF component were measured​

Thanks.

 

[jim hardy]

I am going to venture a GUESS here ---

"Lock In" amplifier i would suspect might mean same thing i learned by name of "Phase Locked".

See if a google on term "Synchronous Demodulation" opens any windows in your analysis.
Here's a device used for such applications:
http://www.analog.com/static/imported-files/data_sheets/AD630.pdf
its datasheet is informative on the technique

and here's the guys who "wrote the book" on phase locked loops:
http://www.scribd.com/doc/55127135/1972-Signetics-PLL-Applications
(at least it's the same book where i learned about them - still have my ancient hardcover copy someplace)

From the tidbits you copied it sounds like they're measuring standing waves in a tube of ionized gas. That'd be a logical approach, capacitive coupling through glass walls of the tube.

But that is a guess on my part. Reason for my guess is this - way back in vacuum tube days we did something similar in my high school electronics class to observe standing waves in an air dielectric transmission line.

Perhaps somebody closer to today's technology will have better advice for you.
old jim

 

[raining Pete]

Can you please provide more information on the lock-in amplifier with a forcing grid and how it is used in this experiment?

Sure, a lock-in amplifier with a forcing grid is a type of electronic instrument used to measure and analyze weak signals in the presence of noise. It works by synchronizing the input signal with a reference signal, and then amplifying and filtering the synchronized signal to reduce the noise.

In this experiment, the lock-in amplifier with a forcing grid is used to measure the wavelength of plasma waves in a glow discharge column. The forcing grid is a small electrode placed near the anode, which applies a small audio frequency (AF) component to the high voltage. This AF component helps to stabilize the signal and allows for more accurate measurements of the plasma waves.

The capacitive antenna is used to detect the density and temperature oscillations in the plasma, while the lock-in amplifier with a forcing grid helps to filter out any noise and extract the desired signal. By measuring the variation in frequencies with different tube lengths, the wavelength of the plasma waves can be accurately determined.

Overall, the lock-in amplifier with a forcing grid is an important tool in this experiment as it helps to improve the signal-to-noise ratio and obtain more precise measurements of the plasma waves.