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.