Le labo de Michel | LDM #259: Magnetic Anomaly Detector AN/ASQ-81(V) - Part 3: Helium discharge lamp @lelabodemichel5162 | Uploaded 2 years ago | Updated 8 minutes ago
This video shows the circuits involved in the Helium discharge lamp drive.
A high voltage oscillator permits the ignition of the lamps. When the intensity of both lamps is enough the igniter turns off and a 49.6 MHz oscillator with regulated amplitude permits to keep the discharge lamps ionized.
A part of this signal is applied to the helium cells in order to keep the Helium atoms inside the cells in the métastable state.
Between the discharge lamp an the cell there is a filter/polarizer.
A large photodiode placed on the other side of the cell permits the measurement of the opacity of the cell.
A coil perpendicularly placed to the coil permits the generation of a weak AC magnetic field. For a given static magnetic field there is a particular frequency of the AC magnetic field generated by the coils which permits the maximum absorption of the light coming from the discharge lamps. When this occurs the transmission of light is minimum.
This detection is performed in 2 steps:
* A vco generates a variable frequency from 400kHz to 2.2 MHz to cover the entire range of the magnetic field to be measured. During this sweep the vco frequency will be closed to the resonant frequency which will activate the second step described below.
* When a drop of the photodiode signal appears the 2nd step occurs. The goal of this second step is to detect the exact resonant frequency. For that purpose the frequency of the vco is frequency modulated around the frequency detected in first step. The deviation is 3kHz and the modulation frequency 430Hz. The photodiode acts like a demodulator. The signal is phase-detected by multiplying the photodiode signal by the modulation signal. When the center frequency of the VCO is identical to the resonant frequency the DC output of the phase detector is null, otherwise the DC value is positive or negative. The output of the phase detector drives an integrator which drives the center frequency of the VCO.
As a result when the system is locked there is a signal which is frequency modulated with a mean frequency proportional to the static magnetic field. A PLL permits to have a signal without FM modulation. The frequency is converted into a voltage which is displayed on a paper chart recorder after correction of altitude and scaling.
This video shows the circuits involved in the Helium discharge lamp drive.
A high voltage oscillator permits the ignition of the lamps. When the intensity of both lamps is enough the igniter turns off and a 49.6 MHz oscillator with regulated amplitude permits to keep the discharge lamps ionized.
A part of this signal is applied to the helium cells in order to keep the Helium atoms inside the cells in the métastable state.
Between the discharge lamp an the cell there is a filter/polarizer.
A large photodiode placed on the other side of the cell permits the measurement of the opacity of the cell.
A coil perpendicularly placed to the coil permits the generation of a weak AC magnetic field. For a given static magnetic field there is a particular frequency of the AC magnetic field generated by the coils which permits the maximum absorption of the light coming from the discharge lamps. When this occurs the transmission of light is minimum.
This detection is performed in 2 steps:
* A vco generates a variable frequency from 400kHz to 2.2 MHz to cover the entire range of the magnetic field to be measured. During this sweep the vco frequency will be closed to the resonant frequency which will activate the second step described below.
* When a drop of the photodiode signal appears the 2nd step occurs. The goal of this second step is to detect the exact resonant frequency. For that purpose the frequency of the vco is frequency modulated around the frequency detected in first step. The deviation is 3kHz and the modulation frequency 430Hz. The photodiode acts like a demodulator. The signal is phase-detected by multiplying the photodiode signal by the modulation signal. When the center frequency of the VCO is identical to the resonant frequency the DC output of the phase detector is null, otherwise the DC value is positive or negative. The output of the phase detector drives an integrator which drives the center frequency of the VCO.
As a result when the system is locked there is a signal which is frequency modulated with a mean frequency proportional to the static magnetic field. A PLL permits to have a signal without FM modulation. The frequency is converted into a voltage which is displayed on a paper chart recorder after correction of altitude and scaling.
