Phase sensitive amplifier

Recorder Low-pass Phase-sensitive Amplifier filter rectifier... 

The phase-sensitive amplifier has a certain response bandwidth and will therefore measure a signal due to the thermal noise from the mixer over that bandwidth, limiting the ultimate signal to noise ratio of the system. The nature of noise in these mixers and detectors is discussed in Section 3.5. 

The main signal detection system was a commercial EG G phase-coherent detector that consisted of a broadband preamplifier of adjustable 30-60 dB gain that transmitted both 1 kHz and 2 kHz signals from the MMW detector, but was selectively tuned to 2 kHz. The outputs from this were split, with one half input to a home-made 1 kHz phase detector. This generated the piezoelectric actuator control signal that scanned the cavity. The other half went to the EG G precision low-noise 2 kHz phase sensitive amplifier that passed the spectral to the computer for processing and display (Figure 2.1). 

As shown in Fig. 19 for solid samples, monochromatic light, chopped at a frequency in the order of magnitude of 10-1000 cps which is low compared with the velocity of deactivation, strikes the solid sample contained in a sample holder. After excitation and relaxation the released heat diffuses to the surface, passes into the gas phase and acts as an acoustic piston which generates a pressure wave detected by the microphone and amplified by a phase-sensitive amplifier locked to the chopping frequency co. Solution of the heat diffusion equation proves that after a distance x from their starting point the heat waves are damped by ... 

A common problem in crossed-beam apparatus is that reaction with the neutral gas may take place not only at the beam intersection, but elsewhere along the track of the ion beam. This difficulty is readily avoided by pulsing the neutral beam using a shutter of some kind. Products resulting from the beam crossing are thus similarly pulsed and may be distinguished from the dc background by a phase-sensitive amplifier. 

Extensive laser-microwave investigations have also been performed in the spectrum of NH2. Hills and Curl, Jr. observed strong electric dipole microwave transitions between a previously unobserved rovibronic level and the J = 1/2 and J = 3/2 spin-rotational levels of ho, A ir(0,10,0), respectively. The NH2 molecules passed through a resonant half-wave microwave cavity with a 60-mW single-mode cw dye laser beam along the axis. The microwave field was amplitude modulated, and the laser-excited fiuorescence signal was detected with a phase sensitive amplifier. Microwave transitions in that part of the NH2 spectrum were used by Hills to assign numerous optical transitions. 

In a magnetic field, the J, I, F) levels of the atom split into components labelled by Mj and Mp quantum numbers (here F = J+I=L+S+1). The magnetic field is swept, and at suitable values of the field the frequencies of the transitions J, Mj, f) if f come into coincidence with the frequency of a fixed-frequency laser. These resonances result in a decrease in detected laser power. In practice, a small modulation (up to 50 G) is added to the magnetic field and the change in transmitted laser power is recorded as a first derivative signal by processing the detector output in a phase-sensitive amplifier. 

The amplified signal is passed to a double-balanced mixer configured as a phase-sensitive detector where the two inputs are the NMR signal (cOq) and the frequency of the synthesizer (03. gf) with the output proportional to cos(coq - co gj.)t + 0) + cos((coq + + 9). The sum frequency is much larger than the total bandwidth of the... 

The detection of the AC component allows one to separate the contributions of the faradaic and charging currents. The former is phase shifted 45° relative to the applied sinusoidal potential, while the background component is 90° out of phase. The charging current is thus rejected using a phase-sensitive lock-in amplifier (able to separate the in-phase and out-of-phase current components). As a result, reversible electrode reactions yield a detection limit around 5 x 10 7m. 

Phase sensitive, but requires 174/50 Interphase and iock-in amplifier... 

To improve the S/N ratio, the modulation signal is processed by amplification with a tuned amplifier using phase-sensitive detection. This means that the detected signal must not only be at the modulation frequency, but must also be in phase with the modulation. Since the amplifier itself can introduce a bit of phase shift, there is a phase control which, in principle, should be adjusted to maximize the signal amplitude. In practice, this control needs to be adjusted only rarely and in most cases the best approach is to leave it alone. 

Photomultipliers are generally used to convert the spectral radiation to an electrical current and often phase-sensitive lock-in amplifiers are used to amplify the resulting current. AES and AFS require similar read-out systems because both methods are measuring small signals. The difficulty associated with both these methods is the separation of the signal for the atomic transition of interest from the background radiation emitted by excited molecular species produced in the atom reservoir. AFS phase locks the amplifier detection circuit to the modulation frequency of the spectral source. Modulation of the source is also used in AAS. 

The audio signal resulting from rf amplification and detection is amplified and detected in a phase sensitive manner by using the original modulation phase as a reference. Both mixer vacuum tubes 6S) and mechanical choppers are used. The resulting DC voltage is fed into a recorder. For sufficiently small audio modulation amplitudes the first derivative of the resonance absorption or dispersion results from the narrow-banding technique. 

A number of different modulation techniques can be used to increase the signal-to-noise ratio (e.g., see Schiff et al., 1994a, 1994b and Brassington, 1995). For example, the laser beam can be mechanically chopped and detected using phase-sensitive detection with a lock-in amplifier. A more commonly used method for accurately measuring small absorbances is to modulate... 

The detector output signal is generated by a current preamplifier for photovoltaic detectors, such as InSb, and by a simple detector bias circuit shown in Fig. 4 for photoconductive detectors, such as PbS and Hg Ge. The voltage signal derived from the bias circuit is normally preamplified and forwarded to a phase-sensitive synchronous detector usually embodied in a lock-in amplifier (Stewart, 1970 Blass, 1976b). 

All the photoconductive devices need to be operated in conjunction with amplifiers, and ac amplification of a chopped signal is most satisfactory. The author also finds chopping the radiation and ac amplification followed by phase-sensitive detection to be the best way of amplifying photomultiplier signals, and it may be of interest to describe the system... 

The second approach is to send the signal to analog, phase sensitive detectors, or lock-in amplifiers, that will analyze the harmonic content. The DC component, in turn,... 

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