Detection bandwidth

As we have seen in Section 9.5.3, in the case of resistance thermometry, the signal produced by a low-temperature thermometer is very low (microvolt range). Low-pass filters are not sufficient to narrow the detection bandwidth in order to get a suitable signal to noise ratio (S/N). Bandpass filters are needed. The most commonly used method is the synchronous demodulation, usually simply called lock-in technique, as shown in the block diagram of Fig. 10.7. 

We now check whether Eq. (1), with S /3 = e2/ and modified as above to account for finite propagation time, can explain our data. The unknown parameters are the resistance Rq and the effective environment noise temperature Tq. We checked that the impedance of the samples was frequency independent up to 1.2 GHz within 5%. Fig. 2 shows the best fits to the theory, Eq. (1), for all our data. The four curves lead to Ro = 42 12, in agreement with the fact that the electromagnetic environment (amplifier, bias tee, coaxial cable, sample holder) was identical for the two samples. We have measured the impedance Zenv seen by the sample. Due to impedance mismatch between the amplifier and the cable, there are standing waves along the cable. This causes Zenv to be complex with a phase that varies with frequency. We measured that the modulus Zenv varies between 30 12 and 70 12 within the detection bandwidth, in reasonable agreement with f o = 42 12 extracted from the fits. 

Many common materials contain nitrogen or one of the many other nuclei from which NQR can be measured [38], Fortunately, Resonances are narrow in NQR and are dispersed over a very large bandwidth so that NQR signals from benign materials do not interfere with the detection of explosives. Based on an NQR database of more than 10 000 compounds [39], no NQR frequencies from benign materials are found within the typical detection bandwidth of an NQR system when it is set to the frequency of any of the explosives detected by NQR. 

By decreasing the detection bandwidth as much as possible, consistent with maintaining a good signal to noise ratio, a limiting condition can be approximated for which the quenching summation varies in a simple manner from flame to flame. In the limit in which only one transition is monitored from the v J state populated by the laser, almost every vibrational or rotational relaxation from that state is an effective quenching collision. Under these conditions the quench summation term approximates to a gas kinetic quench rate. 

The experimental conditions for the excitation and detection of all the species are listed in Table I along with the radiative lifetimes of the excited states. Under the narrow detection bandwidth conditions for these measurements the quench term is much greater than T 1 for the species studied and the fluorescence efficiency varies as x T1/2. Thus with fixed geometry, laser excitation wavelength, and detection parameters, the fluorescence intensity in Equation (1) simplifies to... 

In contrast to the SNMR method, it is crucial to measure an undistorted polycrystaUine static spectrum in order to have a reUable spectral analysis when the effects of finite RF pulses and detection bandwidth are ignored in the numerical simulation. Since Co static powder spectra in general have well-defined singularities and therefore are suitable for lineshape simulation, the issue of spectral distortion seems to be particularly important for Co NMR practitioners. In this section we will give a brief account of lineshape analysis, including experimental considerations and precautions for numerical simulations. 

An example is shown in Fig. 1.6. The same signal was recorded by an oscilloscope (left) and by photon counting (right). The counter binning time and the oscilloscope risetime were adjusted to approximately the same value so that the detection bandwidth was approximately the same. The lower SNR and the wider noise amplitude distribution in the oscilloscope trace is clearly visible. 

The efficiency is further reduced by the efficiency of the grating of the monochromator itself, which is 60 % to 80 % at best. Moreover, the detection bandwidth is usually much smaller than the width of the fluorescence band of the fluorophore. Therefore, the efficiency of a monochromator-based system can be orders of magnitude smaller than that of a filter-based system (see below). 

TCSPC is superior in terms of efficiency and sensitivity. The effective detection bandwidth is much higher than for modulation systems. The IRF can be kept... 

Example 9.18 The shot-noise limit of an optical detector with the quantum efficiency r] < 1 irradiated by N photons per second leads to a minimum relative fluctuation A5/5 of the detector signal 5, which is for a detection bandwidth A/ given by... 

The noise power of both detectors adds quadratically. The signal-to-noise ratio for a detection bandwidth A/... 

Among the variety of fiber optic sensors, Fabry-Perot interferometer (FPI) sensors have shown the best performance in the frequency range up to 100 MHz because of their high sensitivity, broad bandwidth and excellent tolerance to low-frequency ambient vibration. Several FPI designs are used whereas intrinsic FPI sensors with flat mirrors show the best performance and are quite compatible with the mechanical structure of composites. Using this type of sensor for measurement of AEs, the detection bandwidth is 15 MHz to a few GHz. The minimum detectable phase of the current system was mainly limited by electronic noise 4 10 rad/Hz [66]. 

The most widely used and in many cases most sensitive way of recording EPR spectra is continuous-wave (CW) EPR. In this experiment, microwave with constant frequency is irradiated with relatively weak power (typically between 1 jlW to 200 mW) and the magnetic field is swept to Irring the transitions into resonance. As broadband microwave detection by diodes is used, the detection bandwidth has to be limited by other means to avoid accumulation of excessive noise over a wide frequency range. This is achieved by low-frequency (typically 100 kHz) modulation of the magnetic field with an amplitude of 0.01-1 mT and phase-sensitive detection of the signal component modulated with this frequency. As a result, the derivative of an absorption lineshape is measured, which is better resolved than the absorption lineshape itself The technique is disadvantageous if all features of the absorption lineshape are much broader than the maximum modulation amplitude that can be technically achieved. 

One important parameter is the minimum force detectable in a MRFM setup. For NMR QIP applications, this will establish the effective number of qubits detectable in an experiment. So, let rrif be the effective oscillating mass of the cantilever, t its damping time-constant and B the detection bandwidth. The minimum force detectable at temperature T is [10] ... 

Detectivity bandwidth / The operating frequency at which specific detectivity drop is 3 dB, given in Hz 0.707... 

Finally, as an illustration, a frequency dependence of the specific detectivity-bandwidth product for two different types of infrared detectors is given, one of them photonic and the other thermal device. One of the detectors is a HgCdTe... 

A real detection scheme may include aU of the above or only some of them. Actually, only the (d) part—active detection zone— must exist in aU photodetectors, while all of the others are optional and their function is to improve overall performance by increasing the values of different factors in the specific detectivity-bandwidth product. 

The possibility to tailor the composition of Hgi xCdxTe enables a continuous variation of operating temperatures, applied fields and detector dimensions to reach their optimum values for maximum specific detectivity-bandwidth product. A larger carrier depletion within the active area will not only improve signal-to-noise ratio but simultaneously shorten the response time. However, high fields easily cause transition of carriers to hot region. There is an optimum ratio between the field intensities and the beneficial influence of Auger suppression. This ratio is determined by the characteristic curve drawn between the areas A and B in Fig. 3.46. 

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