Mixer detector phase

The initially amplified signal is then usually detected in the receiver using a three-port device acting as a double-balanced mixer. Three port devices, which can be used as gates, mixers or phase-sensitive detectors, consist of a series of diodes and transformer coils. The phase-sensitive detector has two input signals, the NMR signal (wq) and the frequency of the synthesiser (Wref)- The output is proportional to... 

Each of the mixers (called phase sensitive detectors, mixers, or double balanced mixers abbreviated as DBM in the figure) will form the sum and difference of the input frequencies. The IF amplifier is tuned to Vj and all other frequencies will be discriminated against. The result is that the signal... 

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... 

Solvent Mixing. During gradient operation, the HPLC solvents that comprise the mobile phase must be mixed adequately, most commonly with either a static or dynamic mixer. Static mixers rely on laminar-flow dynamics, whereas dynamic mixers use magnetic stirrers. Solvent viscosity affects mixing characteristics inadequate mixing is detected by many UV detectors and may be expressed as an unstable baseline. 

The receiver system consists of a preamplifier (Radiation Devices model BBA-1P, bandwidth 3-500 MHz at 15 dB gain and 3 dB noise), a 10 MHz amplifier (F+H Instruments mixer-amplifier, model 39 A/M), and a phase detector (F+H Instruments phase detector 1-30 MHz). 

A comprehensive overview of frequency-domain DOT techniques is given in [88]. Particular instraments are described in [166, 347, 410]. It is commonly believed that modulation techniques are less expensive and achieve shorter acquisition times, whereas TCSPC delivers a better absolute accuracy of optical tissue properties. It must be doubted that this general statement is correct for any particular instrument. Certainly, relatively inexpensive frequency-domain instruments can be built by using sine-wave-modulated LEDs, standard avalanche photodiodes, and radio or cellphone receiver chips. Instruments of this type usually have a considerable amplitude-phase crosstalk". Amplitude-phase crosstalk is a dependence of the measured phase on the amplitude of the signal. It results from nonlinearity in the detectors, amplifiers, and mixers, and from synchronous signal pickup [6]. This makes it difficult to obtain absolute optical tissue properties. A carefully designed system [382] reached a systematic phase error of 0.5° at 100 MHz. A system that compensates the amplitude-phase crosstalk via a reference channel reached an RMS phase error of 0.2° at 100 MHz [370]. These phase errors correspond to a time shift of 14 ps and 5.5 ps RMS, respectively. 

Amplitude-phase crosstalk is intrinsically low in frequency-domain instraments that use gain-modulated PMTs as detectors and mixers [166]. Results presented in [98, 346] show that optical properties can be obtained with an accuracy comparable to that of TCSPC-based instraments. The modulated-PMT technique is somewhat less efficient than TCSPC and does not work well at extremely low photon rates. Nevertheless, the sensitivity is well within the range required for fluorescence detection in DOT. 

In an FM MMW spectrometer the spectral source frequency is modulated at a certain rate /, typically 1 kHz. This gives rise to sidebands of the spectral source frequency above and below the carrier frequency. The frequency modulated MMW carrier has in its modulation envelope phase and amplitude relationships to the carrier. Mixing in the non-linear junction of the detector yields the modulation signals altered by their interaction with the cavity and gas inside it, with their preserved amplitude and phase relationship to the original modulation signals. Those properties are measured by passing the heterodyne mixer output and the thermal noise contribution from the mixer, to a filtered phase-sensitive detection system, with the original modulation as reference. 

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. 

If a narrow spectral feature is present with linewidth on the order of or less than 03m, the unbalancing of the two sidebands will convert the phase-modulated laser beam into an amplitude-modulated beam which produces a strong oscillating photocurrent at cUm at the detector output. More precisely, the detected signal at the I port of the mixer (in the absorption phase) is proportional to [a(cOc -I- com) - a(wc - (Om)]L where a is the absorption coefficient and L is the sample thickness. Thus the FM signal measures the difference in aL at the two sideband frequencies. For a spectral feature narrower than tUm, two copies of the absorption line appear, one positive and one negative, as each of the two sidebands is swept over the absorption. 

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