Mixer, radio frequency

Schematic diagrams for radio-frequency (r.f.) electronics are shown in Fig. 5.2. The circuit in Fig. 5.2(a) is a simple heterodyne circuit. The pulse length is defined by the switch SI. The speed of this switch determines the minimum pulse length, and hence the minimum lens focal length, and hence the highest frequency of the microscope thus the limit to the resolution is ultimately determined by the highest useful speed of this switch. S2 is the single-pole-double-throw (s.p.d.t.) switch, and Al the low-noise preamplifier. The mixer is a device that takes two inputs, the radio-frequency signal and the...

Fig. 5.9.1 A spectmm of the R(ll) line of ethane on Jupiter recorded with a CO2 laser infrared heterodyne spectrometer. LO marks the frequency of the laser local oscillator. The observed 25 MHz spectmm (histogram) was detected with a 64-channel radio frequency receiver at the output of the infrared detector mixer. The dashed and solid curves are calculations of the line shape (Kostiuk et al, 1987).

The use of the mixer and amplifier combination thus determines which sky frequency is observed. In practice, at a radio telescope, the value of the intermediate frequency is fixed (usually in the range 1.5-8 GHz), and the value of 11.0. is chosen to obtain the desired sky frequency according to the equation ... 

Figure 1. Schematic of the detection system at a radio telescope using an SIS (superconducting-insulating-superconducting) mixer and a HEMT (high electron mobility transistor) amplifier. Here v ky is the sky frequency, Vio ibe local oscillator frequency, and Vif is the intermediate frequency.

The frequency synthesizer is a General Radio (now Gen-Rad) model GR 1164-A with the range 0.01-70 MHz. It is used to provide the frequencies referred to as v+10 MHz and 5 MHz. The 5 MHz is then doubled by a doubler (Relcom D8E) to provide the 10 MHz intermediate frequency. The double balanced mixers are Vari-L model DB-100B. 

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. 

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