Detector noise synchronous

Even more interesting is a new spectrometer that has been built with a novel moving system that synchronizes the movement of the radioactive source with that of the detector (206). This system allows the accumulation of Mossbauer spectra either in constant acceleration or constant velocity modes, therefore giving better signal-to-noise ratios and also reducing the time necessary for accumulation of a spectrum. Narrower line widths were also obtained with this equipment design. 

A second strategy is, of course, to avoid the generation of synchronous noise in the system. Typical sources of coherent noise are cavity dumpers, pulse pickers, and picosecond diode lasers. For the shielding of these devices the same rules should be applied as for detector shielding. 

Lock-in amplifiers provide a very narrow frequency band pass and thus achieve an excellent signal-to-noise ratio. With suitable resistance-capacitance filters, an effective bandpass width of 1 Hz can be obtained. The lock-in amplifier uses a chopper to modulate the energy source. A reference source, usually a small flashlight bulb, is modulated at exactly the same frequency. The two signals are combined in a synchronous detector to produce sum and difference frequencies the sum frequency will be twice the chopping frequency and the difference frequency will be zero. 

However, for the same data collection time, interferometry is more sensitive to multiplicative noise (i.e., noise proportional to the signal) than continuous-scan interferometry [591]. To eliminate the multiplicative and 1// noise, phase modulation (at 400 Hz) of IR radiation in conjunction with LIA demodulation is used [591]. Since the LIA and some IR detectors need the IR signal to be modulated at a single carrier frequency, a mechanical chopper, phase modulation (when at each position the fixed mirror is dithered at a fixed frequency), or modulation of absorption of the sample is used to produce a carrier frequency. In this case, the TR measurement is referred to as a synchronous multiple-modulation experiment. Multiple modulation is unnecessary if the so-called dc coupled detector which does not require a varying signal is used. 

The use of a PLL to provide a stable reference signal for synchronous detection has certain advantages for our application. The voltage-controlled oscillator is set to operate at the intermediate frequency of the detector stages 1 and 2, that is, 70 kHz. The operating range of the VCO, namely provides noise re-... 

The synchronous microvoltmeter detector allows linear (to 1 dB) measurements of signal amplitudes of 1 to 500 pV with a band passing at -3 dB higher at 9 MHz. Between 0.1 and 1 pV, signifieant noise prevents a precise reading of the amplitude, which however remains proportional to the signal to be measured, contrary to the situation with the unmodified spectrometer. 

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