Figure 1 A Simple CarbonResistance Thermometer for Low Temperatures
Related Figures (7)
Fic. 1. Block diagram of the apparatus. CA =current amplifier, LNP = noise preamplifier, WBA = wide-band amplifier, HPF = high- pass filter, IVD = inductive voltage divider, R,, R, standard and sensor resistances, THR = thermoregulator, SYF = synchronous filter. The block diagram of the instrument is presented in Fig. 1. The details of the circuitry are described in Sec. I. The Fic. 2. Detailed circuitry of the oscillator, the current amplifier, and the bridge. where a is the ratio between the voltage V,,, appearing across the R, arm of IVD, and the total driving voltage Vo. The signal at this point is usually buried in thermal white noise: any further nonselective amplification will over- load the amplifier and the information will be lost. On the other hand, conventional narrow bandpass amplifiers intro- duce difficulties with phase instability. A solution to this problem is offered by a double-channel synchronous filter (Fig. 4), which yields good noise rejection and also the split- ting of the in-phase and quadrature component of the signal. The square waves SWF and SWQ obtained at the output of each channel are then amplified and fed into the lock-in am- plifier (Fig. 5). Each channel of the lock-in section is made up of a multiplier, a low-pass filter with selectable time con- stant, a dc amplifier, and a readout meter. The intrinsic resolution of the bridge is limited by the minimum change a in the ratio of the inductive voltage di- vider FIG. 3. Detailed circuitry of the low-noise preamplifier (gain = 100), of the wide-band preamplifier (gain = 1, 10, 100, 1000), and the high-pass filter. Fic. 5. Details of the lock-in amplifier (only the in-phase channel is shown). Fic. 6. The thermoregulator circuit. When set into the automatic mode the base current is modulated by the V- signal and by its time derivative dV,,/ at. Fic. 7. (a) Check of the bridge sensitivity. (b) Check of the amplifier chain stability.
