Synchronous demodulator

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. 

The synchronous demodulator (4) eliminates the quadrature signal. This demodulator multiplies the input signal with a reference signal, which is generated by the sensor drive velocity. The signal conditioner (7) corrects any time delay and phase shift, because the reference signal has to relate in the phase exactly to the velocity of the sensor drive. 

Fig. 7.2.17 1 -4, Piezo element pairs 5, electronics 6, band-pass filter 7, reference phase 8, synchronous demodulator Ua, output ...

Transformer-coupled isolation amplifiers perform on the basis of inductive transmission of a carrier signal that is amplitude modulated by the biosignal. A synchronous demodulator on the output port reconstructs the signal before it is fed through a Bessel response low-pass filter to an output buffer. A power transformer, generally driven by a 400-900 kHz square wave, supphes isolated power to the amplifier. 

The receiver for the synchronous demodulation of double-sideband suppressed-carrier (DSB-SC) waves is shown in Fig. 20.49. In general, it is easy to correct the effect of attenuation or amplitude errors through amplification, filtering, or other processing in the receiver. It is more difficult to correct the effect of phase or frequency errors. 

FIGURE 20.51 An illustration of the effect of frequency error in synchronous demodulation of SSB-SC signals (a) amplitude spedrum of message signal, (b) amplitude spectrum of SSB (USB) signal, (c) amplitude spectrum of demodulated USB signal when A/ < 0, (d) amplitude spectrum of demodulated USB signal when A/ > 0. 

Consider the synchronous demodulation of a DSB-SC signal plus the interfering sinusoid previously described. The same receiver of Fig. 20.49 is employed, but in this case, the phase and frequency errors between the received and the local carriers are ignored. The received signal plus interference r ft), the product modulator output x t), and the demodulated output of the low-pass filter yft), are given, respectively, by... 

Keywords— Electrical Impedance Tomography, Audio Codec, Digital Signal Processor, Synchronous Demodulation, Howland Current Source... 

Fig. 5 synchronous demodulator implemented in the DSP to obtain Vqut real component. 

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