REMOTE THE SENSOR FROM THE DA LOGGER - SYSTEM ANALYSIS & USE OF AMPLITUDE MODULATION
In a previous post we have examined the case of remoting the DA logger together with its companion PC (Slave PC) and connecting the Slave with the local Master PC via an Internet application. We have called this «Solution A.». Here below is a condensed panoramic of some of the solutions possible.
SOLUTION A. - DIGITAL – Internet connection
Pros. (1) All DA logger channels transmitted. (2) No limitation in acquisition speed/accuracy. (3) DA Logger works as if it were directly connected to the Master PC. (4) No special hardware necessary. (5) No extra software necessary.
Cons. (1) Expensive and hardly justified for a small number of channels. (2) Not suitable for harsh environments. (3) Does not provide protection against local noise or ground loop effects. (4) Uses two PCs wide open to Internet viruses and hacking, therefore fully dedicated PCs must be used. (5) Extra care needed with collected data files.
SOLUTION B. – Wire connection
Solution B1 – ANALOGUE - A.M. (Amplitude Modulation)
Pros. (1) Conceptually simple and reliable hardware. (2) Extremely robust and immune from external interference/noise. (3) Provides excellent galvanic isolation between sensor and DA Logger.
Cons. (1) Requires twisted wire connection. (2) Special hardware must be built. (3) Requires initial system calibration on the field. (4) Remotes one channel only. For more channels (8 or 16) additional multiplexing hardware required, plus 3 additional twisted pairs. (5) Reduced acquisition speed.
SOLUTION B2 – ANALOGUE – F.M. (Frequency Modulation)
Pros. (1) Provides yet maximum robustness and immunity from external interference/noise. (2) Provides excellent galvanic isolation between sensor and DA Logger. (3) Does not require system calibration in the field.
Cons. (1) Far more complex hardware for reliable operation. (2) Requires twisted wire connection. (3) Special complex hardware must be built. (4) Remotes one channel only. For more channels (8 or 16) additional multiplexing hardware, plus 3 additional twisted pairs required. (5) Reduced acquisition speed.
SOLUTION B3 – DIGITAL – Microprocessor interconnection
To be discussed in a future post.
In environments not suitable for equipment such as PCs and DA loggers, we must resort to one form of «Solution B. ». Under the circumstances, the best choice seems that of Amplitude Modulation. It is an extremely robust technology, affording excellent reliability in noisy environments. The hardware uses proven, simple and inexpensive components and provides double galvanic isolation between sensor and DA Logger. In its simplest form provides remoting for one DA channel only, but with the addition of Multiplexing (to be discussed in a future post) will accept up to 8 (or 16) DA channels at the price of reduced acquisition speed.
With reference to the Blocks Diagram, the Carrier Generator IC2 outputs a 5Vpp, fixed frequency, 4 Khz square wave carrier (other frequencies possible and given in next post). The main advantage of AM is that carrier frequency drift, such as can be expected by a simple NE 555 oscillator, has no effect on system performance. The carrier also excites the –6V (nominal) Negative Supply Generator necessary to the Level Shifter. Level shifting is necessary to avoid transmission of zero amplitude carrier at zero output from the sensor. With level shifting « zero level » corresponds to a carrier level of 1 V.
Sensor output voltage (0 to 2.5V) reaches the unity gain Level Shifter IC1A, which linearly translates the System Dynamic Range to 1 to 3.5V for the input of the Amplitude Modulator Q2. Q2’s variable squarewave output level depends on the Level Shifter’s output voltage and can have any value between 1 and 3.5 V, which is reduced by the following Carrier Control trimmer and 40 dB attenuator to 10 to 35 mV. The square wave is now processed by a 3 Pole Lowpass Filter IC1b. The symmetrical square wave contains odd order harmonics only, which are attenuated by the lowpass filter, to obtain a sinewave with an harmonic distortion of approx. 5%. Sine wave operation is advisable to avoid possible level distortions in the transmission path. The sine wave carrier is now amplified by 40 dB by Power Amplifier IC3 and the resultant 1 to 3.5 V sine wave feeds the low impedance primary of Line Transformer T1, having a 1:20 turns ratio.
Across T1’s secondary we have a balanced, ground decoupled, signal level of 20 to 70 V, which feeds the wireline. System balance with respect to ground, plus high voltage level provide strong immunity to noise and interference which could be picked up underway. Additional protection is provided at the receiving end, by Line Transformer T2, identical to T1, but with a reversed connection. Across T2’s secondary we now have a 1 to 3.5 V signal, i.e. reduced by a factor of 20 (26 dB).With this arrangement interference picked up in the transmission path will be also attenuated by 26 dB. T2 feeds a 20 dB Resistive Attenuator, so that the nominal level to the DA Pico Logger input is 100 to 350 mV.
Theoretical wireline path loss at audio frequencies, for a length of 1 mile, depending on wire diameter and resistivity, amounts to less than -0.12 dB, or -1.4%. The 20 dB (nominal) attenuator (Rh/Rk) should be used, if at all necessary, to set the AC level at the DA Logger ‘s input in the 100 to 350 mV range.
Circuit diagram and detailed description in the following post.