Sampling Circuits:

Sampling Circuits is one which allows a comparison of instantaneous values derived at different instants of time, thereby dispensing with the need to phase shift and mix signals derived from the primary line quantities.

Figure (10.36) shows a sampling circuits in conjunction with an amplitude pulse width convertor (A/W convertor), this complete circuit forms the measuring unit for a relay. The circuit works as follows.

When i_G is zero, a very high impedance is inserted between the signal voltage V_S and A/W convertor. The maximum V_S must be less than the breakdown voltage of the zener diode, otherwise conduction can take place via the zener diode and the V_G source impedance. When i_G is nonzero, i.e. during the 50μs sampling period provided that i_S<i_G the resistance of the bridge between V_S and the A/W convertor is small.

The current i_G is caused by the application of a rectangular 50μs voltage pulse V_G to the diode bridge, where V_G is large enough to break through the zener diode. It is found that the stray capacitance of the bridge diodes and the zener diode are such as to allow a very fast positive going edge to produce a spike at the output, even though i_G is zero. The circuit comprising

T₁,R₁ and C₁ is added to deal with this contingency. The time constant R₁ C₁ is such that T₁ is held off except when V_S attempts to increase rapidly. Purely by considerations of cause and effect this circuit cannot remove a spike from V_S, but by switching on T₁ and drawing a spike of current through the signal source impedance, the rate of rise of discontinuity is slowed down sufficiently to prevent it getting through the diode bridge.

During the 50μs sampling period, C charges up through the resistance r+R_S, where R_S is the signal source resistance. R>r+R_S and can therefore be neglected during the charging period. At the end of the sampling period, C commences to discharge through R towards the negative supply voltage. The diode bridge resistance is now very much larger than R and can be neglected during the discharge period.

The transistor T₂ is normally on and is switched off at the start of the sampling period as the voltage on C goes positive. It switches on again when the voltage on C is approxi­mately zero. The output at the collector of T₂ is therefore a pulse of length T’ which is proportional to V_S at the instant of Sampling Circuits provided that T′≪CR.