R7 and R8 form a potential divider across the power supply and bias the gate of Q1 at 1/3 supply voltage. Q1, R5 and Q2, R6 form a Wheatstone bridge circuit, the meter M1 and preset PR1 are at the centre of the bridge. With no input signal, VR1 is used so that M1 reads 0V. Q1 and Q2 should be matched as evenly as possible with values drain source current matched within 10% or better.
With the range switch S2 at position 1, apply a calibrated 0.5V DC voltage to the input terminals. Note that this circuits ground is actually at 1/3 the supply voltage which is why it is battery powered and isolated from other circuitry. With a 0.5V input signal adjust PR1 so the meter reads full scale. Remove the input signal and make sure reading is zero, if not adjust VR1 again and repeat procedure.
R1,R2 and R3 should be 1% tolerance resistors and set the input impedance at 11.1M. Any analogue meter responds much faster than a digital multi-meter and can easily detect more than 20 changes per second. For any input signal that changes faster than this then an oscilloscope is required.
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