RF-telemetry transmitter features minimal parts count

The circuit in Figure 1 meets these criteria and uses only three off-the-shelf ICs and a few passive components. Although dedicated to conditioning the low-level signal a strain-gauge bridge produces, the circuit can operate with almost any resistive transducer based on a Wheatstone bridge. The circuit comprises a VFC (voltage-to-frequency converter) that produces a PPM (pulse-position-modulation) output, and an OOK (on/off-keyed) RF transmitter.

To understand the circuit's operation, assume that the comparator's output is high and, consequently, the inverter's output is low. Also assume that the series combination of the strain gauge, RX, and the trimmer, RT, is always equivalent to R(1+X)>R. That is, for linear operation, the sensor's resistance variation represents a small fraction of the arm resistance. Under these conditions, the noninverting input of IC1 biases to VDD/2, and the Wheatstone bridge's active arm drives a positive current, II, into the summing node of IC1. This current causes the integrator's output, VOI, to ramp down toward the low threshold voltage, VTL, of the Schmitt trigger. When VOI=VTL, the comparator's output goes to zero, and the inverter's output consequently rises to VDD. This action inverts the direction of the integrator's input current, causing the integrator's output to ramp upward to the Schmitt trigger's high threshold voltage. Finally, when VOI=VTH, the comparator's output goes high, and the above sequence repeats indefinitely, producing a free-running oscillation in which the integrator's output ramps up and down between the threshold voltages of the Schmitt trigger (Trace 1 in Figure 2). Meanwhile, the comparator's output and the inverter's output deliver two square waves with a 50%-duty-cycle ratio (traces 2 and 3, respectively, in Figure 2) that drive the...

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