Powering portable telemetry systems for long-term monitoring presents interesting design challenges. Batteries are unsuitable for certain critical applications, and, in these circumstances, designers typically use wireless inductive links to transmit both power and data. An inductive link comprises an RF transmitter that drives a fixed primary coil, and a loosely coupled secondary coil that supplies power to the portable circuitry. For design engineers, measuring transmitted power takes on considerable importance because it imposes limits on the amount of circuitry that designers can include in the portable circuitry. Unfortunately, classical test equipment is poorly suited to the task. Standard voltage probes pick up noise that the primary coil induces, and, in some applications, the portable circuits are hermetically encapsulated in small enclosures that prevent entry of a cable or a probe.

The circuit in Figure 1 reduces noise effects because its VFC (voltage-to-frequency converter) produces a PPM (pulse-position-modulated) output signal, V_{OUT}, that integrates, or averages, noise. In addition, the design uses “load modulation” to eliminate wired connections. When the PPM signal drives on MOSFET switch Q_{1}, the switch connects an additional loading network comprising D_{1} and the series combination of R_{SF} and R_{SV }across the secondary coil, L_{S}. A load-modulation receiver connects to the primary coil and recovers the PPM signal. When you build it with surface-mounted components, the VFC circuit occupies a board area of only 238 mm^{2}.

Figure 1. |
A low-power VFC and load modulator measure power generated by a wireless-telemetry power source. |

To understand the circuit's operation, assume that a 125-kHz sinusoidal magnetic field induces approximately 4 to 16 V in secondary coil L_{S}. To improve power-transfer efficiency, L_{S} and C_{S} form a tuned, 125-kHz tank circuit having a loaded Q factor, Q_{L}, of approximately 8. Schottky diode D_{2} rectifies the voltage induced in L_{S}, and C_{1} provides lowpass filtering. The resultant dc voltage, V_{X}, powers low-dropout regulator IC_{1}, which supplies a constant 3 V to VFC IC_{2} and the load resistors, R_{LF} and R_{LV}. Trimmer potentiometer R_{LV} sets the output current at 2.5 to 13.5 mA.

The combined total current drain of the low-dropout regulator and the VFC measures a few tens of microamperes and is negligible compared with the output current. Hence, I_{IN} approximately equals I_{L}. Equation 1 expresses the dc output power that the inductive power supply produces:

(1) |

This equation shows that the output current is constant and therefore the dc output power, P_{X}, is proportional to the dc voltage, V_{X}. After setting a known initial output current adjustment via R_{LV}, you can test the inductive power supply's output ability by measuring the transmitted dc voltage that the VFC digitizes. To minimize power consumption, component count, and pc-board area, a simple passive integrating network comprising R_{C}, R_{D}, and C_{5} replaces the classical op-amp integrator that constitutes a typical VFC's input stage.

The VFC generates a constant-amplitude sawtooth voltage whose rising slope is proportional to V_{C }across integrating capacitor C_{5}. When the capacitor's voltage reaches a high reference voltage, switch Q_{2} rapidly discharges the capacitor to a low reference voltage. This action produces a free-running waveform whose frequency is proportional to the input voltage, V_{X}. A noninverting Schmitt trigger comprising comparator IC_{2}; its positive-feedback network, R_{1}, R_{2}, and C_{3}; and supply-voltage splitter R_{3}, R_{4}, and C_{4} defines the high- and low-level reference voltages, as equations 2 and 3 calculate.

(2) |

(3) |

Equation 3 shows that, to reset the integrated voltage almost to 0 V, the value of R_{1} must be slightly lower than that of R_{2}. Using standard values of E12-series resistors and taking into account power- consumption constraints, select a value of 8.2 MΩ for R_{1} and 10 MΩ for R_{2}. Replacing these values in equations 2 and 3 yields, respectively:

(4) |

To understand the VFC's operation, assume that, at start-up, capacitor C_{5} is fully discharged. Consequently, comparator IC_{2}’s output, V_{OUT}, is low and MOSFET switches Q_{1} and Q_{2} are off. Under these conditions, current through R_{C} and R_{D} begins to charge C_{5} toward V_{X} with a time constant of τ_{C} = (R_{C} + R_{D}) × C_{5}. When capacitor C_{5}’s voltage reaches the Schmitt trigger's upper threshold voltage at time t_{X}, the comparator's output, V_{OUT}, rises to V_{DD} and turns on MOSFET switches Q_{1} and Q_{2}. Switch Q_{2} discharges C_{5 }through R_{D} at time constant of τ_{D} **≈** R_{D} × C_{5}. Simultaneously, Q_{1} generates a load-modulation pulse.

When V_{C} = V_{TL}, the comparator's output drops to zero, restores the initial state, and repeats the sequence. As Trace 1 in Figure 2 shows, the circuit behaves as a free-running oscillator in which the voltage across C_{5} ramps up and down between the Schmitt trigger's threshold voltages. Given that the discharge-time constant, τ_{D}, is much less than the charging-time constant, τ_{C}, the discharge time, t_{ON}, is considerably shorter than the integrating time, t_{X}. As Trace 2 in Figure 2 shows, the comparator's output delivers a PPM signal having a relatively short pulse of approximately 320 µsec.

Figure 2. |
Oscilloscope measurements of the VFC show voltage across capacitorC _{5} (upper trace) and comparator IC_{2}’s output voltage, V_{OUT} (lower trace),for a nominal input voltage of 12 V. |

Equations 5 and 6, respectively, describe the complete expressions for calculating the pulse widths of waveforms t_{X} and t_{ON}:

(5) |

(6) |

These formulas are useful for designing the VFC in Figure 1 but yield little insight into the circuit's global-transfer function. You can apply the following approximations to simplify the calculations: Because t_{X} >> t_{ON}, the PPM output frequency is approximately f_{X} **≈** 1/t_{X}. In normal operation, V_{X} reaches relatively high values when compared with the Schmitt trigger's threshold voltages, and you can linearize the charging law of capacitor C_{5} to a ramp having a constant slope (Equation 7).

(7) |

According to Equation 4, the Schmitt trigger's high and low threshold voltages are, respectively, V_{TH} **≈** V_{DD} and V_{TL} **≈** 0 V. Using these approximations, the PPM output frequency simplifies to:

(8) |

Equation 8 shows that the circuit in Figure 1 exhibits a voltage-to-frequency transfer function, as Figure 3 experimentally confirms. The VFC's power consumption is low; for example, at a dc voltage of 12 V, the VFC's current drain is about 36 µA.

Figure 3. |
The measured transfer function of the VFC exhibits excellent linearityover a wide range of inductively coupled input voltages. |