Diode Charge Pump AM-FM Demodulators


Posted on Feb 4, 2014

Frequency-to-voltage converters form part of a wide variety of instrumentation circuits. They also find use in radio as FM demodulators. One interesting configuration for this application is the Diode Charge Pump circuit (DCP), which also doubles as an AM detector. The DCP is basically a pulse-driven half-wave voltage doubler. Its use as a demodulator derives from the analysis of charge transfer taking place between circuit components.


Diode Charge Pump AM-FM Demodulators
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In this article I will attempt to explain why the demodulation process takes place in the DCP. Following, the circuit will be studied under AC sine-wave excitation. Let s begin then analysing a voltage doubler driven by a periodic train of single-polarity pulses having a duty cycle of 50% (Fig. 1. a). We shall model this situation by a switch that toggles between a battery delivering V1 volts and a resistor R1 connected to ground (Fig. 1. b). The switch stays in each position equal periods of time. When the switch is in position "a" a pulse of height V1 is applied to Cp. The charge received by this capacitor is distributed between Cr and resistor Rr. At the end of the pulse, Cp discharges through R1 and D1 (switch in position "b"). Diode D2 does not conduct (is an open circuit) on this interval. As a consequence, Cr discharges through Rr. When the switch returns to the "a" position, the operation cycle is repeated. If the pulse rate is sufficiently high, Cr`s discharge will be incomplete on each cycle and a continous current will flow through Rr. Here, q1 is the charge received by Cp per pulse; q2 is the charge transferred to Cr, also per pulse (it restores the charge lost by this capacitor in the preceding cycle) and qr is the charge that diverts through Rr (fraction of q1 that doesn t reach Cr). Clearly, a linear relationship exists between Vo and the pulse rate f, and also between the output and the height V1 of the...




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