A
Anonymous
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In a conventional PI, at the end of the flyback pulse, a fixed resistor is used to provide something close to critical damping for the loop LC circuit. After everything settles down, then the receiver is turned on. For the sake of maximum sensitivity, it is desirable to get the receiver turned on as soon as possible, but if it's turned on too soon, you'll be looking at flyback decay (for lack of a better word), which may cause problems with allowable gain or with response to magnetite.
FIX #1: FLYBACK-RESPONSIVE TURN-ON
If you have a comparator detect the end of flyback, you can use this to activate a timer that will turn on the receiver a fixed number of microseconds later.
On the face of it, it seems like you'd have to detect a threshold voltage of several hundred volts, presumably through a resistive voltage divider. However, there are better alternatives.
Alternative #1: use a capacitive voltage divider. Remember to supply bias current to the comparator input.
Alternative #2 (probably the best method): use a differentiator which trips the comparator whenever the rate of change exceeds a threshold value, going in the direction of end-of-flyback. This method can be very fast and virtually noise-free. This method may use a capacitive voltage divider if desired.
If the leading edge of the receiver turn-on is variable, either the timing must be a fixed duration, or the "negative demodulator" will have to be varied in proportion. Otherwise there will be problems with earth field pickup.
FIX #2: DUMP THE CAPACITOR CHARGE
Provide a bipolar fast switching transistor, emitter to ground end of the coil, collector in series with a diode going to the other end. (A MOSFET could be used, but might not be the best choice due to drain capacitance.) When end-of-flyback is detected, turn the transistor on. It will short out the distributed/cable capacitance, bringing the voltage down to one diode drop almost instantly. From then on out, the damping resistor passively takes over. It is not necessary to immediately turn off the switching transistor.
IMPLICATIONS FOR CAPACITANCE
The "capacitor dump" technique makes it possible to tolerate more capacitance than would otherwise be acceptable in the design. In some cases this may allow the use of a different, higher capacitance loop cable, or allow higher inductance in order to reduce power consumption.
FIX #1: FLYBACK-RESPONSIVE TURN-ON
If you have a comparator detect the end of flyback, you can use this to activate a timer that will turn on the receiver a fixed number of microseconds later.
On the face of it, it seems like you'd have to detect a threshold voltage of several hundred volts, presumably through a resistive voltage divider. However, there are better alternatives.
Alternative #1: use a capacitive voltage divider. Remember to supply bias current to the comparator input.
Alternative #2 (probably the best method): use a differentiator which trips the comparator whenever the rate of change exceeds a threshold value, going in the direction of end-of-flyback. This method can be very fast and virtually noise-free. This method may use a capacitive voltage divider if desired.
If the leading edge of the receiver turn-on is variable, either the timing must be a fixed duration, or the "negative demodulator" will have to be varied in proportion. Otherwise there will be problems with earth field pickup.
FIX #2: DUMP THE CAPACITOR CHARGE
Provide a bipolar fast switching transistor, emitter to ground end of the coil, collector in series with a diode going to the other end. (A MOSFET could be used, but might not be the best choice due to drain capacitance.) When end-of-flyback is detected, turn the transistor on. It will short out the distributed/cable capacitance, bringing the voltage down to one diode drop almost instantly. From then on out, the damping resistor passively takes over. It is not necessary to immediately turn off the switching transistor.
IMPLICATIONS FOR CAPACITANCE
The "capacitor dump" technique makes it possible to tolerate more capacitance than would otherwise be acceptable in the design. In some cases this may allow the use of a different, higher capacitance loop cable, or allow higher inductance in order to reduce power consumption.