Impedance compensation circuit
The present invention provides an adjustable radio frequency (“RF”) impedance method. Embodiments of the invention provide voltage adjustable RF impedance termination methods.
1. A radio frequency apparatus comprising:
a signal input and a signal output;
a first variable impedance device loading the signal input;
a second variable impedance device including a variable capacitor with capacitance varying inversely with an applied voltage; and,
the second variable impedance device configured to compensate for variations in impedance at the signal input caused by the first variable impedance.
2. The radio frequency apparatus of claim 1 further comprising:
a Wilkinson 75 ohm hybrid splitter with a splitter input wherein the signal input is the splitter input;
a first splitter output is loaded by the first and second variable impedances; and,
a second splitter output is loaded by a nominal 75 ohm impedance.
3. The radio frequency apparatus of claim 2 further comprising:
a varactor diode configured to provide the variable capacitor of the second variable impedance device.
4. The radio frequency apparatus of claim 3 further comprising:
an externally powered amplifier that includes the first variable impedance device; and,
wherein a tendency of the signal input impedance to change during an amplifier power outage is reduced by a corresponding change of the impedance of the varactor diode.
5. A radio frequency device comprising:
a two way power divider with an input and first and second outputs;
the power divider input for connecting to a cable television distribution system;
a varactor diode shunt circuit extending between the power divider first output and a ground; and,
the varactor shunt circuit configured to compensate for downstream impedance changes tending to vary an impedance at the power divider input.
6. The radio frequency device of claim 5 further comprising:
the varactor diode shunt circuit interconnecting with a filter block;
the filter block including input and output diplexers with high and low frequency signal paths therebetween; and,
the high frequency signal path including an amplifier for amplifying signals derived from the power divider first output signals.
7. The radio frequency device of claim 6 further comprising:
an external power supply for supplying DC voltage to the amplifier;
wherein, an external power supply power outage causes the amplifier input impedance to change; and,
wherein an amplifier impedance change is compensated for by action of the varactor diode shunt circuit.
8. The radio frequency device of claim 7 further comprising:
a low frequency varactor shunt in the varactor diode shunt circuit; and,
a high frequency varactor shunt in the varactor diode shunt circuit.
9. The radio frequency device of claim 8 further comprising:
the varactor diode shunt circuit isolated from a splitter by a first RF bypass capacitor;
the variactor diode shunt circuit isolated from the filter block by a second RF bypass capacitor; and,
a blocking diode interposed between the RF bypass capacitors, the diode cathode interconnecting with the first RF bypass capacitor.
10. The radio frequency device of claim 9 wherein the an external power supply outage results in a varactor bias voltage decrease of at least 6 volts and a corresponding varactor diode shunt capacitance at least 6 times greater than varactor diode shunt capacitance before the outage.
11. A radio frequency apparatus comprising:
a signal input and one or more signal outputs;
a first variable impedance source within the radio frequency device loading the signal input;
a second voltage-controlled variable impedance device having a capacitance varying inversely with an applied voltage; and,
the second variable impedance device compensating for variations in impedance at the signal input caused by the first variable impedance source.