Distortion compensation circuit including one or more phase invertible distortion paths
A distortion compensation circuit compensates for distortion generated by one or more non-linear elements such as a laser device and may include a primary signal path for carrying an input signal and one or more secondary signal paths for generating distortion. The distortion compensation circuit may also include one or more controllable phase inverters on at least one of the paths. For example, the secondary signal path may include a distortion generator to produce distortion products from the input signal and a signal controlled phase inverter that inverts the phase of the distortion products. The distortion generator and phase inverter may be combined as an invertible distortion generator. The phase inversion may be controlled in response to a phase inversion control signal generated based on one or more parameters such as temperature.
1. An optical transmitter comprising:
a RF signal input configured to provide a RF input signal;
a predistortion circuit configured to receive the RF input signal and to generate a predistorted RF input signal, the predistortion circuit comprising a primary signal path configured to receive at least a portion of the RF input signal and a secondary signal path coupled to the primary signal path and configured to receive at least a portion of the input signal, the secondary signal path comprising:
at least one distortion generator configured to produce intermodulation distortion products from the RF input signal on the secondary path; and
at least one signal controlled phase inverter configured to invert the intermodulation distortion products produced by the distortion generator in response to a phase inversion control signal;
a laser configured to receive the predistorted RF input signal and to generate a modulated optical output, wherein the predistorted RF input signal compensates for distortion generated by at least the laser; and
a controller coupled to the predistortion circuit and configured to generate the phase inversion control signal.
2. The optical transmitter of claim 1 wherein the secondary signal path further includes at least one variable attenuator responsive to an attenuation control signal.
3. The optical transmitter of claim 1 wherein the controller is configured to generate the phase inversion control signal to control phase inversion in response to at least one parameter.
4. The optical transmitter of claim 1 wherein the controller is configured to monitor temperature, to determine when a change in temperature results in a change in phase of distortion, and to generate the phase inversion control signal when the change in temperature results in a change in phase.
5. An invertible distortion generator comprising:
a RF input configured to receive a RF input signal;
a distortion output configured to provide a distortion signal generated from the RF input signal;
a distortion generator portion configured to generate distortion products from the RF input signal;
a balun including first and second input terminals coupled to the distortion generator portion and first and second output terminals configured to be coupled to either ground or to the distortion output, wherein a phase of the distortion provided on the distortion output from the second output terminal is inverted relative to a phase of the distortion provided on the distortion output from the first output terminal; and
a switching unit coupled to the balun, the switching unit being configured to, in response to a phase inversion control signal, couple one of the first and second output terminals to ground and the other of the first and second output terminals to the distortion output.
6. The invertible distortion generator of claim 5 wherein the distortion generator portion generates even order intermodulation distortion products.
7. The invertible distortion generator of claim 5 wherein the distortion generator portion includes a diode pair.
8. The invertible distortion generator of claim 7 wherein the diode pair is configured to receive a bias current, and wherein a magnitude of the distortion products is related to the bias current.
9. The invertible distortion generator of claim 5 wherein the switching unit is selected from a group consisting of a RF switch, a RF relay, and a RF MEMS switch.