Indirect reflection detection for receiver circuitry protection in TDD transceivers
In an example, a receiver protection circuit includes a directional coupler configured to be coupled to a receive signal path of a communications device between an isolation device and an amplifier of the receive signal path. The receiver protection circuit further includes a power detector coupled to the directional coupler and configured to measure a power level of a transmit leakage signal in the receive signal path. The receiver protection circuit further includes a comparator configured to compare the measured power level of the transmit leakage signal in the receive signal path to a threshold value and output an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold value.
1. A communications device, comprising:
a transmit signal path;
a receive signal path;
an isolation device coupled to the transmit signal path and the receive signal path, wherein the isolation device is configured to provide an analog transmit signal from the transmit signal path to an antenna port of the isolation device, wherein the isolation device is configured to provide an analog receive signal from the antenna port to the receive signal path;
a coupler coupled to the receive signal path between the isolation device and an amplifier of the receive signal path;
a power detector coupled to the coupler and configured to measure a power level of a transmit leakage signal in the receive signal path; and
a comparator configured to compare the measured power level of the transmit leakage signal to a first threshold value and output a first alarm signal indicating that the measured power level of the transmit leakage signal exceeds the first threshold value;
wherein the communications device is configured to reduce a transmit output power in response to the first alarm signal.
2. The communications device of claim 1 , wherein the communications device is configured to reduce the transmit output power by:
turning off a driver stage in the transmit signal path;
reducing a supply voltage of the driver stage in the transmit signal path;
reducing a supply voltage of a power amplifier in the transmit signal path;
attenuating a signal traversing the transmit signal path; and/or
switching off a bias signal for a power amplifier in the transmit signal path.
3. The communications device of claim 1 , further comprising:
a resistor and a thermistor that form a passive compensation circuit;
wherein the communications device is configured to adjust the first threshold value based on an output of the passive compensation circuit.
4. The communications device of claim 1 , further comprising:
a temperature sensor;
a humidity sensor; and
a controller configured to adjust the first threshold value based on a temperature measurement from the temperature sensor and a humidity measurement from the humidity sensor.
5. The communications device of claim 1 , further comprising a switch in the receive signal path, wherein the switch is positioned between the coupler and the amplifier, wherein the switch is configured to provide the analog receive signal to the amplifier in a first state, wherein the switch is coupled to a termination in a second state;
wherein the communications device is configured to control the switch to be in the first state or the second state based on a time-division duplexing schedule.
6. The communications device of claim 5 , wherein the communications device is configured to control the switch to be in the second state in response to the first alarm signal.
7. The communications device of claim 1 , wherein the first threshold value is determined based on a known relationship between return loss at the antenna port and isolation between the transmit signal path and the receive signal path.
8. The communications device of claim 1 , wherein the isolation device is a circulator.
9. The communications device of claim 1 , wherein the power detector is a peak power detector.
10. The communications device of claim 1 , wherein the communications device is one of:
a remote unit of a distributed antenna system;
a radio frequency repeater;
a radio point for a small cell;
an access point; or
a remote radio head of a base station.
11. The communications device of claim 1 , wherein the comparator is configured to compare the measured power level of the transmit leakage signal to a second threshold value that is different than the first threshold value and output a second alarm signal indicating that the measured power level of the transmit leakage signal exceeds the second threshold value;
wherein the communications device is configured to reduce the transmit output power in response to the second alarm signal.
12. A method of protecting receiver circuitry, comprising:
measuring a power level of a transmit leakage signal in a receive signal path;
comparing the measured power level of the transmit leakage signal to a threshold voltage;
outputting an alarm signal indicating that the measured power level of the transmit leakage signal exceeds the threshold voltage; and
reducing a transmit output power in response to the alarm signal.
13. The method of claim 12 , wherein reducing the transmit output power includes:
turning off a driver stage in a transmit signal path;
reducing a supply voltage of the driver stage in the transmit signal path;
reducing a supply voltage of a power amplifier in the transmit signal path;
attenuating a signal traversing the transmit signal path; and/or
switching off a bias signal for a power amplifier in the transmit signal path.
14. The method of claim 12 , further comprising controlling a switch in the receive signal path to be in a first state or a second state based on a time-division duplexing schedule, wherein the switch is configured to provide an analog receive signal to an amplifier of the receive signal path in the first state, wherein the switch is coupled to a termination in the second state.
15. The method of claim 14 , further comprising controlling the switch to be in the second state in response to the alarm signal.
16. A receiver protection circuit, comprising:
a directional coupler configured to be coupled to a receive signal path of a communications device between an isolation device and an amplifier of the receive signal path;
a power detector coupled to the directional coupler and configured to measure a power level of a transmit leakage signal in the receive signal path; and
a comparator configured to compare the measured power level of the transmit leakage signal in the receive signal path to a first threshold value and output a first alarm signal indicating that the measured power level of the transmit leakage signal exceeds the first threshold value.
17. The receiver protection circuit of claim 16 , further comprising:
a resistor and a thermistor that form a passive compensation circuit;
wherein the first threshold value used by the comparator is configured to be adjusted based on an output of the passive compensation circuit.
18. The receiver protection circuit of claim 16 , further comprising:
a temperature sensor;
a humidity sensor; and
a microcontroller configured to adjust the first threshold value based on a temperature measurement from the temperature sensor and a humidity measurement from the humidity sensor.
19. The receiver protection circuit of claim 16 , wherein the power detector is a peak power detector.
20. The receiver protection circuit of claim 16 , wherein the comparator is configured to compare the measured power level of the transmit leakage signal to a second threshold value that is different than the first threshold value and output a second alarm signal indicating that the measured power level of the transmit leakage signal exceeds the second threshold value;
wherein the communications device is configured to reduce a transmit output power in response to the second alarm signal.