Patent Application
20120329415
The present disclosure is generally related to receiver circuits and methods for receiving radio frequency signals, and more particularly to reducing interference in receivers and associated methods.
For any receiver, the sensitivity performance (noise figure) and the inter-modulation performance (3rd order intercept point) present opposing requirements on the design of the receiver. High values of gain, particularly in the front-end of the receiver, achieve relatively high sensitivity but result in relatively poor inter-modulation (IM) performance. Conversely, moderate to low values of front-end gain produce improved IM performance at the expense of sensitivity.
In an embodiment, a circuit includes a front-end circuit having an input for receiving a radio frequency signal, a control input, and an output. The front-end circuit includes an adjustable gain. The circuit further includes a received signal strength indicator (RSSI) circuit including an input coupled to the output of the front-end circuit and including an output. The circuit also includes a control circuit including an input coupled to the output of the RSSI circuit and an output coupled to the control input of the front-end circuit. The control circuit is configured to iteratively adjust the adjustable gain by a pre-determined adjustment until a difference between a first RSSI measurement and a second RSSI measurement is less than or approximately equal to the pre-determined adjustment.
In another embodiment, a method for adjusting a gain in a front-end circuit of a receiver includes detecting interference in a signal received at an input of a receiver. The method further includes decreasing a gain of the front-end circuit by a pre-determined adjustment when the interference includes inter-modulation interference and increasing the gain of the front-end circuit by at least the pre-determined adjustment when no inter-modulation interference is detected in the interference.
In another embodiment, a receiver includes a front-end circuit including an input, a control input, an output, the input for receiving a signal. The receiver further includes a received signal strength indicator (RSSI) circuit including an input coupled to the output of the front-end circuit and including an output. The RSSI circuit is configured to provide an RSSI value of the signal to the output of the RSSI circuit. The receiver further includes a controller having an input coupled to the output of the RSSI circuit and an output coupled to the control input of the front-end circuit. The controller iteratively adjusts a gain of the front-end circuit by a pre-determined adjustment until a difference between a first RSSI value and a second RSSI value is less than or approximately equal to the pre-determined adjustment.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Further, the drawings represent block diagrams to facilitate the discussion and are not intended to show every detail, such as implementation specific details.
Embodiments of a receiver and methods described in this document make use of the fact that inter-modulation products may not always be harmful. The receiver is configured to determine if inter-modulation products are in the band of interest. If not, the receiver can aggressively reduce the attenuation to a low level, such as zero decibels (0 dB), and the gain of the low noise amplifier can be set to a relatively high level to provide enhanced sensitivity, and the receiver can utilize channel filtering to remove the inter-modulation products. If, however, the inter-modulation products are in the band of interest, the receiver dynamically reduces the gain, which helps to make the front-end circuitry more linear and hence more robust against inter-modulation distortion.
In particular, a receiver includes front-end circuitry including an input for receiving a radio frequency (RF) signal and an output for providing a gain adjusted and filtered signal. The receiver further includes an received signal strength indicator (RSSI) circuit to measure the gain adjusted RF signal to produce an RSSI signal and a controller coupled to the RSSI circuit and to the front-end circuit and configured to selectively adjust a gain of the front-end circuit in response to receiving the RSSI signal.
When the controller detects that there is no harmful interference (i.e., no inter-modulation interference in the band of interest) in the gain adjusted RF signal, the controller adjusts the gain of the front-end circuit to a relatively high level, enhancing sensitivity of the receiver. If the controller detects harmful interference in the gain adjusted RF signal, the controller reduces the gain of the front-end circuit incrementally to make the front-end circuit more linear and robust against inter-modulation distortion. However, if the incremental adjustment does not reduce the RSSI by more than the incremental adjustment, the harmful interference is not inter-modulation noise. In this instance, after a period of time, the controller increases the gain of the front-end circuit. If the incremental adjustment reduces the RSSI by more than the incremental adjustment, the receiver continues adjusting the gain until the RSSI is reduced to a level that is less than or approximately equal to the gain adjustment.
In a particular embodiment, the front-end circuit includes an adjustable attenuator (such as a pin diode or other attenuator) in front of a low-noise amplifier. The adjustable attenuator has an input for receiving the RF signal and an output for providing an attenuated RF signal to an input of a low-noise amplifier (LNA), which provides a gain-adjusted RF signal to a mixer of the receiver. Further, the adjustable attenuator includes a control input for receiving an adjustment signal from a controller. The receiver circuit uses the front-end circuit to take advantage of the effect of the gain adjustment on higher order inter-modulation products. In particular, higher order inter-modulation products are reduced N-times more than the gain reduction of the gain of the front-end circuit, where N represents the order of the inter-modulation product. In an example, a gain reduction of 3 dB produces a 3 dB times N reduction in higher order inter-modulation products at the output of the LNA. In this example, a third order inter-modulation product will be reduced by 9 dB when the gain of the front-end is reduced by 3 dB. However, if such higher order inter-modulation products are not present in the RF signal, a 3 dB adjustment causes a 3 dB reduction in RSSI.
The controller of the receiver is configured to adjust the gain of the front-end circuit in small increments or adjustments (such as 3 dB). The controller compares the RSSI measurements taken before and after applying the gain adjustment, and if the measured RSSI interference is greater than the gain step, the controller determines that the RF signal includes harmful interference. In response to detecting harmful interference, the controller iteratively decreases the gain of the front-end and compares the RSSI to the previous RSSI value until there is no significant difference between the gain step and the measured RSSI difference.
In an example, the receiver periodically checks for harmful interference by altering the gain of the front-end circuit and measuring the RSSI change. In a packet-based transmission system, it may be desirable to adjust the gain of the front-end gain during idle times when no packets are being received. An example of a receiver circuit configured to adjust the front-end gain in response to the RSSI signal is described below with respect to
System 100 further includes a demodulator 110 including an input coupled to output terminal 108 and an output for providing a demodulated signal related to an RF signal received at the antenna. System 100 further includes a preamble detector 112 including an input coupled to the output of demodulator 110 and an output coupled to a feedback input terminal 114.
Receiver 102 includes a low-noise amplifier (LNA) 118 having an input coupled to input 104, a control input, and an output coupled to an input of a mixer 120, which has an input for receiving a signal from a local oscillator 121 and an output coupled to an input of an intermediate frequency (IF) filter 122. IF filter 122 can include a programmable gain amplifier and filters and includes an output coupled to the output terminal 108. Receiver 102 further includes an RSSI circuit 126 including an input coupled to the output of IF filter 122 and an output coupled to a first input of a controller 128. Receiver 102 further includes a radio frequency (RF) peak detector 124 including an input coupled to the output of LNA 118 and an output coupled to a second input of controller 128. Controller 128 includes a third input coupled to feedback input terminal 114, a first control output coupled to the control input of LNA 118, and a second control output coupled to control output terminal 116. Controller 128 further includes a timer 130.
In the illustrated example, system 100 includes front-end circuitry 103 that includes at least a portion of receiver 102 (such as LNA 118, mixer 120, optionally local oscillator 121, IF filter 122, and RF peak detector 124) and external attenuation circuitry (such as attenuator 106), if such external attenuation circuitry is provided. Alternatively, in some instances, attenuator 106 may be included within receiver 102 between input terminal 104 and the input of LNA 118. In some instances, in addition to including attenuator 106 within receiver 102, addition circuitry (such as a transformer and/or other circuitry) may also be included between the antenna and input terminal 104.
Controller 128 receives an RF peak detection signal from RF peak detector 124 and receives an RSSI measurement from RSSI circuit 126. Controller 128 controls one of attenuator 106 and LNA 118 to adjust a gain of front-end circuit 103 by a pre-determined adjustment and receives a second RSSI measurement from RSSI circuit 126 in response thereto. If the difference between the RSSI measurements (i.e., the current and previous RSSI measurements) is less than or approximately equal to the pre-determined adjustment of the gain adjustment, controller 128 determines that there is no harmful inter-modulation interference, and controller 128 can control attenuator 106 to reduce the attenuation to a relatively low level or to no attenuation. In a particular instance, attenuator 106 may be omitted or bypassed to provide relatively low attenuation for front-end circuit 103, increasing the sensitivity of receiver 102. In an example, controller 128 initiates timer 130 when the difference is less than or approximately equal to the pre-determined adjustment of the gain adjustment. Once the timer reaches a pre-determined timer threshold, controller 128 receives a third RSSI measurement, adjusts the gain of the front-end circuit 103 by a pre-determined adjustment, and receives a fourth RSSI measurement in response thereto. Controller 128 iteratively increases and/or decreases the gain and compares RSSI measurements until the RSSI difference is less than or approximately equal to the pre-determined adjustment of the gain adjustment. Thereafter, controller 128 periodically captures an RSSI measurement from RSSI circuit 126, adjusts the gain of front-end circuit 103, captures another RSSI measurement, determines whether inter-modulation interference is present, and repeats the above-process.
Controller 128 takes advantage of the fact that higher order inter-modulation products are reduced N times more than the gain reduction of front-end circuit 103, where N is the order of the inter-modulation product. In an example, a third-order inter-modulation product will be reduced by nine decibels (9 dB) when the gain of front-end circuit 103 is reduced only three decibels (3 dB). Controller 128 reduces the gain of front-end circuit 103 by a small amount (e.g., 3 dB) and receives the RSSI measurement before and after the gain adjustment. If the measured RSSI difference is significantly greater than the gain adjustment, one or more inter-modulation products have reached the input of demodulator 110, which one or more inter-modulation products constitute “harmful interference.” In response to detecting harmful interference, controller 128 reduces the gain of front-end circuit 103 incrementally until there is no significant difference between the gain adjustment and the measured RSSI difference. In an example, when the measured RSSI difference is less than or approximately equal to the gain adjustment, the difference is not significant because further adjustment to the attenuation will not further improve the signal-to-noise ratio. Once the controller 128 reaches this state, controller 128 can stop adjusting the gain and can periodically check if the harmful interference is increased or reduced by reducing or increasing the gain and measuring the RSSI change.
In some instances, receiver 102 includes an automatic gain control (AGC) circuit. In this instance, the front-end gain (i.e., the maximum front-end gain) of the receiver 102 can be adjusted and/or limited according to the aforementioned algorithm. An example of a method of adjusting the gain of a receiver to reduce inter-modulation interference is described below with respect to
Advancing to 206, if the interference is in-band, the method 200 advances to 212 and controller 128 selectively adjusts the gain of the front-end circuit 103. The method 200 returns to 204 and controller 128 looks for interference in the RF signal.
Otherwise, at 206, if the interference is not in-band, the method 200 advances to 208 and controller 128 reduces the attenuation to a relatively low level (increasing the adjustable gain of front-end circuit 103). Continuing 210, receiver 102 filters the interference using a channel filter, for example. The method 200 returns to 204 and controller 128 looks for interference in the RF signal.
In an example, receiver 102 makes use of the fact that higher order inter-modulation products are reduced N times more than the gain reduction of the front-end circuitry 103, where N is the order of the inter-modulation product. Thus, a third order inter-modulation product will be reduced by 9 dB when the gain of the front-end circuit 103 is reduced only 3 dB. An example of a method for determining whether interference in the RF signal is harmful is described below with respect to
Continuing to 206, the controller determines if the interference is in-band or not. At 310, if the difference is less than or approximately equal to the pre-determined adjustment, the method 300 advances to 312 and controller 128 determines that the RF signal has no harmful (in-band) interference. Otherwise, if the difference is greater than the pre-determined adjustment at 310, the method 300 advances to 314 and controller 128 determines that the RF signal includes harmful (in-band) interference.
As mentioned above, if the difference is less than or equal to the pre-determined adjustment, controller 128 may reduce the attenuation of front-end circuitry 103 to increase (or maximize) the gain. An example of a method of adjusting the gain to reduce inter-modulation effects is described below with respect to
If the interference is determined not to be in-band, the method 400 advances to 404 and controller 128 starts a timer. Advancing to 406, if elapsed time of the timer is less than a pre-determined threshold, the method 400 continues to monitor the timer until the timer is equal to or exceeds the pre-determined threshold. At 406, when the timer is equal to or exceeds the pre-determined threshold, the method 400 returns to 204 and controller 128 detects interference in the RF signal.
In conjunction with the system and methods described above with respect to
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention.
