TECHNICAL FIELD

[0001] A radio frequency transmitter uses an advantageous arrangement for isolating and splitting a monitoring signal from a pulsed radio signal to be transmitted, for use in monitoring emitted power during off periods of the pulsed radio signal.

BACKGROUND

[0002] High power radios that emit periodical radio pulses may require embedded selfmonitoring of antenna output power levels. The self-monitoring is needed so that the radio amplifier does not drastically degrade receive sensitivity of incoming weak radio signals between strong radio bursts. This is especially important for pulsed radar and military radios that employ weak or stealth communication of sensitive return or incoming radio signals between strong radio pulses. The method and apparatus of the present invention empowers high-power radio amplifiers with minimum loss of forward transmit power and at the same time capable of selfmonitoring weak off-period output emission when bursting at very high output power level. The requirement of self-emission monitoring at very high on and off emission ratio is also part of Federal Aviation Administration (FA A) regulation specific to military Link- 16 radios.

[0003] Particularly, Link-16 terminals are subject to requirements set out in Department of Defense (DoD) Regulation 4650.1-R1. The regulation specifies a minimum 80 dB dynamic range between on and off periods of the pulsed radio signals transmitted according to defined signal structure of Link- 16 communications. Terminals are required to immediately turn off their radio transmitters when self-detecting unwanted output exceeding mandated levels. However, the intrusive nature of high-power self-monitoring can degrade the transmitted output power level and, as a general proposition, reliably and unobtrusively monitoring radio signals having extreme dynamic range imposes numerous design challenges.

SUMMARY

[0004] A method and apparatus for power detection in a radio transmitter include isolating forward signal path components from impedance variations that arise at the output of a power amplifier used to amplify a pulsed radio signal to be transmitted. The method and apparatus further embody an advantageous monitoring circuit arrangement for providing reverse isolation to power-monitoring circuitry that is configured to allow a single monitoring signal to be split into respective off-period and on-period power detection circuits. [0005] In an example embodiment, a radio frequency (RF) transmitter includes a forward signal path comprising a power amplifier that is configured to amplify a pulsed radio signal having on periods and off periods. The forward signal path further includes an isolator that is coupled to an output of the power amplifier and isolates a remainder of the forward signal path from source impedance variations arising at the output of the power amplifier. The remainder of the forward signal path includes a directional coupler having an input port coupled to an output port of the isolator, a transmitted port that outputs a first fraction of the amplified pulsed radio signal as a coupled pulsed radio signal, and a coupled port that outputs a second fraction of the amplified pulsed radio signal as a monitoring signal. Further included is a dual-junction circulator having a first port coupled to the transmitted port of the directional coupler, to receive the coupled pulsed radio signal, and, in a circulation direction of the dual-junction circulator, a second port coupled to an antenna, for transmission of the coupled pulsed radio signal, and a third port for outputting signals received via the antenna.

[0006] The RF transmitter according to the example embodiment further comprises a monitoring signal path that includes a splitter configured to divide the monitoring signal into first and second monitoring signals. An off-period power detection circuit is configured for power detection with respect to signal levels associated with the off periods and comprise a monitoringsignal amplifier, a power limiter, and a first power detector. The monitoring-signal amplifier has an input coupled to receive the first monitoring signal and an output coupled to an input of the power limiter, and an output of the power limiter is coupled to an input of the first power detector. An on-period power detection circuit is configured for power detection with respect to signal levels associated with the on periods and comprises a fixed attenuator having an input coupled to receive the second monitoring signal, and a second power detector having an input coupled to an output of the fixed attenuator.

[0007] Another example embodiment comprises a method of operating a RF transmitter. The method includes amplifying a pulsed radio signal via a power amplifier, to produce an amplified pulsed radio signal, where the pulsed radio signal has on periods and off periods, and where the method further includes coupling the amplified pulsed radio signal to a directional coupler through an isolator, the directional coupler outputting a first and second fractions of the amplified pulsed radio signal. Further, the method includes coupling the first fraction of the amplified pulsed radio signal through a dual-junction circulator, for transmission from an antenna, where the dual-junction circulator provides reverse isolation with respect to external radio signals impinging on the antenna. Still further, the method includes splitting the second fraction of the amplified pulsed radio signal into first and second monitoring signals, and detecting the power of the first monitoring signal via an off period power detection circuit configured for power detection with respect to signal levels associated with the off periods, and detecting the power of the second monitoring signal via an on period power detection circuit configured for power detection with respect to signal levels associated with the on periods. [0008] Of course, the present invention is not limited to the above features and advantages. Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a block diagram of a radio frequency (RF) transmitter, according to one embodiment.

[0010] Figure 2 is a block diagram of a tactical data link (TDL) radio, according to one embodiment.

[0011] Figure 3 is a block diagram of control circuitry for a RF transmitter, according to one embodiment.

[0012] Figure 4 is a logic flow diagram of a method of operating a RF transmitter, according to one embodiment.

DETAILED DESCRIPTION

[0013] Figure 1 illustrates a radio frequency (RF) transmitter 10 according to one or more embodiments. The RF transmitter 10 includes a forward signal path 12 comprising a power amplifier 14 that is configured to amplify a pulsed radio signal 16 having on periods and off periods, and further comprising an isolator 18 that is coupled to an output 20 of the power amplifier 14. The isolator 18 isolates a remainder of the forward signal path 12 from source impedance variations arising at the output 20 of the power amplifier 14.

[0014] The remainder of the forward signal path 12 includes a directional coupler 22 having an input port 24 coupled to an output port 26 of the isolator 18, a transmitted port 28 that outputs a first fraction of the amplified pulsed radio signal 30 as a coupled pulsed radio signal 32, and a coupled port 34 that outputs a second fraction of the amplified pulsed radio signal 30 as a monitoring signal 36. Further included in the remainder portion of the forward signal path 12 is a dual-junction circulator 40 — cascaded circulators — having a first port 42 coupled to the transmitted port 28 of the directional coupler 22, to receive the coupled pulsed radio signal 32. In a circulation direction of the dual-junction circulator 40, the dual-junction circulator 40 further includes a second port 44 coupled to an antenna 46, for transmission of the coupled pulsed radio signal 32, and a third port 48 for outputting signals 50 received via the antenna 46.

[0015] The RF transmitter 10 further comprises a monitoring signal path 60. The monitoring signal path 60 includes a splitter 62 that is configured to divide the monitoring signal 36 into first and second monitoring signals 64 and 66. An off-period power detection circuit 68 is configured for power detection with respect to signal levels associated with the off periods and includes a monitoring-signal amplifier 70, a power limiter 72, and a first power detector 74. The monitoring-signal amplifier 70 has an input 76 coupled to receive the first monitoring signal 64 and an output 78 coupled to an input 80 of the power limiter 72, and an output 82 of the power limiter 72 is coupled to an input 84 of the first power detector 74. An output 86 of the first power detector 74 outputs a power detection signal 88, e.g., for evaluating emitted power during off periods of the amplified pulsed radio signal 30.

[0016] An on-period power detection circuit 90 is configured for power detection with respect to signal levels associated with the on periods. The on-period power detection circuit 90 includes a fixed attenuator 92 having an input 94 coupled to receive the second monitoring signal 66, and a second power detector 96 having an input 98 coupled to an output 100 of the fixed attenuator 92. An output 102 of the second power detector 96 provides a power detection signal 104, e.g., for evaluating emitted power during on periods of the amplified pulsed radio signal 30. [0017] In at least one embodiment, the power amplifier 14 includes or is associated with a gate bias controller 110 that performs gate bias voltage switching for the power amplifier 14 with respect to the on and off periods of the pulsed radio signal 16. The isolator 18 advantageously isolates the remainder of the forward signal path 12 from source impedance variations arising at the output 20 of the power amplifier 14, arising at least in part because of the gate bias voltage switching.

[0018] According to one or more embodiments, the directional coupler 22 has a coupling ratio that, for a maximum on-period power of the amplified pulsed radio signal 30, prevents the first monitoring signal 64 from exceeding an input power rating of the monitoring-signal amplifier 70.

[0019] The monitoring- signal amplifier 70 is a Gallium Nitride (GaN) Low Noise Amplifier (LNA) in one or more embodiments. Among the several advantages gained from this configuration is a higher input power rating for the monitoring- signal amplifier 70, as compared to non-GaN amplifiers. Higher input power ratings for the monitoring- signal amplifier 70 mean that the coupling ratio of directional coupler 22 may be lower — i.e., higher signal levels are permissible for the monitoring signal 36 during on periods and that allows higher signal levels for the first monitoring signal 64 during the off periods and, correspondingly, more reliable and accurate emissions-level monitoring during the off periods.

[0020] A gain of the monitoring-signal amplifier 70 is set, for example, according to a signal level of the first monitoring signal 64 that corresponds to a permissible maximum emission level of the RF transmitter 10 during the off periods. The off-period signal level of the amplified pulsed radio signal 30 and the coupling ratio of the directional coupler 22 determine the off- period signal level of the first monitoring signal 64.

[0021] In one or more embodiments, the RF transmitter 10 further comprises a control circuit 120 that is configured to decide whether emissions of the RF transmitter 10 during the off periods are compliant with an applicable emissions limit. The decision is based on the control circuit 120 evaluating a detected power of the first monitoring signal 64 for one or more of the off periods with respect to a defined threshold that corresponds with the emissions limit. The control circuit 120 in at least one embodiment is further configured to evaluate a detected power of the second monitoring signal 66 for one or more of the on periods.

[0022] In at least one embodiment, the control circuit 120 comprises or is associated with one or more analog-to digital converters (ADCs) 122 to convert power detection signals 88 and 104 output by the first and second power detectors 74 and 96 into respective digital samples. Correspondingly, processing circuitry 124 comprised in the control circuit 120 evaluates the detected powers of the first and second monitoring signals 64 and 66 by evaluating the respective digital samples.

[0023] According to at least one embodiment, the directional coupler 22 further comprises an isolated port 130 that outputs a leakage radio signal 132 arising from external radio signals impinging on the antenna 46 and leaking from the third port 48 of the dual-j unction circulator 40 to the first port 42 of the dual-junction circulator 40. The RF transmitter 10 in at least one such embodiment includes a third power detector 134 coupled to the isolated port 130 of the directional coupler 22. The third power detector 134 outputs a power detection signal 136, for detecting the power of the leakage radio signal 132. The control circuit 120 in at least one such embodiment conditions the evaluation of the first monitoring signal 64 on whether the detected power of the leakage radio signal 132 exceeds a corresponding defined threshold during the one or more off periods.

[0024] As a particular example, the control circuit 120 determines whether the detected power of the first monitoring signal 64 for the one or more off periods evaluated exceeds a defined power threshold corresponding to a maximum permissible emissions level for off periods. However, the control circuit 120 conditions this evaluation according to the detected power of the leakage radio sign al 132, e.g., the control circuit 120 determines whether excessive detected power for the first monitoring signal 64 during the one or more off periods is coincident with the detected power of the leakage radio signal 132 exceeding its corresponding defined threshold and, if so, the control circuit 120 deems the excessive detected power for the first monitoring signal 64 to be attributable to interfering signals incident on the antenna 46 rather than an emissions violation of the RF transmitter 10. [0025] The control circuit 120 may receive or output control signaling 140 in one or more embodiments. For example, it may output signaling responsive to detecting excess emitted power during off periods of the RF transmitter 10. Such signaling may go to other processing or control entities included in an overall assembly that contains the RF transmitter 10, e.g., to log the detected violations, to initiate shut down, or to initiate another control or mitigation response. [0026] In at least one embodiment, the RF transmitter 10 is an RF transmitter of a tactical data link (TDL) radio having a specified permissible level of emissions during the off periods. For example, the TDL radio is a Link- 16 radio.

[0027] Figure 2 illustrates an example tactical data link radio 150 that incorporates the RF transmitter 10 as part of its overall complement of components or sub-systems. The TDL radio 150 includes, in addition to the RF transmitter 10, a receiver 52 arranged with respect to the antenna 46 as suggested in Figure 1. In particular, the RF receiver 52 has a receiver input coupled to the third port 48 of the dual-junction circulator 40 included in the forward signal path 12 .

[0028] The TDL radio 150 further includes transmit (TX) signal modulation circuitry 152, receive (RX) demodulation circuitry 154, a baseband processor 156, a systems processor 158, and interface circuitry 160. The baseband processor 156 is configured as a radio modem that performs transmit-signal processing and received- signal processing in the digital domain, whereas the system processor 158 provides overall radio functionality, e.g., based on signals provided via the interface circuitry 160.

[0029] Figure 3 illustrates an example implementation of the control circuit 120, where the functionality of the control circuit 120 is based on one or more microprocessors or other digital processing circuits 170. Such circuitry is specially adapted to operate as the control circuitry 120, based on the execution of computer program instructions 174 held in storage 172, which comprises one or more types of memory or other computer readable media. The storage 172 also holds configuration data 176 in one or more embodiments, such as numeric values or other data that define the various thresholds used by the control circuit 120 to evaluate the detected power levels of any one or more of the first monitoring signal 64, the second monitoring signal 66, or the leakage radio signal 132. As non-limiting examples, the control circuit 120 may be implemented in the baseband processor 156 of the TDL radio 150 or in the systems processor 158 of the TDL radio 150 or separate from those two processors.

[0030] Figure 4 illustrates an example method 400 of operating a RF transmitter, the method 400 includes: amplifying (Block 402) a pulsed radio signal via a power amplifier, to produce an amplified pulsed radio signal, the pulsed radio signal having on periods and off periods; coupling (Block 404) the amplified pulsed radio signal to a directional coupler through an isolator, the directional coupler outputting a first and second fractions of the amplified pulsed radio signal; coupling (Block 406) the first fraction of the amplified pulsed radio signal through a dualjunction circulator for transmission from an antenna; splitting (Block 408) the second fraction of the amplified pulsed radio signal into first and second monitoring signals; and detecting (Block 410) the power of the first monitoring signal via an off period power detection circuit.

[0031] The off period power detection circuit is configured for power detection with respect to signal levels associated with the off periods, e.g., it has sufficient gain or sensitivity for accurate power detection with respect to the level of the first monitoring signal seen during the off periods. The method 400 may also include detecting the power of the second monitoring signal with an on period power detection circuit that is configured for power detection with respect to signal levels associated with the on periods.

[0032] The method 400 includes, in one or more embodiments, using a coupling ratio in the directional coupler such that, for a maximum on-period power of the amplified pulsed radio signal, the first monitoring signal does not exceed an input power rating of a monitoring-signal amplifier used in the off period power detection circuit. In at least one such embodiment, the method 400 further includes setting a gain of the monitoring-signal amplifier according to a signal level of the first monitoring signal that corresponds to a permissible maximum emission level of the RF transmitter during the off periods.

[0033] In at least one embodiment, the method 400 includes deciding whether emissions of the RF transmitter during the off periods are compliant with an applicable emissions limit, based on evaluating a detected power of the first monitoring signal for one or more of the off periods with respect to a defined threshold that corresponds with the emissions limit. The method 400 may further include evaluating a detected power of the second monitoring signal for one or more of the on periods. One or more implementations of the evaluating operations include digitizing power detection signals that indicate the detected powers of the first and second monitoring signals and evaluating the powers of the first and second monitoring signals according to the respective digital samples.

[0034] In at least one embodiment, the method 400 further includes detecting the power of a leakage radio signal that indicates the presence of external radio signals impinging on the antenna and conditioning the evaluation of the first monitoring signal on whether the detected power of the leakage radio signal exceeds a corresponding defined threshold during the one or more off periods.

[0035] Further, carrying out the method 400 according to at least one embodiment comprises operating the radio transmitter as a tactical data link transmitter. For example, the radio transmitter is operated as a Link- 16 transmitter. [0036] Turning back to Figure 1, in an example implementation, the RF transmitter 10 in one or more embodiments employs on/off switching of gate bias voltages applied to power transistors comprised in the power amplifier 14, to produce the amplified pulsed radio signal 30. For example, a gate bias controller 110 is included in the power amplifier 14 or coupled to bias inputs of the power amplifier 14. With its power transistor(s) biased off during off periods of the pulsed radio signal 16, the RF source impedance seen at the output 20 of the power amplifier 14 varies wildly between on periods and off periods.

[0037] The source impedance variations of the power amplifier 14 between on and off periods, if not mitigated, upsets balanced reverse isolation of all RF devices on the forward signal path 12. For example, the dual-junction circulator 40 and the directional coupler 22 require carefully balanced 50 ohm RF trace impedance to achieve high reverse isolation and directivity. Coupling the isolator 18 between the output 20 of the power amplifier 14 and the input of the isolator 18 mitigates trace impedance imbalance originating from power transistor source impedance variation due to gate bias switching.

[0038] Further, as noted before, the coupling ratio of the directional coupler 22 is configured to match or otherwise complement the safe input power level of the monitoring-signal amplifier 70 in the off period power detection circuit 68. For silicon-based amplifiers, the maximum input level may be in the range of +20 dBm. However, for GaN-based amplifiers, the maximum may be about +35 dBm. Thus, the directional coupler 22 taps off a fraction of the amplified pulsed radio signal for use in self-monitoring of transmitter emissions. The coupling ratio also presents itself as the power attenuation of the minimum detection level. Higher coupling ratios attenuate low-level sensing ability, which increases signal-detection challenges in the off period power detection circuit 68.

[0039] In a well-balanced implementation of the forward signal path 12 with 50 Ohms of reference impedance, the dual-junction circulator 40 provides high reverse isolation against detection of external interfering transmitters. The directional coupler 22 provides additional reverse isolation through directivity when, again, the RF path is well balanced at 50 ohm of reference impedance. As noted, the disclosed arrangement of the isolator 18 prevents the power amplifier 14 from negatively deteriorating balanced reference impedance of the remainder of the forward signal path 12. In so doing, while maintaining the balanced 50 ohm reference impedance, a dual-junction circulator 40 can provide about 40 dB of reverse isolation and the directional coupler 22 can provide about 20 dB of directivity. In combination, the arrangement in one or more embodiments provides reverse isolation better than 60 dB.

[0040] As for splitting the monitoring signal 36 into two paths using the splitter 62, which is a RF power splitter, one path is used for low-level power detection and the other path is used for high-level power detection. Particularly, the off period power detection circuitry 68 is configured for low-level power detection and includes, for example, an RF LNA as the monitoring- signal amplifier 70. In one or more embodiments, the monitoring- signal amplifier 70 is a GaN LNA that has a high input-power rating and, therefore, can handle very high input power levels while maintaining a 50 Ohm reference input impedance. GaN LNAs, in particular, can operate up to +35 dBm without damage while maintaining a constant 50 Ohm input impedance.

[0041] The power limiter 72 following the monitoring- signal amplifier 70 provides amplitude-limiting to prevent damage to the first power detector 74. The monitoring- signal amplifier 70 increases the power level of the first monitoring signal 64, thus improving the ability of the off period power detection circuit 68 to detect the power of the first monitoring signal 64 during the off periods. As a further advantageous configuration, the fixed attenuator 92 provides adequate RF attenuation in advance of the second power detector 96, to prevent damage to the second power detector 96 during on periods.

[0042] Also, as noted earlier herein, one or more embodiments of the RF transmitter 10 include a third power detector 134, to reduce false detection alarms triggered by external interfering transmitters. Co-site or hostile transmitters external to the RF transmitter 10 can inject strong radio signals at the antenna port. Despite the reverse isolation and directivity provided by the dual-junction circulator 40 and the directional coupler 22, very strong external RF interferers impinging on the antenna 46 may result in a leakage radio signal 132 that has power levels sufficient to interfere with accurate sensing of the power of the first monitoring signal 64 during off periods. Thus, the detected power level of the leakage radio signal 132 serves as an input to the evaluation operations carried out by the control circuit 120, to reduce false alarm detection from very strong external transmitters. A “false alarm” here is mistakenly deciding that detecting the power of the first monitoring signal 64 above a defined threshold during an off period means an impermissible emissions level from the RF transmitter 10 rather than indicating the presence of strong external interference.

[0043] Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.