Patent Description:
Some microwave signal receivers are known to use power limiters to protect the receiver components from relatively high power incident signals. This happens, for example, in radar system receivers in which the delicate and sensitive components of the receiver must be protected from direct returns from the radar transmitter. Excessively high power signals can impair both the proper functioning of the receiver components and cause irreversible damage to them.

In radar systems, waveguide power limiters are known to be used to protect the receiver components from high power incident signals. A microwave waveguide power limiter is described in the article "<NPL>.

The waveguide power limiters have some drawbacks. Indeed, the waveguide power limiters in some applications, e.g. in radar integrated into missile seekers, have a relatively large footprint and weight. Furthermore, the integration and testing operations of a radar system receiver are particularly complex when the receiver has a waveguide limiter operationally connected to a subsequent circuit section of the receiver made as a microstrip, for example. Indeed, separate testing activities are initially required for the waveguide section and the microstrip section of the receiver. After assembly, the entire receiver must be tested both individually and after integration into the radar system.

In general, a microwave signal receiver comprising a waveguide power limiter is bulky, expensive, heavy. Document <CIT> describes embodiments of a radar receiver in which the use of a limiter is provided before an SPST (Single Pole Single Throw) switch with an internal resistor or an SPDT (Single Pole Double Throw) switch with an external bypass resistor. In any event, an LNA amplifier is placed immediately downstream of the SPDT or SPDT switch. Due to the presence of the limiter, the aforesaid receiver has relatively long response times because the recovery time of the limiter from saturation state to the linearity state is typically 75ns-100ns.

It is the object of the present invention to provide a microwave signal receiver which makes it possible to solve or at least in part reduce the drawbacks described above with reference to the receivers of the known art described above.

Such an object is achieved by a microwave signal receiver as defined in general in claim <NUM>. Alternative preferred and advantageous embodiments of the aforesaid microwave signal receiver are defined in the accompanying dependent claims.

The invention will be better understood by the following detailed description of a particular embodiment, made by way of example and consequently not limiting in any manner, with reference to the accompanying drawings which are briefly described in the following paragraph. The same or similar elements are identified in such drawings with the same symbols or reference numbers.

<FIG> shows a non-exhaustive, non-limiting embodiment of a radar system <NUM>, comprising a transmitter <NUM> adapted to emit a radar signal s_tx and a microwave signal receiver <NUM> adapted to receive a radar return signal s_rx. The radar <NUM> is, for example, intended to be used aboard a missile or rocket and is integrated into the seeker of the missile or rocket.

The radar system <NUM> further comprises a transmitter antenna <NUM> operationally connected to the transmitter <NUM> for remote and radio transmission of the signal produced by the transmitter <NUM>. The transmitting antenna <NUM> comprises e.g. one or more arrays of radiant antenna elements, e.g. one or more arrays of slot radiant elements. The assembly comprising the transmitter <NUM> and the transmitter antenna <NUM> is the transmission front end of the radar system <NUM>.

The radar <NUM> further comprises a local oscillator <NUM> adapted to synthesize and provide an oscillator signal s_ol sent in input to both the transmitter <NUM> and the microwave signal receiver <NUM>. For example, the transmitter <NUM> comprises a phase modulator, e.g. adapted to modulate the oscillator signal s_ol with a phase code so that it provides in output a signal having a coded pulse sequence which is fed to transmitter antenna <NUM>. Furthermore, the transmitter <NUM> may comprise one or more amplifiers.

The microwave radio signal output from the transmitting antenna s_tx represents the transmitted radar signal s_tx. For example, the transmitted radar signal s_tx is a signal with a phase coded pulse sequence. As known, the received signal s_rx, also called radar return signal, includes a possible useful signal component, i.e. the signal reflected by a possible T target, and a disturbance signal component, essentially represented by undesired reflections on the ground, water, vegetation or infrastructure and direct returns. The aforesaid radar return signal s_rx further comprises a component due to a direct return in the receiver <NUM> of the signal s_tx transmitted by the transmitting antenna <NUM>.

The radar system <NUM> comprises a radio frequency reception front end <NUM>,<NUM> for receiving the radar return signal s_rx, adapted to process the radar return signal received to produce an output signal which is preferably an intermediate frequency signal s_if. In addition to the receiver <NUM>, the radio frequency reception front end <NUM>,<NUM> comprises a receiver antenna <NUM> for receiving the radar return signal s_rx. For example, the receiving antenna <NUM> comprises one or more arrays of slot antennas. For the sake of simplicity, the output signal from the antenna <NUM> was also named return signal s_rx.

Although a radar <NUM> system comprising a dedicated antenna <NUM> for transmission and a dedicated reception antenna <NUM> has been described so far, it is apparent to a person skilled in the art that, e.g. by means of a circulator, the same antenna <NUM> or antenna system used in transmission can be used for the reception of the radar return signal s_rx.

The radar system <NUM> also comprises a radar processor <NUM> operationally connected to both the transmitter <NUM> and the microwave signal receiver <NUM>. For example, the radar processor <NUM> is a hardware and software system, comprising at least one processing unit, such as a general purpose or dedicated processor or an FPGA or DSP.

The radar system <NUM> preferably comprises a frequency conversion module <NUM> to produce a baseband signal s_bb from the intermediate frequency signal s_if. Furthermore, the radar system <NUM> preferably comprises an analog to digital converter <NUM> to convert the baseband s_bb signal produced at the receiver <NUM> output into a digital sample stream s_d. The analog to digital converter <NUM> is operationally connected to radar processor <NUM> to provide the latter with the digital sample stream s_d. According to an alternative embodiment, the analog to digital converter <NUM> could be integrated into the receiver <NUM> or the radar processor <NUM>. The radar processor <NUM> is configured to control the transmitter <NUM> and the receiver <NUM> and is preferably also programmed to receive and process the digital sample stream s_d to extract information regarding the presence and/or features of a target T.

A preferred, non-limiting embodiment of a microwave signal receiver <NUM> will now be described with reference to <FIG>. The receiver <NUM> is represented as a single channel receiver in the functional block chart in <FIG>. However, the receiver <NUM> may have a higher number of channels, mutually in parallel, replicating the diagram in <FIG>. For example, if radar system <NUM> is a monopulse radar system, the receiver <NUM> could have four parallel channels to process four signals from four respective portions of receiver antenna <NUM>.

According to an embodiment, the microwave signal receiver <NUM> is such to provide an L-band microwave signal as output. Preferably, the signal s_tx provided in output from the transmission front-end <NUM>, <NUM> is a Ka-band signal, instead. The same goes for the radar return signal s_rx.

Although the receiver <NUM> is described in this document as being part of the radar system <NUM>, the teachings of the present description are not limited to this type of receiver <NUM> and extend to microwave signal receivers not belonging to a radar system, e.g. data telecommunication receivers.

According to the invention, the microwave signal receiver <NUM> comprises a printed circuit board <NUM>, in particular a microstrip circuit board, comprising an input port <NUM> adapted to be operatively connected to an antenna <NUM>, and in the example to the receiving antenna <NUM> of the radar system <NUM>, to receive in input a microwave signal picked up by the antenna <NUM>, which in this example is the radar return signal s rx.

For example, the output signal from the antenna <NUM> of the receiving front end <NUM>,<NUM> propagates within a waveguide, e.g. a WR28 waveguide, and the receiver <NUM> further comprises a guide-to-microstrip transition.

The receiver <NUM> further comprises at least one power dissipator R1 and at least one electronically controllable switch <NUM> adapted to receive in input the microwave signal s_rx and a first control signal s_blk. For example, the first control signal s_blk is a blanking signal, e.g. supplied to receiver <NUM> by the radar processor <NUM>. As known, a blanking signal in a radar system makes it possible to mutually synchronize the operation of the transmitter <NUM> and the receiver <NUM>. According to an advantageous embodiment, the receiver <NUM> is devoid of a limiter, or any limiter, upstream of the electronically controllable switch <NUM>. In other words, the microwave signal s_rx received at the input of the electronically controllable switch <NUM> does not pass through any limiter before it is received by the electronically controllable switch <NUM>.

The power dissipator R1 is a termination dissipator (as clearly shown in the diagram in <FIG>) and the electronically controllable switch <NUM> comprises a first output port p1 and the electronically controllable switch <NUM> is configured to assume according to the control signal s_blk:.

If the receiver <NUM> is integrated into a radar system <NUM>, it is thus apparent that when the electronically controllable switch <NUM> is in the conduction state, the radar <NUM> system is in the receiving configuration and when the electronically controllable switch <NUM> is in the protection state the radar <NUM> system is in the transmitting configuration.

The microwave signal receiver <NUM> further comprises at least one low-noise amplifier <NUM>, i.e. an LNA (Low-Noise Amplifier), comprising an input port operationally connected to the first output port p1 of the electronically controllable switch <NUM> to receive the microwave signal s_rx in input and further comprising an output port to provide an amplified microwave signal s_a in output.

According to a particularly advantageous embodiment, the electronically controllable switch <NUM> is a PIN diode semiconductor switch. In this manner, a high switching speed of the electronically controllable switch <NUM> between the protection state and the conduction state and vice versa can be guaranteed. Preferably, the rise and fall times of the electronically controllable switch <NUM> are less than <NUM> ns.

Conveniently, the electronically controllable switch <NUM> has a high power handling, e.g. above 12W, e.g. equal to 13W.

According to a preferred embodiment, the electronically controllable switch <NUM>, when in the protection state, guarantees insulation between the input and the first output port p1 of more than 35dB, e.g. either equal to about, or higher than, 40dB.

According to an advantageous embodiment, the power dissipator R1 comprises a resistor external to the electronically controllable switch <NUM>, preferably a resistor which allows high power to be dissipated in a small volume. For example, such a resistor is an aluminum nitride resistor and is preferably capable of dissipating at least 30W, preferably at least or approximately 40W, in Ka-band at a temperature of <NUM>.

According to the invention, the electronically controllable switch <NUM> is an SPDT - Single Pole Double Throw Switch - having a second output port p2 operationally connected to the power dissipator R1. In the protection state, the electronically controllable switch <NUM> is configured to make the microwave signal s_rx available to the second output port p2 so that the microwave signal s_rx can be routed to the dissipator R1.

According to a particularly advantageous embodiment, which makes it possible to increase the robustness of the receiver <NUM> relative to received high power signals to a greater extent, the low-noise amplifier <NUM> is adapted to assume an on and off state and is configured to assume:.

For example, the low-noise amplifier <NUM> is also adapted and configured to receive the aforesaid first control signal s_blk, or in any event, a control signal correlated to it and is configured to assume the on state or off state based on the first control signal s_blk. As explained above, the first control signal s_blk can be a blanking signal.

According to the invention, the microwave signal receiver <NUM> also comprises an electronically controllable attenuator <NUM> operationally interposed between the electronically controllable switch <NUM> and the low-noise amplifier <NUM> to attenuate the microwave signal s_rx. In this case, the microwave signal receiver <NUM> is configured to control the electronically controllable attenuator <NUM> so that it assumes a first state in which the attenuator <NUM> has a relatively high attenuation and a second state in which attenuator <NUM> has relatively low attenuation, and in which the electronically controllable attenuator <NUM> assumes the first state when the electronically controllable switch <NUM> assumes the protection state.

For example, the electronically controllable attenuator <NUM> is adapted and configured to receive the aforesaid first control signal s_blk, or in any event, a control signal correlated to it and is configured to assume the relatively high attenuation state or the relatively low attenuation state based on the first control signal s_blk. As explained above, the first control signal can be a blanking signal.

For example, the electronically controllable attenuator <NUM> in the relatively high attenuation state has an attenuation greater than 30dB, e.g. equal to 40dB, or about 40dB. For example, in the relatively high attenuation state, the attenuator <NUM> is controlled to provide its maximum attenuation.

Preferably, in the relatively low attenuation state, the attenuator <NUM> is controlled to minimize its insertion loss.

Conveniently, the electronically controllable attenuator <NUM> is a voltage controllable variable attenuator.

Again with reference to the diagram in <FIG>, if the receiver <NUM> is the receiver of a radar system <NUM>, during the transmission of the signal s_tx by the transmission front-end <NUM>,<NUM>, the receiver <NUM> can be controlled by the first control signal s_blk, which is, for example, a blanking signal, to bring the electronically controllable switch <NUM> to the protection state, the electronically controllable attenuator <NUM> to the relatively high attenuation state and the low-noise amplifier <NUM> to off state at the same time. In this manner, very strong attenuations were surprisingly achieved, even above 90dB, e.g. about 100dB.

When the electronically controllable switch <NUM> is in the conduction state, the provision of an electronically controllable attenuator <NUM> also makes it advantageously possible to manage the dynamics of the microwave signal receiver <NUM> so that it can operate in a linearity condition, i.e. to prevent the receiver <NUM> from operating in a saturation condition. In this regard, in an advantageous embodiment, the receiver <NUM> comprises a saturation estimate module <NUM>, D1 of the receiver <NUM> adapted to detect whether the receiver <NUM> is in a saturation state or a linearity state. The receiver <NUM> is configured to control the electronically controllable attenuator <NUM> so that it assumes a third state in which it has an intermediate attenuation between the relatively high attenuation and the relatively low attenuation, e.g. equal to about 10dB. When the electronically controllable switch <NUM> is in the conduction state, the receiver <NUM> is configured to control the electronically controllable attenuator <NUM> so that it assumes the second state if the receiver <NUM> is in the saturation state. Instead, when the electronically controllable switch <NUM> is in the conduction state, the receiver <NUM> is configured to control the electronically controllable attenuator <NUM> so that it assumes the third state if the receiver <NUM> is in the saturation state.

Preferably, the saturation estimation module <NUM>, D1 is such to detect a saturating signal to supply a second control signal s_sat to the electronically controllable attenuator <NUM>. Therefore, according to a particularly advantageous embodiment, the electronically controllable attenuator <NUM> is adapted and configured to receive both the first control signal s_blk and the second control signal s_sat, e.g. with an input port which produces the OR logic function between such control signals. Clearly, from a practical point of view, it is also possible for the electronically controllable attenuator <NUM> to receive a single control signal obtained from the first control signal s_blk and the second control signal s_sat by means of an appropriate logic.

In the particular example shown, the saturation estimation block <NUM>, D1 comprises a coupler <NUM> adapted and configured to tap a part of the received signal from the reception chain. It is possible to estimate the strength of such a tapped signal to detect whether the receiver <NUM> is in linearity or saturation. For example, the saturation estimation block <NUM>, D1 comprises a detector diode D1 which provides a voltage output s_v starting from the tapped signal. By comparing this voltage s_v with a threshold voltage Vth it is possible to produce the second s_sat control signal as a binary signal, which assumes a first value when the voltage s_v is higher than the threshold voltage and a second value otherwise. It is, therefore, possible to establish that the receiver <NUM> is in a linearity condition when the second control signal s_sat assumes the first value and the receiver <NUM> is in a linearity condition when the second control signal s_sat assumes the second value.

According to a preferred embodiment, the receiver <NUM> further comprises a further electronically controllable attenuator <NUM> which is arranged downstream of the low-noise amplifier <NUM>. In the particular example shown in <FIG>, this additional attenuator <NUM> is only controlled based on the second control signal s_sat to manage the dynamics of the receiver <NUM> more flexibly and to ensure that the receiver works in linearity conditions when the electrically controllable switch <NUM> is in the conduction state.

According to a preferred, non-limiting embodiment, the receiver <NUM> further comprises an additional low-noise amplifier <NUM> and a fixed attenuator <NUM> downstream of the low-noise amplifier <NUM>, in particular downstream of the additional attenuator <NUM>. For example, the latter is intended to achieve a tuning gain of the receiver <NUM>.

Conveniently, the receiver <NUM> comprises downstream of the low-noise amplifier <NUM>, in the example in <FIG> downstream of the fixed attenuator <NUM>, a band-pass filter <NUM> for image signal rejection, preferably followed by an additional fixed attenuator <NUM>.

According to an embodiment, the receiver <NUM> further comprises a frequency converter <NUM>, such as a mixer, having an input port operationally connected to the output port of the low-noise amplifier <NUM> to convert the amplified microwave signal s_a, and in the example also attenuated and filtered, into an intermediate frequency signal s_if.

In a particularly advantageous embodiment, the saturation estimation module <NUM>, D1 is such to detect whether the receiver <NUM> is in a saturation state or a linearity state starting from an amplitude or signal power measurement at intermediate frequency s_if. This is in accordance with the example shown in <FIG>, where the saturation estimation module <NUM>, D1, the operation of which is shown above, is located downstream of the frequency converter <NUM>.

According to a particularly advantageous embodiment, the frequency converter <NUM> is fed with the oscillator signal s_ol supplied by a local oscillator <NUM> and the receiver <NUM> comprises a bandpass filter <NUM> operationally interposed between the frequency converter <NUM> and the local oscillator <NUM> to filter the oscillator signal s_ol before supplying in input to the frequency converter <NUM>. This device advantageously reduces the spurious components caused by the s_ol oscillator signal generated by local oscillator <NUM>. Preferably, between bandpass filter <NUM> and frequency converter <NUM>, receiver <NUM> comprises an amplifier <NUM> which then acts as a driver for frequency converter <NUM>.

by virtue of the presence of the bandpass filter <NUM>, the receiver <NUM> makes it possible to reduce the spurious components of the oscillator signal s_ol at the frequencies Fol+Fif, Fol-Fif and Fif.

Such spurious components are particularly harmful to receiver <NUM> because they can generate residues at the intermediate frequency Fif of the receiver <NUM> and thus in case of weak signals such spurious components may overlap the useful signal masking it and thus reducing the sensitivity of the receiver <NUM>.

For this reason, the band-pass filter <NUM>, e.g. made as a microstrip, is preferably centered at the frequency Fol. Preferably, such a bandpass filter <NUM> is adapted and configured to attenuate the spurious components present at the frequencies Frf±Fif by more than <NUM> dB and spurious components present at the frequency Fif by about <NUM> dB.

According to an embodiment, the receiver <NUM> further comprises an intermediate frequency amplifier <NUM> and a step attenuator <NUM> downstream of the frequency converter <NUM> to allow further adjustment of the reception chain gain.

From the above, it is apparent that a microwave signal receiver of the type described above makes it possible to fully achieve the set objects in terms of overcoming the drawbacks of the prior art.

Notwithstanding the principle of the invention, embodiments and details may be greatly varied with respect to what has been described and illustrated herein exclusively by way of non-limiting example.

Claim 1:
A microwave signal receiver (<NUM>) comprising a printed circuit board (<NUM>), in particular a microstrip circuit board, comprising an input port adapted to be operatively connected to an antenna (<NUM>) for receiving in input a microwave signal (s_rx) picked up by the antenna (<NUM>), wherein the microwave signal receiver (<NUM>) comprises:
- at least one power dissipator (R1);
- an electronically controllable switch (<NUM>) adapted to receive in input the microwave signal (s_rx) and a first control signal (s_blk), wherein the electronically controllable switch (<NUM>) comprises a first output port (p1) and wherein, based on the control signal, the electronically controllable switch (<NUM>) is configured to assume a conduction state to supply the microwave signal (s_rx) to the first output port (p1), and a protection state in which the electronically controllable switch (<NUM>) is such as to send the microwave signal (s_rx) to the power dissipator (R1);
- at least one low-noise figure amplifier (<NUM>) comprising an input port operatively connected to the first output port (p1)and further comprising an output port for supplying in output an amplified microwave signal (s_a);
wherein:
- the electronically controllable switch (<NUM>) is an SPDT - Single Pole Double Throw - switch having a second output port (p2) operationally connected to the power dissipator (R1) and wherein in the protection state the electronically controllable switch (<NUM>) is configured to make the microwave signal (s_rx) available to the second output port (p2);
- the power dissipator (R1) is a termination dissipator; characterized in that the microwave signal receiver (<NUM>) further comprises an electronically controllable attenuator (<NUM>) operatively interposed between the electronically controllable switch (<NUM>) and the low-noise figure amplifier (<NUM>) for attenuating the microwave signal (s_rx), and wherein the receiver (<NUM>) is configured to control the attenuator (<NUM>) so that the latter assumes a first state in which it has a relatively high attenuation, and a second state in which it has relatively low attenuation, and wherein the electronically controllable attenuator (<NUM>) assumes the first state when the electronically controllable switch (<NUM>) assumes the protection state.