CN-115047928-B - Calibration of RF attenuators

Abstract

Embodiments of the present disclosure relate to calibration of RF attenuators. The present disclosure relates to a circuit comprising an input terminal configured to receive a first signal at a first frequency, a demodulation chain connected to the input terminal and comprising a low noise amplifier having an input coupled to the terminal, a controllable variable impedance connected between a first node and a node configured to receive a reference potential, the first node being connected to the input terminal and/or the amplifier input, and a current source configured to pass a current at the first frequency to the first node.

Inventors

• M. Eloud
• P. Lewell

Assignees

• 意法半导体(格勒诺布尔2)公司
• 意法半导体(格勒诺布尔2)公司

Dates

Publication Date

20260421

Application Date

20220225

Priority Date

20210226

Claims (20)

1. 1. A circuit, comprising: an input terminal configured to receive a first signal at a first frequency; a demodulation chain comprising a low noise amplifier having an input coupled to the input terminal; A controllable variable impedance connected between a first node and a node configured to receive a reference potential, wherein the first node is connected to the input terminal and/or the amplifier input, and A current source configured to pass current at the first frequency to the first node.
2. 2. The circuit of claim 1, wherein the current source comprises: a first circuit configured to pass a second signal at a frequency of a local oscillator of the demodulation chain; a second circuit configured to pass a third signal at an intermediate frequency of the demodulation chain; A mixer configured to receive the second signal and the third signal, wherein an output of the mixer is coupled to an internal node of the current source, and A resistor couples the internal node to the first node.
3. 3. The circuit of claim 2, wherein the third signal is a square wave signal and the mixer is a switch mode mixer controlled by the third signal.
4. 4. The circuit of claim 3, wherein the second circuit comprises an oscillator configured to pass a signal at a frequency greater than an intermediate frequency of the demodulation chain, and a frequency divider configured to pass the third signal from the signal passed by the oscillator.
5. 5. The circuit of claim 4, wherein the oscillator of the second circuit is a quartz oscillator.
6. 6. The circuit of claim 3, wherein the mixer comprises a first switch connected between the output of the mixer and a node configured to receive the second signal, and a second switch connected between the output of the mixer and the node configured to receive the reference potential, wherein the first switch and the second switch are configured to be controlled to be in anti-phase according to the third signal.
7. 7. The circuit of claim 2, wherein the first circuit comprises: a circuit configured to pass a fourth square wave signal at a frequency equal to four times the frequency of the local oscillator; a first frequency divider configured to divide a frequency of the fourth wave signal by two; A second frequency divider configured to divide the frequency of the fourth wave signal by four; A dual input gate configured to receive the output signal of the first frequency divider and the output signal of the second frequency divider, wherein the gate is configured to implement an exclusive-or function between signals received by its inputs; a first resistor coupling the output of the second frequency divider to the output of the first circuit, and A second resistor couples the output of the gate to the output of the first circuit.
8. 8. The circuit of claim 7, wherein a ratio between a value of the first resistor and a value of the second resistor is equal to 0.348/0.84.
9. 9. The circuit of claim 2, wherein the current source comprises a common mode rejection capacitive element, wherein the resistor coupling the internal node of the current source to the first node is connected in series with the common mode rejection capacitive element between the internal node and the first node.
10. 10. The circuit of claim 1, wherein the current source is further configured to selectively turn on or off.
11. 11. A method of operating a circuit, the method comprising: a) Receiving a first signal at a first frequency through an input of a low noise amplifier in a demodulation chain; b) Passing a current at the first frequency through a current source to a first node coupled to an input of the low noise amplifier; c) Selecting a value of a controlled variable impedance coupled between the first node and a reference potential node; d) Acquiring a signal at an output of the demodulation chain while the current is being delivered by the current source to the first node, and E) Determining attenuation values introduced by the variable impedance into the demodulation chain from at least the signal acquired at step d) for the value of the variable impedance selected at step c).
12. 12. The method of claim 11, further comprising: Step d ') between steps c) and e), step d') comprising obtaining a further signal at the output of the demodulation chain when the current source is turned off, and At step e), the attenuation value is determined at least from the signal acquired at step d) and the further signal acquired at step d').
13. 13. The method of claim 11, further comprising repeating steps c) and e) for each of a plurality of values of the variable impedance.
14. 14. The method of claim 13, wherein at one of step c), the variable impedance is equivalent to an open circuit for the selected value.
15. 15. The method of claim 14, further comprising determining, at each step e), the attenuation value from at least the signal observed at the corresponding step d) and the signal observed at step d) when the variable impedance is equivalent to the open circuit.
16. 16. The method of claim 11, further comprising: Passing a second signal at the frequency of a local oscillator of the demodulation chain through a first circuit of the current source; passing a third signal at an intermediate frequency of the demodulation chain through a second circuit of the current source; Receiving the second signal and the third signal via a mixer having an output coupled to an internal node of the current source, and The internal node is coupled to the first node through a resistor.
17. 17. The method of claim 16, wherein the third signal is a square wave signal and the mixer is a switch mode mixer controlled by the third signal.
18. 18. The method of claim 17, further comprising: delivering a signal at a frequency greater than said intermediate frequency of said demodulation chain through an oscillator of said second circuit, and The third signal is transferred according to the signal transferred by the oscillator through a frequency divider of the second circuit.
19. 19. The method of claim 17, further comprising controlling first and second switches of the mixer in anti-phase according to the third signal, the first switch being connected between an output of the mixer and a node receiving the second signal, and the second switch being connected between the output of the mixer and the reference potential node.
20. 20. The method of claim 16, further comprising: passing, by a fourth circuit of the first circuit, a fourth square wave signal at a frequency equal to four times the frequency of the local oscillator; dividing, by a first frequency divider of the first circuit, a frequency of the fourth wave signal by two; Dividing the frequency of the fourth wave signal by four by a second frequency divider of the first circuit; Receiving the output signal of the first frequency divider and the output signal of the second frequency divider through a double-input gate of the first circuit, -Implementing an exclusive or function between said signals received by its inputs through said gates; coupling the output of the second frequency divider to the output of the first circuit through a first resistor of the first circuit, and The output of the gate is coupled to the output of the first circuit through a second resistor of the first circuit.

Description

Calibration of RF attenuators Cross Reference to Related Applications The present application claims the benefit of french patent application No.2101889 filed at 26, 2, 2021, which is incorporated herein by reference. Technical Field The present description relates generally to electronic circuits, and more particularly to wireless receive circuits for sub-GHz radio frequency signals. Background The sub-GHz radio frequency signal has a frequency, for example, in the range from 300MHz to 1 GHz. These sub-GHz signals are used to wirelessly transmit data between a transmit circuit configured to transmit the sub-GHz signals and a receive circuit configured to receive the sub-GHz signals. The receiving circuit is coupled to the antenna through an impedance matching network. The receiving circuit comprises a demodulation chain for extracting data from the signal supplied to the receiving circuit by the antenna. Demodulation chains, also known as receive chains, typically include low noise amplifiers. In order to avoid saturation of the receive chain when the power of the signal received by the antenna is too high, or to avoid too low a gain of the receive chain when the power of the signal received by the antenna is too low, the receive chain comprises a controllable attenuator. The gain of the receive chain is then related to the attenuator control or, in other words, to the attenuation applied by the controllable attenuator to the sub-GHz signal received by the receive circuit. By properly controlling the attenuator, the gain of the receive chain may be adapted for the power of the sub-GHz signal received by the antenna. In practice, in order to control the attenuator, a calibration phase of the attenuator is provided. In this calibration phase, the sub-GHz signal is passed to the antenna and the value of the corresponding signal at the output of the receive chain is observed by changing the value of the attenuator impedance. Attenuation introduced by the attenuator on the sub-GHz signal received by the demodulation chain is then derived for each of these impedance values. Thus, when the receiving circuit is operating, the attenuation value is selected in dependence on the amplitude of the output signal of the chain, and the attenuator impedance value corresponding to this attenuation during the calibration phase is selected. However, this calibration phase is cumbersome to implement, especially because it requires accurate transmission of sub-GHz signals in an anechoic environment. Furthermore, this step does not take into account the interference experienced by the antenna and its impedance matching network in its environment of use, especially when the antenna is arranged close to the conductive element. Disclosure of Invention Accordingly, there is a need to overcome all or part of the disadvantages of known methods of calibrating attenuators of radio frequency signal receiving chains, and more particularly sub-GHz signal receiving chains. It is also desirable to overcome all or part of the disadvantages of known circuits comprising a receive chain, wherein such a calibration method is implemented. One embodiment overcomes all or part of the shortcomings of known calibration methods of attenuators of radio frequency signal receiving chains, and more particularly sub-GHz signal receiving chains, and known circuits configured to implement such known methods. One embodiment provides a circuit comprising an input terminal configured to receive a first signal at a first frequency, a demodulation chain connected to the input terminal and comprising a low noise amplifier having an input coupled (preferably connected) to the terminal, a controllable variable impedance connected between a first node and a node configured to receive a reference potential, the first node being connected to the input terminal and/or the amplifier input, and a current source configured to pass a current at the first frequency to the first node. According to one embodiment, the current source comprises a first circuit configured to pass a second signal at the frequency of a local oscillator of the demodulation chain, a second circuit configured to pass a third signal at an intermediate frequency of the demodulation chain, a mixer configured to receive the second signal and the third signal, an output of the mixer being coupled (preferably connected) to an internal node of the current source, and a resistor coupling the internal node to the first node. According to one embodiment, the third signal is a square wave signal and the mixer is a switch mode mixer controlled by the third signal. According to one embodiment, the second circuit comprises an oscillator configured to pass a signal at a frequency greater than the intermediate frequency of the demodulation chain and a frequency divider configured to pass a third signal in dependence on the signal passed by the oscillator. According to one embodiment, the oscillator of the second circu