Search

EP-4297354-B1 - A METHOD FOR CALIBRATING AN SSB RECEIVER

EP4297354B1EP 4297354 B1EP4297354 B1EP 4297354B1EP-4297354-B1

Inventors

  • SCHIDL, Stefan
  • Krapfenbauer, Markus

Dates

Publication Date
20260506
Application Date
20220624

Claims (12)

  1. A method for calibrating a single sideband, SSB, receiver (1) having an input (2) for receiving a signal (S), an I/Q mixer (3) for converting the signal (S) into an I-signal (I) in an inphase path (8) and a Q-signal (Q) in a quadrature path (11), a phase shifter (4) for mutually phase shifting the I- and Q-signals (I, Q) by an adjustable phase shift (ΔΘ), and a combiner (5) for combining the mutually phase shifted I- and Q-signals (I', Q') to an SSB signal (R), the method (M) comprising: a) adjusting the phase shift (ΔΘ) to a first phase shift value (ΔΘ 1 ); the method (M) being characterised by : b) successively feeding a first, a second and a third test signal (S 1 , S 2 , S 3 ) to the input (2), the first and second test signals (S 1 , S 2 ) having a predetermined phase offset (ΔΦ) and the second and third test signals (S 2 , S 3 ) having the same predetermined phase offset (ΔΦ), to obtain respective first, second and third SSB signals (R 1 , R 2 , R 3 ), and measuring a first phase difference (Δφ 12 ) between the first and second SSB signals (R 1 , R 2 ) and a second phase difference (Δφ 23 ) between the second and third SSB signals (R 2 , R 3 ); c) calculating a first phase error (E φ,1 ) on the basis of the first and second phase differences (Δφ 12 , Δφ 23 ); d) repeating steps a) - c) with a second phase shift value (ΔΘ 2 ) in step a) to obtain a second phase error (E φ,2 ); and e) calibrating the SSB receiver (1) by using that one of the first and second phase shift values (ΔΘ 1 , ΔΘ 2 ) that has yielded the smaller one of the first and second phase errors (E φ,1 , E φ,2 ).
  2. The method according to claim 1, wherein the first and second phase errors (E φ,1 , E φ,2 ) are calculated each as E φ , j = Δ φ 12 − Δ φ 23 with E φ,j ........... the respective first (j = 1) and second (j = 2) phase error, Δφ 12 , Δφ 23 ..... the first and second phase differences, and |·| ...........denoting the absolute value.
  3. The method according to claim 1 or 2, wherein the predetermined phase offset (ΔΦ) is in the range of 10° to 80°, preferably in the range of 25° to 65°, particularly preferably 45°.
  4. The method according to any one of claims 1 to 3, wherein the phase shifter (4) comprises, in one of the inphase and quadrature paths (8, 11), a filter (28) with at least one variable capacitor or inductor.
  5. The method according to claim 4, wherein the filter (28) comprises a capacitor bank (30) having parallel capacitors (31 n ) that can be selectively connected to said one path (8, 11) to adjust the phase shift (ΔΘ).
  6. The method according to claim 5, wherein capacitances of the capacitors (31 n ) of said capacitor bank (30) form a geometric sequence with a common ratio of two.
  7. The method according to any one of claims 4 to 6, wherein the phase shifter (4) comprises another filter (29) in the other one of said paths (8, 11).
  8. The method according to any one of claims 1 to 7, wherein the SSB receiver (1) further comprises an amplitude adjuster (33) for mutually adjusting the amplitudes of the I- and Q-signals (I, Q) by an adjustable amplitude gain (Γ), wherein in step a) and its repetition in step d) also the amplitude gain (Γ) is adjusted to a first and a second amplitude gain value (Γ 1 , Γ 2 ), respectively, and in step e) the SSB receiver (1) is calibrated by using that combination of first phase shift and amplitude gain values (ΔΘ 1 , Γ 1 ) and second phase shift and amplitude gain values (ΔΘ 2 , Γ 2 ) that has yielded the smaller one of the first and second phase errors (E φ,1 , E φ,2 ).
  9. The method according to claim 8, wherein the amplitude adjuster (33) comprises a resistor bank (34) having parallel resistors (35 m ) that can be selectively connected to one of the inphase and quadrature paths (8, 11) to adjust the amplitude gain.
  10. The method according to claim 9, wherein resistances of the resistors (35 m ) of said resistor bank (34) form a geometric sequence with a common ratio of two.
  11. The method according to claim 9 or 10, wherein the resistor bank further has a respective capacitor (37 m ) in series to each selectively connectable resistor (35 m ).
  12. The method according to any one of claims 1 to 11, wherein the test signals (S i ) are generated by amplitude modulation.

Description

The present invention relates to a method for calibrating a single sideband (SSB) receiver. In typical SSB communications an SSB transmitter sends a signal with frequency components in one sideband only to reduce power and bandwidth required for signal transmission. The sent signal is received and processed to a baseband SSB signal by the SSB receiver. To this end the SSB receiver has an input for receiving the signal, an I/Q mixer for converting the signal into an I- and a Q-signal, a phase shifter for mutually phase shifting the I- and Q-signals by 90° and a combiner for combining the I- and Q-signals to an SSB signal. In this receiver structure, an unwanted sideband (the "image signal"), e.g. originating from an imperfect SSB transmitter, superposes and impedes the SSB signal and should, thus, ideally be cancelled after phase shifting and combining the I- and Q-signals. However, due to residual misalignments of the I- and Q-signals, e.g. caused by component ageing or temperature, most often a residual image signal remains and impedes the SSB signal also in the single sideband of interest by distorting phases and amplitudes of its frequency components. The level of the residual image signal impacts the received SSB signal and, consequently, the accuracy of the application, wherefore a proper receiver calibration is crucial. In road tolling applications, for instance, road-sided localisation transceivers comprise an array of SSB receivers and localise a passing vehicle carrying an on-board unit (OBU) via SSB communications between the array and the OBU. To localise the vehicle, a phase of a received SSB signal is measured at a target frequency by each SSB receiver of the array, and an angle of arrival of the SSB signal and the position of the OBU is determined from differences of the measured phases. Any residual image signals arising in the SSB receivers result in an incorrect measurement of the phase of each received SSB signal, in incorrect phase differences and, hence, in an incorrect localisation of the vehicle. Consequently, a receiver calibration which achieves an efficient image cancellation is desirable to correctly localise vehicles, e.g. in tolling or lane control applications. A variety of calibration methods for cancelling the image signal in an SSB receiver are known in the art, see, e.g., US 2002/0055347 A1, US 6,714,776 B1, US 2006/0281411 A1 and US 2011/0182388 A1. However, current receiver calibrations still suffer from a too high residual image signal distorting the received SSB signal and/or a high complexity necessitating expensive hardware. It is an object of the present invention to provide a method for calibrating an SSB receiver which overcomes the drawbacks of the state of the art and which is both efficient in image signal cancellation and easy to implement. This object is achieved by a method for calibrating an SSB receiver having an input for receiving a signal, an I/Q mixer for converting the signal into an I-signal in an inphase path and a Q-signal in a quadrature path, a phase shifter for mutually phase shifting the I- and Q-signals by an adjustable phase shift, and a combiner for combining the mutually phase shifted I- and Q-signals to an SSB signal, the method comprising: a) adjusting the phase shift to a first phase shift value;b) successively feeding a first, a second and a third test signal to the input, the first and second test signals having a predetermined phase offset and the second and third test signals having the same predetermined phase offset, to obtain respective first, second and third SSB signals, and measuring a first phase difference between the first and second SSB signals and a second phase difference between the second and third SSB signals;c) calculating a first phase error on the basis of the first and second phase differences;d) repeating steps a) - c) with a second phase shift value in step a) to obtain a second phase error; ande) calibrating the SSB receiver by using that one of the first and second phase shift values that has yielded the smaller one of the first and second phase errors. The inventive calibration method is based on adjusting an adjustable phase shift in the phase shifter in order to minimise an asymmetry of measured phase differences between three (the first, second and third) SSB signals originating from three (the first, second and third) test signals which have been generated with predetermined symmetric phase offsets around a phase of the second test signal. As the asymmetry of the measured phase differences indicates the strength of the unwanted residual image signal, a minimisation of the phase error calculated from the measured phase differences minimises the residual image signal and allows for an accurate image signal cancellation in the combiner. By repeatedly carrying out steps a) to c) for each of the two (optionally for more than two) phase shift values, that phase shift value which minimises the calculated phase erro