Passband reflectometer
View Patent ↗A time-domain (TD) reflectometer that is designed to operate based on probe and response signals that are substantially fully spectrally confined to a designated frequency passband. In one embodiment, the TD reflectometer uses a passband transmitter to generate the probe signal based on a pseudo-random bit sequence and a passband receiver to demodulate the response signal. The TD reflectometer determines the impulse response of a channel under test based on cross-correlation of the transmitter and receiver baseband signals. In various embodiments, the TD reflectometer can be designed to operate in an acoustic-frequency range, a radio-frequency range, or an optical-frequency range. Due to its passband configuration, the TD reflectometer is advantageously capable of determining impulse responses without disrupting the operation and/or interfering with normal functions of the tested channel.
1. An apparatus comprising:
a passband transmitter configured to generate a drive signal based on a bit sequence;
an interface configured to convert the drive signal into a probe signal, apply the probe signal to a channel under test, and receive a response signal corresponding to the probe signal back from said channel;
a passband receiver configured to receive, from the interface, an input signal corresponding to the response signal and to convert said input signal into a receiver-baseband signal; and
a high-pass filter coupled between the interface and the passband receiver, wherein:
the probe signal is spectrally limited to a spectral band located at frequencies higher than a first threshold frequency, wherein the first threshold frequency is a nonzero frequency;
the apparatus is configured to characterize an impulse response of the channel under test based on the bit sequence and the receiver-baseband signal; and
the high-pass filter has a cut-off frequency that is lower than or about the same as the first threshold frequency.
2. The apparatus of claim 1 , wherein the bit sequence is a maximum-length sequence.
3. The apparatus of claim 1 , wherein the passband transmitter comprises:
a coder module configured to convert the bit sequence into a corresponding sequence of constellation symbols; and
a transmit filter configured to convert the sequence of constellation symbols into a transmitter-baseband signal, wherein:
the passband transmitter is configured to generate the drive signal based on the transmitter-baseband signal; and
the apparatus is configured to determine the impulse response based on a correlation between the receiver-baseband signal and the transmitter-baseband signal.
4. The apparatus of claim 3 , wherein the coder module is configured to convert the bit sequence into the sequence of constellation symbols using a constellation selected from a set consisting of a binary phase-shift keying (BPSK) constellation, a quadrature phase-shift keying (QPSK) constellation, an amplitude-shift keying (ASK) constellation, and a quadrature amplitude modulation (QAM) constellation.
5. The apparatus of claim 3 , wherein the passband transmitter further comprises:
a frequency up-converter configured to inject a carrier frequency into the transmitter-baseband signal to generate a transmitter-passband signal; and
a drive circuit configured to generate the drive signal based on the transmitter-passband signal.
6. The apparatus of claim 5 , wherein:
the interface comprises an ultrasonic transducer configured to convert the drive signal into the probe signal; and
the carrier frequency is selected from an ultrasonic frequency range.
7. The apparatus of claim 5 , wherein:
the interface comprises an antenna or a radio-frequency coupler configured to convert the drive signal into the probe signal; and
the carrier frequency is selected from a radio-frequency range.
8. The apparatus of claim 3 , wherein:
the interface comprises an optical modulator coupled to a laser and configured to generate the probe signal based on the drive signal and a carrier-frequency signal provided by the laser; and
the carrier frequency is in an optical-frequency range.
9. The apparatus of claim 3 , further comprising a correlation module configured to:
generate a Fourier transform of the receiver-baseband signal;
generate a correlation function by multiplying the Fourier transform of the receiver-baseband signal and a Fourier transform of the transmitter-baseband signal;
generate an inverse Fourier transform of the correlation function; and
determine the impulse response based on the inverse Fourier transform.
10. The apparatus of claim 1 , wherein the passband receiver comprises:
a receive filter configured to correct the input signal for filtering effects of the interface; and
a demodulator configured to down-convert the corrected input signal to generate the receiver-baseband signal.
11. The apparatus of claim 1 , wherein:
the interface is designed to be coupled to a human vocal tract; and
the apparatus is an acoustic reflectometer.
12. The apparatus of claim 1 , wherein the impulse response is a reflected impulse response.
13. A method of characterizing an impulse response, the method comprising:
generating a drive signal based on a bit sequence using a passband transmitter;
converting the drive signal into a probe signal using an interface, wherein the probe signal is spectrally limited to a spectral band located at frequencies higher than a first threshold frequency, wherein the first threshold frequency is a nonzero frequency;
applying the probe signal to a channel under test using the interface;
receiving a response signal corresponding to the probe signal back from said channel, wherein the interface is configured to receive said response signal;
generating a receiver-baseband signal based on the response signal using a passband receiver; and
characterizing an impulse response of the channel based on the bit sequence and the receiver-baseband signal; and
wherein the step of generating the drive signal using the passband transmitter comprises:
converting the bit sequence into a corresponding sequence of constellation symbols; and
converting the sequence of constellation symbols into a transmitter-baseband signal, wherein:
the drive signal is generated based on the transmitter-baseband signal; and
the impulse response is determined based on a correlation between the receiver-baseband signal and the transmitter-baseband signal.
14. The method of claim 13 , wherein the bit sequence is a maximum-length sequence.
15. The method of claim 13 , wherein the bit sequence is converted into the sequence of constellation symbols using a constellation selected from a set consisting of a binary phase-shift keying (BPSK) constellation, a quadrature phase-shift keying (QPSK) constellation, an amplitude-shift keying (ASK) constellation, and a quadrature amplitude modulation (QAM) constellation.
16. The method of claim 13 , wherein the step of generating the drive signal further comprises:
frequency up-converting the transmitter-baseband signal by injecting a carrier frequency into the transmitter-baseband signal to generate a transmitter-passband signal; and
generating the drive signal based on the transmitter-passband signal.
17. The method of claim 13 , further comprising generating a carrier-frequency signal using a laser, wherein:
the probe signal is generated using an optical modulator configured to modulate the carrier-frequency signal based on the drive signal; and
the carrier frequency is in an optical-frequency range.
18. The method of claim 13 , further comprising:
generating a Fourier transform of the receiver-baseband signal;
generating a correlation function by multiplying the Fourier transform of the receiver-baseband signal and a Fourier transform of the transmitter-baseband signal;
generating an inverse Fourier transform of the correlation function; and
determining the impulse response based on the inverse Fourier transform.
19. An apparatus comprising:
a passband transmitter configured to generate a drive signal based on a bit sequence;
an interface configured to convert the drive signal into a probe signal, apply the probe signal to a channel under test, and receive a response signal corresponding to the probe signal back from said channel; and
a passband receiver configured to receive, from the interface, an input signal corresponding to the response signal and to convert said input signal into a receiver-baseband signal, wherein:
the probe signal is spectrally limited to a spectral band located at frequencies higher than a first threshold frequency, wherein the first threshold frequency is a nonzero frequency;
the apparatus is configured to characterize an impulse response of the channel under test based on the bit sequence and the receiver-baseband signal; and
the passband transmitter comprises:
a coder module configured to convert the bit sequence into a corresponding sequence of constellation symbols; and
a transmit filter configured to convert the sequence of constellation symbols into a transmitter-baseband signal;
the passband transmitter is configured to generate the drive signal based on the transmitter-baseband signal; and
the apparatus is configured to determine the impulse response based on a correlation between the receiver-baseband signal and the transmitter-baseband signal.
20. An apparatus comprising:
a passband transmitter configured to generate a drive signal based on a bit sequence;
an interface configured to convert the drive signal into a probe signal, apply the probe signal to a channel under test, and receive a response signal corresponding to the probe signal back from said channel; and
a passband receiver configured to receive, from the interface, an input signal corresponding to the response signal and to convert said input signal into a receiver-baseband signal, wherein:
the probe signal is spectrally limited to a spectral band located at frequencies higher than a first threshold frequency, wherein the first threshold frequency is a nonzero frequency;
the apparatus is configured to characterize an impulse response of the channel under test based on the bit sequence and the receiver-baseband signal; and
the passband receiver comprises:
a receive filter configured to correct the input signal for filtering effects of the interface; and
a demodulator configured to down-convert the corrected input signal to generate the receiver-baseband signal.
21. An apparatus comprising:
a passband transmitter configured to generate a drive signal based on a bit sequence;
an interface configured to convert the drive signal into a probe signal, apply the probe signal to a channel under test, and receive a response signal corresponding to the probe signal back from said channel; and
a passband receiver configured to receive, from the interface, an input signal corresponding to the response signal and to convert said input signal into a receiver-baseband signal, wherein:
the probe signal is spectrally limited to a spectral band located at frequencies higher than a first threshold frequency, wherein the first threshold frequency is a nonzero frequency;
the apparatus is configured to characterize an impulse response of the channel under test based on the bit sequence and the receiver-baseband signal;
the interface is designed to be coupled to a human vocal tract; and
the apparatus is an acoustic reflectometer.