Method and system for characterizing phase dispersion in intermediate frequency channel of receiver

• US 9,520,954 B1
• Filed: 01/28/2016
• Issued: 12/13/2016
• Est. Priority Date: 01/28/2016
• Status: Active Grant

• Current Assignee: Keysight Technologies, Inc.
• Original Assignee: Keysight Technologies, Inc.
• Inventors: Verspecht, Jan, Anderson, Keith F.
• Primary Examiner(s): Vuong, Quochien B
• Application Number:
  US15/009,309
• Time in Patent Office: 320 Days
• Field of Search: 455/67.11, 455/67.14, 455/131, 455/307, 455/313, 455/323, 455/334, 455/339, 375/316, 375/324
• US Class Current: 1/1
• CPC Class Codes: H04B 1/26 for superheterodyne receivers multiple frequency-changing H03D7/16
• CPC Class Codes: H04B 17/0085 using test signal generators
• CPC Class Codes: H04B 17/20 of receivers

Abstract

A system and method supply a test signal having a first tone at a first RF frequency and a second tone at a second RF frequency to a frequency converter; provide a local oscillator (LO) signal to the frequency converter, wherein an IF output signal of the frequency converter is supplied to an input of an intermediate frequency (IF) filter, in response to which the IF filter provides a filtered IF output signal; for each of N>1 different LO frequencies, measure the filtered IF output signal at a pair of IF frequencies corresponding to differences between the first and second RF frequencies and the LO frequency, where the measurements of the filtered IF output signal measure time-invariant phase; and ascertain N−1 values of phase dispersion D of the IF filter at N−1 corresponding IF frequencies from the N measurements of the filtered IF output signal at the N different LO frequencies.

Claims

1. A method, comprising:

generating a test signal having a first tone at a first radio frequency (RF) frequency and a second tone at a second RF frequency;

using a local oscillator (LO) signal to convert the test signal to an intermediate frequency (IF) signal;

filtering the IF signal with an IF filter to produce a filtered IF output signal;

setting the LO signal to a first LO frequency;

measuring the filtered IF output signal at first and second IF frequencies while the LO signal is set to the first LO frequency, wherein the first IF frequency is a difference between the second RF frequency and the first LO frequency, and the second IF frequency is a difference between the first RF frequency and the first LO frequency;

setting the LO signal to a second LO frequency, wherein a frequency difference between the second LO frequency and the first LO frequency is equal to a frequency difference Δ

f between the second RF frequency and the first RF frequency of the test signal;

measuring the filtered IF output signal at the second IF frequency and at a third IF frequency while the LO signal is set to the second LO frequency, wherein the third IF frequency is a difference between the first RF frequency and the second LO frequency; and

ascertaining a value D of a phase dispersion of the IF filter at the second IF frequency from the measurements of the filtered IF output signal,

where the measurements of the filtered IF output signal measure time-invariant phase.

2. The method of claim 1, wherein measuring the filtered IF output signal includes:

measuring a phasor M1[1] of the filtered IF output signal at the second IF frequency while the LO signal is set to the first LO frequency,

measuring a phasor M2[1] of the filtered IF output signal at the first IF frequency while the LO signal is set to the first LO frequency,

measuring a phasor M1[2] of the filtered IF output signal at the third IF frequency while the LO signal is set to the second LO frequency, and

measuring a phasor M2[2] of the filtered IF output signal at the second IF frequency while the LO signal is set to the second LO frequency; and

wherein ascertaining the value D of the phase dispersion of the IF filter at the second IF frequency comprises ascertaining

3. The method of claim 1, further comprising measuring the filtered IF output signal at a plurality of additional IF frequencies for a plurality of first RF frequencies, second RF frequencies, LO frequencies, and frequency differences Δ

f, and ascertaining a phase dispersion characteristic of the IF filter as a function of IF frequency from the measurements of the filtered IF output signal.

4. The method of claim 1, wherein measuring the filtered IF output signal includes:

digitizing the filtered IF output with an analog-to-digital converter (ADC) to produce digital data; and

applying a discrete Fourier transform to the digital data to obtain a digital representation of the filtered IF output signal as a function of frequency.

5. The method of claim 4, wherein the first RF frequency, second RF frequency, first LO frequency, second LO frequency, and the ADC are all synchronized to a common reference clock.

6. The method of claim 4, further comprising:

providing a phase slope reference signal, the phase slope reference signal synchronized to the first and second RF frequencies and having a frequency equal to a difference between the first and second RF frequencies;

ascertaining phases of the phase slope reference signal corresponding to the times when measuring the filtered IF output signal at the first and second IF frequencies while the LO signal is set to the first LO frequency and corresponding to the times when measuring the filtered IF output at the second IF frequency and at a third IF frequency while the LO signal is set to the second LO frequency; and

adjusting the measurements of the filtered IF output signal by the ascertained phases of the phase slope reference signal.

7. The method of claim 1, wherein using the local oscillator (LO) signal to convert the test signal to an intermediate frequency (IF) signal comprises:

supplying the test signal to an RF filter; and

supplying the LO signal and an output of the RF filter and to a mixer.

8. The method of claim 1, wherein the test signal further has a third RF frequency, the method further comprising:

measuring the filtered IF output signal at the third IF frequency while the LO signal is set to the first LO frequency;

measuring the filtered IF output at the a fourth IF frequency while the LO signal is set to the second LO frequency; and

ascertaining a phase dispersion of the IF filter at the third IF frequency from the measurements of the filtered IF output signal.

9. The method of claim 1, wherein generating the test signal having the first tone at the first RF frequency and the second tone at the second RF frequency comprises generating a pulsed RF signal having at least the first tone and the second tone.

10. A system, comprising:

a radio frequency (RF) signal generator configured to produce a test signal having a first tone at a first RF frequency and a second tone at a second RF frequency and further configured to supply the test signal to a superheterodyne receiver, the superheterodyne receiver also receiving a local oscillator (LO) signal and in response to the test signal and the LO signal supplying an intermediate signal (IF) output signal to an input of an IF filter, in response to which the IF filter outputs a filtered IF output signal;

an IF signal processor configured to process the filtered IF signal; and

a controller configured to control the system to;

set the LO signal to a first LO frequency;

measure the filtered IF output signal at first and second IF frequencies while the LO signal is set to the first LO frequency;

set the LO signal to a second LO frequency, wherein a frequency difference between the second LO frequency and the first LO frequency is equal to a frequency difference Δ

f between the second RF frequency and the first RF frequency of the test signal;

measure the filtered IF output at the second IF frequency and at a third IF frequency while the LO signal is set to the second LO frequency, where the third IF frequency is a difference between the first RF frequency and the second LO frequency; and

ascertain a value D of a phase dispersion of the IF filter at the second IF frequency from the measurements of the filtered IF output signal,

where the measurements of the filtered IF output signal measure time-invariant phase.

11. The system of claim 10, wherein measuring the filtered IF output signal includes:

measuring a phasor M1[1] of the filtered IF output signal at the second IF frequency while the LO signal is set to the first LO frequency,

measuring a phasor M2[1] of the filtered IF output signal at the first IF frequency while the LO signal is set to the first LO frequency,

measuring a phasor M1[2] of the filtered IF output signal at the third IF frequency while the LO signal is set to the second LO frequency, and

measuring a phasor M2[2] of the filtered IF output signal at the second IF frequency while the LO signal is set to the second LO frequency; and

wherein ascertaining the phase dispersion D of the IF filter at the second IF frequency comprises ascertaining

12. The system of claim 10, wherein the controller is further configured to control the RF generator to adjust the first and second RF frequencies and to control the LO to adjust the LO frequencies, and to measure the filtered IF output signal at a plurality of additional IF frequencies for a corresponding plurality of first RF frequencies, second RF frequencies, LO frequencies, and frequency differences Δ

f, and to ascertain a phase dispersion characteristic of the IF filter as a function of IF frequency from the measurements of the filtered IF output signal.

13. The system of claim 10, wherein the IF signal processor includes:

an analog-to-digital-converter (ADC) configured to digitize the filtered IF signal to produce digital data; and

a digital-signal-processor configured to apply a discrete Fourier transform to the digital data to obtain a digital representation of the filtered IF output signal as a function of frequency, and to process the digital representation of the filtered IF output signal as a function of frequency to measure the filtered IF output signal.

14. The system of claim 13, wherein the first RF frequency, second RF frequency, first LO frequency, second LO frequency, and the ADC are all synchronized to a common reference clock.

15. The system of claim 10, further comprising a phase slope reference signal generator configured to generate a phase slope reference signal synchronized to the first and second RF frequencies and having a frequency equal to a difference between the first and second RF frequencies,

wherein the controller is configured to control the system to ascertain phases of the phase slope reference signal corresponding to the times when measuring the filtered IF output signal at the first and second IF frequencies while the LO signal is set to the first LO frequency and corresponding to the times when measuring the filtered IF output at the second IF frequency and at a third IF frequency while the LO signal is set to the second LO frequency, and to adjust the measurements of the filtered IF output signal by the ascertained phases of the phase slope reference signal.

16. The system of claim 15, wherein the phase slope reference signal generator comprises a numerically controlled oscillator (NCO).

17. The system of claim 10, wherein the test signal further has a third tone at a third RF frequency, wherein the controller is further configured to control the system to:

measure the filtered IF output signal at the third IF frequency while the LO signal is set to the first LO frequency;

measure the filtered IF output at a fourth IF frequency while the LO signal is set to the second LO frequency; and

ascertain a phase dispersion of the IF filter at the third IF frequency from the measurements of the filtered IF output signal.

18. The system of claim 10, wherein the RF signal generator comprises a pulse modulated signal generator generating a pulsed RF signal having at least the first tone and the second tone.

19. A method, comprising:

supplying a test signal having a first tone at a first radio frequency (RF) frequency and a second tone at a second RF frequency to a frequency converter;

controlling a local oscillator (LO) frequency of an LO signal provided to the frequency converter, wherein an IF output signal of the frequency converter is supplied to an input of an intermediate frequency (IF) filter, in response to which the IF filter provides a filtered IF output signal;

for each of N>

1 different LO frequencies, measuring the filtered IF output signal at a pair of IF frequencies corresponding to differences between the first and second RF frequencies and the LO frequency, where the measurements of the filtered IF output signal measure time-invariant phase; and

ascertaining N−

1 values of a phase dispersion response of the IF filter at N−

1 corresponding IF frequencies from the N measurements of the filtered IF output signal at the N different LO frequencies.

20. The method of claim 19, wherein measuring the filtered IF output signal includes:

measuring a phasor M1[1] of the filtered IF output signal at the second IF frequency while the LO signal is set to a first LO frequency,

measuring a phasor M2[1] of the filtered IF output signal at the first IF frequency while the LO signal is set to the first LO frequency,

measuring a phasor M1[2] of the filtered IF output signal at the third IF frequency while the LO signal is set to a second LO frequency, and

measuring a phasor M2[2] of the filtered IF output signal at the second IF frequency while the LO signal is set to the second LO frequency; and

wherein ascertaining the phase dispersion D of the IF filter at the second IF frequency comprises ascertaining

21. The method of claim 19, wherein measuring the filtered IF output signal includes:

digitizing the filtered IF output with an analog-to-digital converter (ADC) to produce digital data; and

applying a discrete Fourier transform to the digital data to obtain a digital representation of the filtered IF output signal as a function of frequency,

wherein the first RF frequency, second RF frequency, first LO frequency, second LO frequency, and the ADC are all synchronized to a common reference clock.

22. The method of claim 19, wherein the test signal further has a third RF frequency, the method further comprising:

for each of N>

1 different LO frequencies, measuring the filtered IF output signal at a third IF frequency corresponding to a difference between the third RF frequency and the LO frequency; and

ascertaining an Nth value of phase dispersion D of the IF filter an Nth IF frequency from the N measurements of the filtered IF output signal at the N different LO frequencies.

23. The method of claim 19, wherein supplying to the frequency converter the test signal having the first tone at the first RF frequency and the second tone at the second RF frequency comprises generating a pulsed RF signal having at least the first tone and the second tone.

24. A method, comprising:

supplying a first signal to a first input of a frequency converter and supplying a second signal to a second input of the frequency converter, wherein an intermediate frequency (IF) output signal of the frequency converter is supplied to an input of an IF filter, in response to which the IF filter provides a filtered IF output signal;

setting the first signal to a first frequency and the second signal to a second frequency, and measuring a first instantaneous phase the filtered IF output signal at a first IF frequency corresponding to a difference between the first frequency and second frequency;

changing the first signal to a third frequency while leaving the second signal at the second frequency, and measuring a second instantaneous phase of the filtered IF output signal at a second IF frequency corresponding to a difference between the third frequency and the second frequency;

changing the second signal to a fourth frequency while leaving the first signal at the third frequency, and measuring a third instantaneous phase of the filtered IF output signal at a third IF frequency corresponding to a difference between the third frequency and the fourth frequency;

changing the first signal to the first frequency while leaving the second signal at the fourth frequency, and measuring a fourth instantaneous phase of the filtered IF output signal at the second IF frequency corresponding to the difference between the third frequency and the second frequency, wherein a phase of the first signal is controlled to be the same while measuring the fourth instantaneous phase of the filtered IF output signal as a phase of the first signal while measuring the first instantaneous phase of the filtered IF output signal; and

combining the measured first instantaneous phase of the filtered IF output signal, the measured second instantaneous phase of the filtered IF output signal, the measured third instantaneous phase of the filtered IF output signal, and the measured fourth instantaneous phase of the filtered IF output signal, and normalizing the instantaneous phase of the filtered IF output signal at the first IF frequency and at the third IF frequency, to obtain a phase dispersion of the IF filter at the second IF frequency.

25. The method of claim 24, wherein the frequency converter comprises a mixer having a radio frequency (RF) input port and a local oscillator (LO) input port, and wherein the first signal is applied to the RF input port, and the second signal is applied to the LO input port.

26. The method of claim 24, wherein the frequency converter comprises a mixer having a radio frequency (RF) input port and a local oscillator (LO) input port, and wherein the first signal is applied to the LO input port, and the second signal is applied to the RF input port.