HALF BIN LINEAR FREQUENCY DISCRIMINATOR

• US 20090273372A1
• Filed: 05/04/2009
• Published: 11/05/2009
• Est. Priority Date: 02/25/2005
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Brenner, Joel
• Granted Patent: US 7,843,228 B2
• Time in Patent Office: Days
• US Class Current: 327/39
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: H03D 13/00 Circuits for comparing the phase or frequency of two mutually-independent oscillations measuring phase G01R25/00; phase-discriminators with yes/no output G01R25/005

Abstract

Frequency discriminator based on a variant of the DFT transform in which the usual twiddle factors are replaced with twiddle factors as for a DFT on a number of points which is the double as the actual number of sample points. The DFT so modified allows half-bin frequency discrimination, with few added computational burden. Two DFT shifted of half bin with respect to the zero frequency provide a linear response of the discrimination and good immunity to noise. The discriminator is particularly useful in FLL for tracking signals in a GPS receiver.

Claims

1. Method of obtaining a frequency difference between an input signal (x) and a reference frequency, comprising the steps of:

applying a first operator (H_D) for extracting a discrete spectral component of the input signal at a lower frequency (f_D) and a second operator (H_U) for extracting a discrete spectral component of the input signal at a upper frequency (f_U) the upper frequency (f_U) and the lower frequency (f_D) being placed above and under the reference frequency, whereas each of the first and second operators (H_D, H_U) has a maximum of its response for the corresponding spectral component;

calculating the difference of the two spectral component'"'"'s frequencies for obtaining a error value (65) depending on the distance between the input signal'"'"'s fundamental frequency and the reference frequency.

2. Method according to the preceding claim, further characterized in that:

the response of first and second operators (H_D, H_U) does not go to zero for any intermediate frequency comprised between the upper frequency (f_D) and the lower frequency (f_U).

3. Method according to the preceding claim, characterized in that the upper frequency (f_D) and the lower frequency (f_U) are symmetrically placed around the reference frequency.

4. Method according to claim 1, characterized in that the error value (65) is linearly dependent from the distance between the input signal'"'"'s fundamental frequency and the reference frequency.

5. Method according to claim 1, characterized in that the reference frequency is the zero frequency.

6. Method according to claim 1, characterized in that the two operators (H_D, H_U) comprise a step of calculating an absolute value of the output.

7. Method according to claim 1, comprising a step of dividing the error value (65) by the sum of the two spectral components from the first and second operators.

8. Method according to claim 1, characterized in that the input signal (x) comprises a number of N successive samples (x_i), and in that first and second operators (H_D, H_U) comprise a linear combination of the samples (x_i), with weight (W) factors taken from the 2N distinct complex roots of unity of order 2N.

9. Method according to the preceding claim, in which the first and second operators (H_D, H_U) are given by the formula

15. Computer program for carrying out the steps of methods of claim 1.

10. Discriminator device, comprising an input, for receiving an input signal, and frequency discriminator means (70), for generating an output signal (65) dependent on the difference from the fundamental frequency of the input signal and a reference frequency;

wherein the frequency discriminator means (70) comprise frequency extraction means (702, 704) for extracting a discrete spectral components of the input signal, at a lower frequency (f_D) and at a upper frequency (f_U) the upper frequency (f_U) and the lower frequency (f_D) being placed above and under the reference frequency, whereas the frequency discriminator means (702, 704) has a maximum of its response for the corresponding spectral component;

characterized in that;

the response of the frequency discriminator means (702, 704) does not go to zero for any intermediate frequency between f_D and f_U.

11. Discriminator device according to the preceding claim, wherein the reference frequency is the zero frequency.

12. Frequency control device, comprising a variable frequency source (44), a mixer (45) for combining an input frequency (42) with an output of the variable frequency source (44), a discriminator according to claim 10 comparing an output signal of the mixer and generating a frequency error signal, for driving the variable frequency source (44) and locking it to the input frequency (42).

13. GPS receiver, comprising a discriminator according to claim 10.

14. GPS receiver, comprising a frequency control device according to claim 12.