Sampling system for radio frequency receiver

• US 5,841,814 A
• Filed: 10/17/1995
• Issued: 11/24/1998
• Est. Priority Date: 10/17/1995
• Status: Expired due to Term

• Current Assignee: Chesapeake Virginia Communications LP (Rembrandt IP Management, LLC)
• Original Assignee: Paradyne Corporation (Dasan Networks Incorporated)
• Inventors: Cupo, Robert L.
• Primary Examiner(s): LE, AMANDA T
• Application Number:
  US08/544,244
• Time in Patent Office: 1,134 Days
• Field of Search: 375/261, 375/264, 375/270, 375/277, 375/316, 375/320-322, 375/324, 375/328, 375/340, 375/344, 348/725, 348/726, 455/204, 455/205, 455/207-209, 455/214, 455/313-315, 455/323, 455/324, 329/304, 329/316, 329/323, 329/348, 329/357, 329/358, 329/359, 329/361
• US Class Current: 375/321
• CPC Class Codes: H04L 27/0008 arrangements for allowing a transmitter or receiver to use more than one type of modulation negotiating modulation type for two-way transmission paths H04L5/1453
• CPC Class Codes: H04L 27/063 Superheterodyne receivers
• CPC Class Codes: H04L 27/3827 in which the carrier is recovered using only the demodulated baseband signals
• CPC Class Codes: H04N 5/46 for receiving on more than one standard at will deflecting circuits of multi-standard receivers H04N3/27

Abstract

A radio frequency (rf) receiver adapted to receive a number of different digitally modulated rf input signals such as quadrature amplitude modulated (QAM) and vestigial side band (VSB) rf input signals includes circuitry for down converting the rf input signals to an intermediate frequency (IF) range having a center frequency fc2 and a bandwidth of Bhz and converter circuitry for sampling the IF signals and then producing corresponding baseband signals. In a preferred embodiment, the intermediate frequency signals are applied to a sample and hold circuit which is sampled at a frequency fs and whose output is coupled via a low pass filter to an analog-to-digital converter whose output is then applied to a Hilbert filter for demodulating the sampled signals and producing baseband signals. In-phase (I) and quadrature (Q) signals are produced whose phase and amplitude are not a function of different components and their tolerance of different conduction paths, as in the prior art.

Claims

1. A radio frequency (RF) receiver comprising:

an input terminal adapted to receive one or more of a number of different digitally modulated RF input signals;

a converting means having an input and an output;

means coupling the input of said converting means to said input terminal for down converting one of said RF input signals and producing at the output of said converting means an intermediate frequency (IF) signal;

a sampling means having an input coupled to the output of said converting means for digitally sampling the IF signal and having an output for producing thereat a baseband signal in response to one of said RF input signals; and

said sample means further comprising a sample and hold circuit (S/H) having an input, an output and a clocking input;

a low pass filter having an input and an output;

an analog-to-digital converter (ADC) having an input, an output and a clocking input;

means coupling the input of the S/H to the output of the converting means and the output of the S/H to the input of the low pass filter;

means coupling the output of the low pass filter to the input of the ADC; and

, means for applying a sampling signal having a frequency f3 to the clocking input of the S/H and for applying a sampling signal having a frequency f4 to the clocking input of the ADC, wherein f3 is a rational multiple of f4.

2. A radio frequency receiver as claimed in claim 1 wherein the frequency of f3 is two times the frequency of f4 (i.e., f3=2f4).

3. A radio frequency (rf) receiver comprising:

an input terminal adapted to receive one or more of a number of different digitally modulated rf input signals, including quadrature amplitude modulated (QAM) and vestigial side band (VSB) rf input signals;

a converting means having an input and an output, said converting means including a first mixer and a second mixer, each of said mixers having an input and an output, a first local oscillator (LO1) and a second local oscillator (LO2), each local oscillator having an output at which is produced a frequency signal, means coupling an output of LO1 and the rf input signals to the input of the first mixer, means coupling the output of the first mixer to an input of a first bandpass filter (BPF1) having a first center frequency (fc1) and a bandwidth of Bhz to produce at an output of BPF1 a first output signal (fo1) having a first center frequency of fc1 with a bandwidth of Bhz, means coupling the output of BPF1 and an output of LO2 to an input of the second mixer for down converting the rf signal, and, means coupling the output of the second mixer to an input of a second bandpass filter (BPF2), said BPF2 having a second center frequency (fc2) and a bandwidth of Bhz, and BPF2 having an output defining the output of said converting means for producing thereat an intermediate frequency (IF) signal whose center frequency is equal to fc2 and whose bandwidth is equal to Bhz; and

a sampling means having an input coupled to the output of said converting means for digitally sampling the intermediate frequency signal and having an output for producing thereat base band signals in response to the rf input signals, said sampling means including a sample and hold circuit (S/H) having an input, an output and a clocking input, a low pass filter having an input and an output, an analog-to-digital converter (ADC) having an input, an output and a clocking input, means coupling the input of the S/H to the output of the converting means and the output of the S/H to the input of the low pass filter, means coupling the output of the low pass filter to the input of the ADC, and, means for applying a sampling signal having a frequency f3 to the clocking input of the S/H and for applying a sampling signal having a frequency f4 to the clocking input of the ADC, wherein f is a rational multiple of f4, and, wherein said sampling means is capable of producing at its output an in-phase (I) base band signal and a quadrature (Q) base band signal in response to a QAM rf input signal and an in-phase (I) baseband signal in response to a VSB rf input signal.

4. A radio frequency receiver as claimed in claim 3 wherein the frequency of f3 is two times the frequency of f4 (i.e., f3=2f4).

5. A radio frequency receiver as claimed in claim 3, wherein the center frequency of the first bandpass filter minus the frequency of the signal at the output of the second local oscillator coupled to the second mixer is equal to the center frequency of the second bandpass filter.

6. A radio frequency receiver as claimed in claim 5, wherein the frequency of the sampling signal applied to the S/H circuit is equal to, or greater than, the sum of the center frequency fc2 of BPF2 plus a frequency fA;

where fA is equal to either one half of the bandwidth of the QAM signal or one half of the bandwidth of the VSB signal.

7. A radio frequency receiver as claimed in claim 6, wherein said sampling means further comprises a Hilbert filter having an input, a real output, and an imaginary output, said output of the ADC being coupled to said input of said Hilbert filter, said Hilbert filter for producing at the output of the Hilbert filter the real and imaginary components of the sampled signals.

8. A radio frequency receiver as claimed in claim 7, wherein said real and imaginary components of the sampled signals at the output of the Hilbert filter are used to produce said I and Q base band signals responsive to said QAM rf input signal and said I baseband signal responsive to said VSB rf input signal.

9. A radio frequency receiver as claimed in claim 8, wherein the demodulator includes means for digitally demodulating the carrier signals and completing the transfer of the received signal to baseband at the output of the demodulator.

10. A radio frequency receiver as claimed in claim 9, wherein the center frequency (fc2) of BPF2 is selected such that the sampling frequency (fs) applied to the S/H circuit is equal to, or greater than, the sum of fc2 plus a frequency fA;

where fA is either one half the bandwidth of the QAM signal or one half the bandwidth of the VSB signal.

11. A radio frequency receiver as claimed in claim 7, wherein there is also included means for adjusting the frequency of the second local oscillator;

said means for adjusting including a local oscillator adjust circuit having an input coupled to the output of the Hilbert filter and having an output coupled to the second local oscillator for adjusting its frequency.

12. A radio frequency receiver as claimed in claim 3, wherein the output of the ADC is coupled to a Hilbert filter for producing at a first output a first signal which represents the real portion of the signal and at a second output a second signal which represents the imaginary portion of the signal.

13. A radio frequency (RF) receiver adapted to receive digitally modulated signals including quadrature amplitude modulated (QAM) and vestigial side band (VSB) RF signals, comprising:

an input terminal adapted to receive a digitally modulated RF signal, said input terminal having an output which comprises said RF signal with a frequency band centered at a first center frequency;

a signal conditioning circuit electrically coupled to said input terminal, said signal conditioning circuit receiving said RF signal and producing a first intermediate frequency (IF) signal centered at a first intermediate center frequency;

a sample and hold circuit electrically coupled to said signal conditioning circuit, said sampling and hold circuit producing a second IF signal centered at a second intermediate center frequency;

a filter electrically coupled to said sample and hold circuit, said filter isolating said second IF signal;

an analog to digital (A/D) converter electrically coupled to said filter, said A/D converter producing a digitized IF signal from said second IF signal; and

a demodulator circuit electrically coupled to said A/D converter, said demodulator circuit adapted to demodulate said digitized IF signal, producing a baseband signal.

14. The receiver of claim 13, further comprising:

a first sampling signal at a first frequency applied to a clocking input of said sample and hold circuit; and

a second sampling signal at a second frequency applied to a clocking input of said A/D converter, wherein said first frequency is a multiple of said second frequency.

15. The receiver of claim 13, wherein

said digital IF signal is centered at said second intermediate center frequency;

and

said A/D converter samples said second IF signal at a predetermined sampling frequency, wherein said sampling frequency is a multiple of said second intermediate center frequency.

16. The receiver of claim 13, wherein said demodulation of said digital IF signal further comprises the conversion of said digital IF signal into a baseband signal using a demodulation angle which is a function of e.sup.(-jω

.sbsp.c^nT.sbsp.s.sub.) where ω

_c =2π

f_c, T_s =1/f_s, n is an integer, f_c is said second intermediate center frequency, and f_s is the sampling frequency at which said A/D converter samples said second IF signal, wherein f_s is equal to Kf_c, where K is a rational number, thereby reducing said demodulation to ##EQU1##

17. The receiver of claim 16, wherein said intermediate center frequency is approximately equal to 37.66 Megahertz.

18. The receiver of claim 17, wherein f_c is approximately equal to 2.69 MHz. for VSB signals.

19. The receiver of claim 17, wherein f_c is approximately equal to 5.38 MHz. for QAM signals.

20. The receiver of claim 17, wherein K=4 for QAM signals, said demodulation angle being a function of ##EQU2##

21. The receiver of claim 17, wherein K=8 for VSB signals, said demodulation angle being a function of ##EQU3##

22. A radio frequency (RF) receiver, comprising:

an input terminal adapted to receive a digitally modulated RF signal, said input terminal having an output which comprises said RF signal with a frequency band centered at an initial center frequency;

a signal conditioning circuit electrically coupled to said input terminal, said signal conditioning circuit receiving said RF signal and producing a first intermediate frequency (IF) signal centered at a first intermediate center frequency;

an analog to digital (AID) converter electrically coupled to said signal conditioning circuit, said A/D converter receiving said first IF signal and producing a digitized IF signal centered at a second intermediate center frequency; and

a demodulator circuit electrically coupled to said A/D converter, said demodulator circuit adapted to demodulate said digitized IF signal by converting said digitized IF signal into a baseband signal using a demodulation angle which is a function of e.sup.(-jω

.sbsp.c_nT.sbsp.s.sub.) where ω

_c =2π

f_c, T_s =1/f_s, n is an integer, f_c is said second intermediate center frequency, and f_s is the sampling frequency at which said A/D converter samples said second IF signal, wherein f_s is equal to Kf_c, where K is a rational number, thereby reducing said demodulation angle to ##EQU4##

23. The receiver of claim 22, wherein said first intermediate center frequency is approximately equal to 37.66 Megahertz.

24. The receiver of claim 23, wherein f_c is approximately equal to 2.69 MHz. for VSB signals.

25. The receiver of claim 23, wherein f_c is approximately equal to 5.38 MHz for QAM signals.

26. The receiver of claim 23, wherein K=4 for QAM signals, said demodulation angle being a function of ##EQU5##

27. The receiver of claim 23, wherein K=8 for VSB signals, said demodulation angle being a function of ##EQU6##

28. A down converting circuit for the conversion of an intermediate radio frequency (RF) signal converted from a RF signal input to a second intermediate frequency (IF) signal which is located at a substantially lower center frequency than said intermediate radio frequency, said second intermediate frequency (IF) signal being processed thereafter by a demodulating circuit in a RF receiver, comprising:

a sample and hold circuit with a signal input, a signal output and a clocking input, wherein said intermediate RF signal input is applied to said signal input;

an anti-aliasing low pass filter (AALPF) having an input coupled to the output of said sample and hold circuit and having an output;

an analog to digital (A/D) converter having a signal input coupled to the output of said AALPF, a signal output coupled to said demodulating circuit, and a clocking input; and

a first sampling signal with a frequency f₁ applied to the clocking input of said sample and hold circuit, and, a second sampling signal with a frequency f₂ applied to the clocking input of said A/D converter, where f₁ is a multiple of f₂.