DC offset-free RF front-end circuits and systems for direct conversion receivers

• US 20050143028A1
• Filed: 12/24/2003
• Published: 06/30/2005
• Est. Priority Date: 12/24/2003
• Status: Active Grant

• Current Assignee: Agency For Science Technology and Research
• Original Assignee: Agency For Science Technology and Research
• Inventors: Wong, Sheng Jau, Zheng, Yuanjin, Tear, Chin Boon
• Granted Patent: US 7,164,901 B2
• Time in Patent Office: Days
• US Class Current: 455/127.100
• CPC Class Codes: H03D 3/008 Compensating DC offsets
• CPC Class Codes: H03G 3/3052 in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver H03G3/32, H03G3/34 take precedence
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A DCR RF front-end removes the DC term resulting from mixer port to port leakage and feeds RF and LO signals into the mixers with different phase combinations. The signal inputs to the mixers result in the intermediate frequency (IF) output of mixers being 180 degrees out of phase and their DC terms in phase. When subtracting the two outputs from each other the IF signals add up while the DC terms cancel giving a DC free output. A DC offset feedback tuning loop detects the DC offset in the IF domain and then feeds back the tuning voltage to current injection mixers. A multiplier cancels the IF signals and amplifies the DC offset and a comparator compares the amplified DC offset with a threshold voltage and controls the gain of current injection mixers. The feedback tuning loop thus tracks time-varying DC offsets in real-time.

Claims

1. A DC offset-free direct conversion receiver RF front end, comprising:

an I/Q splitting network receiving a radio-frequency signal X(t) and producing a first output signal Y_I(t) and a second output signal Y_Q(t), where said Y_Q(t) signal is 90 degrees out of phase from said Y_I(t) signal;

a polyphase filter receiving a local oscillator signal X_LO(t), said polyphase filter having outputs producing output signals with 0 degree, 90 degree, and 180 degree phase differences relative to said local oscillator signal;

mixers receiving said output signals of said I/Q splitting network and said output signals of said polyphase filter, said mixers having outputs producing intermediate-frequency signals with I and Q components; and

combiner and buffer means having inputs and outputs, each of said inputs coupled to the output of one of said mixers, where at one output of said combiner and buffer means the I component intermediate-frequency signals are added, and the I component intermediate-frequency signal DC terms are subtracted, and where at another output of said combiner and buffer means the Q component intermediate-frequency signals are added and the Q component intermediate-frequency signal DC terms are subtracted.

2. The DC offset-free direct conversion receiver RF front end of claim 1, wherein addition of said I and Q component intermediate-frequency signals enhances said signals at said outputs of said combiner and buffer.

3. The DC offset-free direct conversion receiver RF front end of claim 1, wherein subtraction of said I and Q component intermediate-frequency signal DC terms cancels said DC terms at said outputs of said combiner and buffer.

4. The DC offset-free direct conversion receiver RF front end of claim 1, wherein the output intermediate-frequency signals of said mixers are 180 degrees out of phase and where the DC terms of the intermediate-frequency signals are in phase.

5. A DC offset-free direct conversion receiver RF front end, comprising:

an I/Q splitting network receiving a radio-frequency signal X(t) and producing a first output signal Y_I(t) and a second output signal Y_Q(t), where said first output signal Y_I (t) and said second output signal Y_Q(t) are the I and Q components of said signal X(t), respectively, where said Q component is 90 degrees out of phase relative to said I component;

a polyphase filter receiving a local oscillator signal X_LO(t), said polyphase filter having outputs producing a first output signal with a 90 degree phase difference, a second output signal with a 0 degree phase difference, a third output signal with a 180 degree phase difference, and a fourth output signal with a 90 degree phase difference relative to said local oscillator signal;

a first mixer receiving said first output signal Y_I(t) and said first polyphase filter output signal, the output of said first mixer producing an I component intermediate-frequency signal;

a second mixer receiving said first output signal Y_I(t) and said third polyphase filter output signal, the output of said second mixer producing an I component intermediate-frequency signal;

a third mixer receiving said second output signal Y_Q(t) and said second polyphase filter output signal, the output of said third mixer producing a Q component intermediate-frequency signal;

a fourth mixer receiving said second output signal Y_Q(t) and said fourth polyphase filter output signal, the output of said fourth mixer producing a Q component intermediate-frequency signal;

a first combiner and buffer means having a first and a second input and an output, said first input coupled to the output said first mixer, said second input coupled to the output of said third mixer, where at said output the I component intermediate-frequency signals add up and the I component intermediate-frequency signal DC terms are subtracted; and

a second combiner and buffer means having a first and a second input and an output, said first input coupled to the output said second mixer, said second input coupled to the output of said fourth mixer, where at said output the Q component intermediate-frequency signals add up and the Q component intermediate-frequency signal DC terms are subtracted.

6. The DC offset-free direct conversion receiver RF front end of claim 5, wherein said first, second, third, and fourth mixers are fully symmetrical mixers.

7. The DC offset-free direct conversion receiver RF front end of claim 5, wherein the outputs of said first and said second combiner and buffer means produce the difference between signals applied to said first and said second input.

8. The DC offset-free direct conversion receiver RF front end of claim 5, wherein said I and Q component intermediate-frequency signals are enhanced.

9. The DC offset-free direct conversion receiver RF front end of claim 5, wherein said I and Q component intermediate-frequency signal DC terms are canceled.

10. The DC offset-free direct conversion receiver RF front end of claim 5, wherein the output intermediate-frequency signals of said mixers are 180 degrees out of phase and where the DC terms of the intermediate-frequency signals are in phase.

11. A front-end DC offset feedback tuning loop to detect the DC offsets of intermediate-frequency signals having I and Q components and to cancel said DC offsets by feeding back a tuning voltage to current injection mixers, comprising:

a multiplier coupled to combiner and buffer means, said multiplier sensing said DC offsets at its inputs and producing at its output an amplified DC offset;

a comparator having a first input in communication with the output of said multiplier and a second input coupled to a threshold voltage, said comparator comparing the signal applied at its inputs and providing said tuning voltage at said comparator output; and

said current injection mixers canceling said DC offsets by receiving RF signals, local oscillator signals, and said tuning voltage from said comparator, where said tuning voltage changes the conversion gain of said current injection mixers, the outputs of said current injection mixers coupled to the inputs of said combiner and buffer means.

12. The front-end DC offset feedback tuning loop of claim 11, wherein said intermediate-frequency signals with I and Q components having a 90 degree phase difference will be canceled at said output of said multiplier.

13. The front-end DC offset feedback tuning loop of claim 11, wherein said DC offset sensed by said multiplier is a time-varying DC offset.

14. The front-end DC offset feedback tuning loop of claim 11, wherein means are provided to determine the tuning sensitivity and convergence of said feedback tuning loop.

15. The front-end DC offset feedback tuning loop of claim 11, wherein said conversion gain of each of said current injection mixers changes linearly with said tuning voltage.

16. A front-end DC offset feedback tuning loop to detect the DC offsets of an intermediate-frequency signal with an I component and an intermediate-frequency signal with a Q component and to cancel said DC offsets by feeding back a tuning voltage, comprising:

a multiplier having a first and a second input and outputs, said first input receiving said intermediate-frequency signal with an I component, said second input receiving said intermediate-frequency signal with an Q component, said multiplier sensing the time-varying DC offset at said first and second input and producing at said outputs amplified time-varying DC offsets;

an adaptor having inputs and an output, said inputs coupled to said outputs of said multiplier, said adaptor providing a low frequency voltage gain ranging from 80 to 120 dB and converting said input signals to a single ended output signal;

a limiter having an input and an output, said input receiving said single ended output signal of said adaptor, said limiter limiting at its output the magnitude of its input signal;

a comparator having a first and a second input and an output, said first input coupled to the output of said limiter, said second input coupled to a threshold voltage, said comparator comparing the signal applied at said first input to the threshold voltage applied to said second input and providing a tuning voltage at said comparator output; and

current injection mixers, each having a first, a second and a third input, and an output, said first input receiving an RF signal, said second input receiving a local oscillator signal and said third input receiving said tuning voltage of said comparator, where said tuning voltage changes the conversion gain of each of said current injection mixers thus canceling said DC offset, where each of said outputs of said current mixer injectors is coupled to the input of a combiner and buffer.

17. The front-end DC offset feedback tuning loop of claim 16, wherein the multiplied value of said intermediate-frequency signal with an I component and a Q component and having a 90 degree phase difference, when applied to said multiplier will be canceled at said output of said multiplier.

18. The front-end DC offset feedback tuning loop of claim 16, wherein said adaptor determines the tuning sensitivity and convergence of said feedback tuning loop.

19. The front-end DC offset feedback tuning loop of claim 16, wherein said conversion gain of each of said current injection mixers changes linearly with said tuning voltage.

20. The front-end DC offset feedback tuning loop of claim 16, wherein said DC offset is tracked and canceled in real time.

21. A DC offset-free RF front-end circuit, comprising:

an RF front-end system, further comprising;

a low noise amplifier to amplify a received RF signal;

a first polyphase filter array coupled to said low noise amplifier to generate I and Q signals;

a second polyphase filter array to generate LO signals with phase offsets;

a first set of mixers in the I path to mix I signals with a first group of said LO signals;

a second set of mixers, having variable gain, in the Q path to mix Q signals with a second group of said LO signals;

a first combiner to combine signals coming from a first mixer of each of said first and second set of mixers, thus combining I and Q signals;

a second combiner to combine signals coming from a second mixer of each of said first and second set of mixers, thus combining I and Q signals;

a DC offset tuning feedback tuning loop system, further comprising;

an analog multiplier coupled to said first and second combiner, to form a detection signal generating a DC offset signal;

a comparator to compare said DC offset signal against a threshold voltage, to generate a current tuning signal; and

where said second set of mixers with variable gain are Gilbert cell mixers coupled to said current tuning signal.

22. The DC offset-free RF front-end circuit of claim 21, wherein said detection signal identifies the strength of said DC offset.

23. The DC offset-free RF front-end circuit of claim 21, wherein an adaptor is coupled between said multiplier and said comparator to provide tuning sensitivity and convergence for said feedback tuning loop.

24. The DC offset-free RF front-end circuit of claim 21, wherein said Gilbert cell mixers eliminate said DC offset.

25. A DC offset-free RF front-end circuit, comprising:

a DC offset-free direct conversion receiver RF front end, further comprising;

an I/Q splitting network receiving a radio-frequency signal X(t) splitting said signal X(t) into output signals X_I(t) and X_Q(t);

a polyphase filter receiving a local oscillator signal X_LO(t), having outputs producing output signals having phase differences relative to said local oscillator signal;

a first set of mixers coupled to said output signal X_I(t) and said polyphase filter, said mixers having outputs producing intermediate-frequency signals with an I component;

a second set of mixers coupled to said output signal X_Q(t) and said polyphase filter, said mixers having outputs producing intermediate-frequency signals with a Q component, said second set of mixers further coupled to the output of a comparator;

a combiner and buffer coupled to the outputs of said first and second set of mixers, where a first output of said combiner and buffer produces a DC-free I component intermediate-frequency signal and where a second output of said combiner and buffer produces a DC-free Q component intermediate-frequency signal;

a feedback tuning loop to cancel a DC offset of said I and Q component intermediate-frequency signals, said feedback tuning loop further comprising;

a multiplier coupled to said outputs of said combiner and buffer to amplify said DC offset;

an adaptor/limiter coupled to the output of said multiplier to limit and further amplify said DC offset;

said comparator having a first and a second input, said first input coupled to the output of said adaptor/limiter;

to compare said first input against said second input, said comparator producing a tuning voltage at its output; and

said second set of mixers receiving said tuning voltage from said comparator.

26. The DC offset-free RF front-end circuit of claim 25, wherein said I/Q splitting network receives a radio-frequency signal X(t) and producing a first output signal Y_I(t) and a second output signal Y_Q(t), where said Y_Q(t) signal is 90 degrees out of phase from said Y_I(t) signal.

27. The DC offset-free RF front-end circuit of claim 25, wherein said polyphase filter receives a local oscillator signal X_LO(t), said polyphase filter having outputs producing output signals with 0 degree, 90 degree, and 180 degree phase differences relative to said local oscillator signal.

28. The DC offset-free RF front-end circuit of claim 25, wherein said first set of mixers produces intermediate-frequency signals with I and Q components.

29. The DC offset-free RF front-end circuit of claim 25, wherein said second set of mixers produces intermediate-frequency signals with I and Q components, where said tuning voltage changes the conversion gain of said second set of mixers.

30. The DC offset-free RF front-end circuit of claim 25, wherein at said first output of said combiner and buffer the I component intermediate-frequency signals are added, and the I component intermediate-frequency signal DC terms are subtracted.

31. The DC offset-free RF front-end circuit of claim 25, wherein at said second output of said combiner and buffer the Q component intermediate-frequency signals are added and the Q component intermediate-frequency signal DC terms are subtracted.

32. The DC offset-free RF front-end circuit of claim 25, wherein said multiplier is coupled to said combiner and buffer, said multiplier sensing the DC offset at its inputs and producing at its output an amplified DC offset;

33. The DC offset-free RF front-end circuit of claim 25, wherein said adaptor/limiter is coupled to the output of said multiplier, said adaptor/limiter further amplifying the output signal of said multiplier and providing a signal for tuning the gain of current injection mixers;

34. The DC offset-free RF front-end circuit of claim 25, wherein said second input of said comparator is coupled to a threshold voltage.

35. The method of providing a DC offset-free DCR RF front-end system, comprising the steps of:

1) splitting an RF signal into I and Q components;

2) creating LO signals with 0 degree, 90 degree, and 180 degree phases;

3) mixing the RF and LO signals in a first and second set of mixers and creating at the output of each set of mixers IF signals which are 180 degrees out of phase and where the DC terms of the IF signals are in phase;

4) combining the output of the first set of mixers in a combiner and producing an IF signal with an I component such that the IF signals are added up and the DC terms are subtracted; and

5) combining the output of the second set of mixers in a combiner and producing an IF signal with a Q component such that the IF signals are added up and the DC terms are subtracted.

36. The method of providing a DC offset-free DCR RF front-end system of claim 35, wherein said first and second set of mixers are fully symmetrical mixers.

37. The method of providing a DC offset-free DCR RF front-end system of claim 35, wherein subtraction of said DC terms at the outputs of said combiners cancels said DC terms.

38. The method of providing a DC offset-free DCR RF front-end system of claim 35, wherein said IF signal with a Q component has a 90 degree offset from said IF signal with an I component.

39. The method of providing a DC offset feedback tuning loop, comprising the steps of:

1) multiplying in a multiplier an IF signal with an I component and an IF signal with a Q component where the IF signals have a DC offset;

2) canceling in the multiplier the IF signals and detecting and amplifying the DC offsets in the IF domain and providing an output signal;

3) comparing the output signal in a comparator against a threshold voltage and generating a tuning voltage;

4) feeding back the tuning voltage to the current injection mixers, thereby controlling their gain in the RF domain;

5) applying the output of each mixer to its associated combiner; and

6) feeding back the output of the combiners to the multiplier in step 1) to complete the feedback loop.

40. The method of providing a DC offset feedback tuning loop of claim 39, wherein said IF signal with a Q component has a 90 degree offset from said IF signal with an I component.

41. The method of providing a DC offset feedback tuning loop of claim 39, wherein said mixers are fully symmetrical mixers.

42. The method of providing a DC offset feedback tuning loop of claim 39, wherein said mixers receive a LO signal with a phase of 0 degrees and 90 degrees, respectively, and a RF signal with a Q component.