Direct conversion receiver

• US 6,192,225 B1
• Filed: 04/22/1998
• Issued: 02/20/2001
• Est. Priority Date: 04/22/1998
• Status: Expired due to Term

• Current Assignee: Unwired Planet LLC (JP Morgan Chase & Co)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Gore, Charles Jr., Arpaia, Domenico
• Primary Examiner(s): Bost, Dwayne D.
• Assistant Examiner(s): GELIN, JEAN ALLAND
• Application Number:
  US09/064,930
• Time in Patent Office: 1,035 Days
• Field of Search: 455/324, 455/303, 455/304, 455/255, 455/256, 455/257, 455/196.1, 455/296, 455/302, 455/313, 455/316, 455/264, 455/265, 455/186.1, 455/314, 455/317, 375/316, 375/328, 375/329, 375/334, 375/335, 375/259, 375/260, 375/261, 375/349, 375/344, 375/346, 375/371
• US Class Current: 455/196.1
• CPC Class Codes: H03D 1/22 Homodyne or synchrodyne circuits receiver circuits H04B1/30
• CPC Class Codes: H03D 1/2245 using two quadrature channels H03D1/2209 takes precedence
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

A homodyne receiver according to the present invention includes a signal input and first and second mixers coupled to the signal input. A local oscillator provides a reference signal at the frequency of modulation of the input signal. The reference signal output by the local oscillator is passed through a switchable phase change element, the output of which is coupled directly to the first mixer and indirectly to the second mixer through a second phase change element. The switchable phase change element changes the phase of an input signal by a predetermined phase based on the state of a control input, such that the output of the phase change element in one state is π radians out of phase with the output of the phase change element in the other state. The second phase change element changes the phase of an input signal by π/2 radians. The outputs of the mixers are coupled to a pair of switchable inverters and low pass filters. Switching of the switchable phase change element and the switchable inverters is controlled by a switching oscillator.

Claims

1. An apparatus for receiving a radio frequency (RF) signal, comprising:

an RF input for receiving a modulated RF signal;

a mixer having first and second inputs and an output, the first input of said mixer electrically coupled to said RF input;

a local oscillator for generating a reference signal;

a switching oscillator for generating a switching signal having a first state and a second state;

a switchable phase change element having a signal input, a control input and an output, wherein said signal input is electrically coupled with said local oscillator, said control input is electrically coupled to said switching oscillator, the output of said phase change element is electrically coupled to the second input of said mixer;

a switchable inverter having a signal input, a control input and an output, the output of said mixer is electrically coupled to the signal input of said inverter and the output of said switching oscillator is coupled to the control input of said inverter; and

a first low pass filter electrically coupled to the output of said first switchable inverter, the first low pass filter having a bandwidth less than a switching frequency of the switchable inverter.

2. An apparatus as recited in claim 1 further comprising:

a second mixer having a first input, a second input and an output, wherein the first input is electrically coupled to said input means;

a fixed phase change element having an input and an output, wherein the input of said fixed phase change element is electrically coupled with the output of said phase change element and the output of said fixed phase change element is electrically coupled with the second input of said second mixer; and

a second switchable inverter having a signal input, a control input and an output, such that the output of said second mixer is electrically coupled to the signal input of said second switchable inverter and the output of said switching oscillator is electrically coupled to the control input of said second switchable inverter.

3. An apparatus as recited in claim 2 wherein said switchable phase change element has a first phase change state and a second phase change state, such that said switchable phase change element shifts the phase of an input signal by a first phase shift in said first phase change state and a second phase shift offset by 180 degrees from said first phase shift in said second phase change state.

4. An apparatus as recited in claim 2, further comprising a second low pass filter electrically coupled to the output of said second switchable inverter, the second low pass filter having a bandwidth less than a switching frequency of the second switchable inverter.

5. An apparatus as recited in claim 2 wherein the frequency of the signal generated by said switching oscillator is variable.

6. An apparatus as recited in claim 2 wherein said first and second switchable inverters comprise double voltage switches.

7. An apparatus as recited in claim 4 further comprising a preselector filter having a lower cutoff frequency and an upper cutoff frequency and electrically coupled to said first and second mixers.

8. An apparatus as recited in claim 7 wherein each of said first and second low pass filters has a corner frequency at least equal to the difference between the upper and lower cutoff frequencies of said preselector filter.

9. An apparatus as recited in claim 7 further comprising an amplifier electrically connected in series between said preselector filter and said first and second mixers.

10. An apparatus as recited in claim 9 further comprising:

a second amplifier electrically connected in series between said first inverter and the first output of the system; and

a third amplifier electrically connected in series between said second inverter and the second output of the system.

11. An apparatus as recited in claim 7 wherein the frequency of the signal generated by said switching oscillator is greater than the bandwidth of said preselector filter.

12. An apparatus as recited in claim 4 further comprising:

a third low pass filter connected in series between said first mixer and said first inverter, and having a group delay equal to time τ

;

a fourth low pass filter connected in series between said second mixer and said second inverter, and having a group delay equal to time τ

; and

a delay element connected in series between said inverters and said switching oscillator such that the switching signal sent to said inverters is delayed by time τ

.

13. An apparatus as recited in claim 5, wherein the frequency of the signal generated by said switching oscillator is altered according to a predetermined pseudorandom pattern.

14. An apparatus as recited in claim 5, wherein the signal generated by said switching oscillator is a square wave.

15. A method of reducing distortion due to second-order non-linearities and local oscillator leakage in a direct conversion receiver comprising:

receiving a radio frequency (RF) signal propagated at a known frequency;

generating a sinusoid with the same frequency as the received RF signal by a local oscillator;

changing the phase of the generated sinusoid between a first phase change amount and a second phase change amount such that the difference between the first phase change amount and the second phase change amount is 180 degrees;

mixing the received RF signal and the phase-changed signal in a mixer;

synchronously inverting the output of the mixer at a switching frequency when the generated sinusoid was changed by the first phase change amount and passing the output of the mixer when the generated sinusoid was changed by the second phase change amount; and

filtering the signal at a filter bandwidth less than the switching frequency.

16. A method of reducing distortion due to second-order non-linearities and local oscillator leakage in a direct conversion receiver comprising:

receiving a radio frequency (RF) signal propagated at a known frequency;

splitting the received RF signal into a first channel and a second channel;

generating a sinusoid with the same frequency as the received RF signal by a local oscillator;

changing the phase of the generated signal between a first phase change amount and a second phase change amount such that the difference between the first phase change amount and the second phase change amount is 180 degrees;

splitting the generated signal into a third and fourth channel;

changing the phase of the signal propagated in the third channel by 90 degrees;

mixing the received signal propagated in the first channel with the signal propagated in the third channel to form a sixth channel;

mixing the received signal propagated in the second channel with the signal propagated in the fourth channel to form a seventh channel;

synchronously inverting the signal propagated in the sixth and seventh channels at a switching frequency when the generated sinusoid was changed by the first phase change amount and passing the signal propagated in the sixth and seventh channels when the generated sinusoid was changed by the second phase change amount; and

filtering the signals propagated in the sixth and seventh channels at a filter bandwidth less than the switching frequency.

17. An apparatus for receiving an RF signal propagated at a known frequency comprising:

an input means for receiving an RF signal;

a first mixer having a first input, a second input and an output, such that the first input is electrically coupled to said input means;

a second mixer having a first input, a second input and an output, such that the first input is electrically coupled to said input means;

a local oscillator tunable to produce an output signal having the same frequency as the desired received signal and directly coupled with said first mixer;

a first phase change element having an input and an output, such that the input is electrically coupled with said local oscillator and the output is electrically coupled to said first mixer, whereby said first phase change element changes the phase of the input signal by +90 degrees;

a second phase change element having an input and an output, such that the input is electrically coupled with said local oscillator and the output is electrically coupled to said second mixer, whereby said second phase change element changes the phase of the input signal by −

90 degrees;

a first low pass filter electrically coupled to output of said first mixer;

a second low pass filter electrically coupled to output of said second mixer; and

a subtractor element having a first input and a second input and one output such that the first input is electrically coupled to the output of said first low pass filter and said input is electrically coupled to output of said second low pass filter, and whereby said subtractor element subtracts the signal received in its second input from the signal received in its first input.

18. An apparatus as recited in claim 17 further comprising:

a third mixer having a first input, a second input and an output, such that the first input is electrically coupled to said input means;

a fourth mixer having a first input, a second input and an output, such that the first input is electrically coupled to said input means and said second input is directly connected to output of said local oscillator;

a third phase change element having an input and an output, such that the input is electrically coupled with said local oscillator and the output is electrically coupled to the second input of said third mixer, whereby said phase change element changes the phase of the input signal by 180 degrees;

a third low pass filter electrically coupled to output of said third mixer;

a fourth low pass filter electrically coupled to output of said fourth mixer; and

a second subtractor element having a first input and a second input and one output such that the first input is electrically coupled to the output of said third low pass filter and second input is electrically coupled to output of said fourth low pass filter, and whereby said second subtractor element subtracts the signal received in its second input from the signal received in its first input.

19. An apparatus for reducing spurious emissions from a receiver comprising:

a preselector configured to receive an input signal at an input frequency;

a switchable phase change element coupled to an output of a local oscillator; and

a switching oscillator having a switching frequency f_S coupled to a control input of said switchable phase change element, the switching frequency f_S being greater than the input frequency.

20. An apparatus as recited in claim 19, further comprising:

means for varying the switching frequency f_S of said switching oscillator.

21. An apparatus as recited in claim 20, wherein said means for varying the switching frequency varies the switching frequency f_S according to a pseudorandom pattern.

22. A method for reducing spurious emissions from a receiver having a local oscillator, comprising:

generating a local oscillator signal;

passing the local oscillator signal through a switchable phase change element; and

periodically changing the phase shift provided by the switchable phase change element.