Down conversion methodology and topology which compensates for spurious response
There is a need for an inexpensive, high-performance, fully-integrable, multistandard transceiver, which suppresses spurious noise signals. The invention provides a topology that satisfies this need, providing a first mixer for receiving an input signal x(t), and mixing it with a multi-tonal mixing signal φ 1 to generate an output signal φ 1 x(t), and providing a second mixer for receiving the φ 1 x(t) signal, and mixing it with a mono-tonal mixing signal φ 2, to generate an output signal φ 1 φ2 x(t). The two mixing signals emulate an LO signal because φ 1*φ2 has significant power at the frequency of the LO signal being emulated. The topology also includes a power measurement circuit for measuring the power of the output signal φ 1 φ2 x(t). This power output signal is used to vary the characteristics of the mono-tonal mixing signal φ 2 to reduce the power level of said output signal.
1 . A demodulator circuit for emulating the down conversion of an input signal x(t) with a local oscillator (LO) signal, said demodulator circuit comprising:
a first mixer for receiving said input signal x(t), and mixing said input signal x(t) with a multi-tonal mixing signal φ 1 , to generate an output signal φ 1 x(t);
a second mixer for receiving said signal φ 1 x(t) as an input, and mixing said signal φ 1 x(t) with a mono-tonal mixing signal φ 2 , to generate an output signal φ 1 φ 2 x(t);
a first signal generator for generating said multi-tonal mixing signal φ 1 ;
a second signal generator for generating said mono-tonal mixing signal φ 2 , where φ 1 *φ 2 has significant power at the frequency of said local oscillator signal being emulated; and
a power measurement circuit for measuring the power of said output signal φ 1 φ 2 x(t);
said second signal generator receiving a power level signal output from said power measurement circuit, and varying the characteristics of said mono-tonal mixing signal φ 2 to reduce the power level of said output signal φ 1 φ 2 x(t).
2 . The circuit of claim 1 wherein said second signal generator varies the frequency of said φ 2 signal.
3 . The circuit of claim 2 wherein said second signal generator comprises a voltage controlled oscillator (VCO).
4 . The circuit of claim 3 wherein said second signal generator comprises:
a means for detecting changes in output power over time; and
a frequency control circuit which directs said VCO to incrementally adjust the frequency of said φ 2 signal in response to changes in output power over time.
5 . The circuit of claim 4 wherein said frequency control circuit responds to a failing trend in said power level over time by directing said VCO to continue adjusting the frequency of said φ 2 signal in the same manner that it has been.
6 . The circuit of claim 4 wherein said frequency control circuit responds to a rising trend in said power level over time by directing said VCO to invert the sense of the incremental adjustments being made to the frequency of said φ 2 signal.
7 . The circuit of claim 4 wherein said frequency control circuit further comprises means for smoothing changes in values of said output power, improving stability.
8 . The circuit of claim 4 wherein said means for detecting changes in output power comprises:
a power measurement device with digital output;
a time delay device for receiving said digital output from said power measurement device and delaying said digital output; and
a comparator for comparing a current digital output to a delayed digital output, thereby determining whether power level is rising or falling over time.
9 . The circuit of claim 5 further comprising a means for setting initial conditions of said frequency control circuit.
10 . The circuit of claim 5 further comprising a clock which establishes timing for sampling and processing of output power signals for said frequency control circuit.
11 . The circuit of claim 2 wherein neither of said φ 1 nor said φ 2 signals have significant power at the carrier frequency of said input signal x(t).
12 . The circuit of claim 11 wherein neither of said φ 1 nor said φ 2 signals have significant power at the carrier frequency of said LO signal being emulated.
13 . The circuit of claim 1 wherein said first signal generator comprises a signal generator for generating square wave signals.
14 . The circuit of claim 1 wherein said second signal generator comprises a signal generator for generating square wave signals.
15 . The circuit of claim 1 wherein unwanted power at baseband is minimized by adjusting the frequency of said φ 2 signal such that unwanted RF tones do not fall within the frequency range of the desired signal at baseband.
16 . The circuit of claim 1 wherein unwanted power at baseband is minimized by adjusting the frequency of said φ 2 signal so that the probability of unwanted RF tones falling within the frequency range of φ 1 *φ 2 x(t) is significantly reduced.
17 . The circuit of claim 1 wherein said second signal generator varies the phase of said φ 2 signal.
18 . The circuit of claim 1 wherein said second signal generator is responsive to noise in said output signal φ 1 φ 2 x(t) by adjusting the frequency of φ 2 .
19 . The circuit of claim 3 wherein said first mixer comprises an active mixer.
20 . The circuit of claim 19 wherein said first mixer comprises an active mixer having adjustable performance.
21 . The circuit of claim 19 further comprising a high pass filter electrically connected between said first mixer and said second mixer.
22 . The circuit of claim 21 wherein said second mixer comprises a passive mixer.
23 . The circuit of claim 22 , wherein each of said active mixer, said high pass filter and said passive mixer is a differential device.
24 . A method of emulating the demodulation of an input signal x(t) to the product of said input signal with a local oscillator (LO) signal, said method comprising the steps of:
generating a multi-tonal mixing signal φ 1 ;
generating a mono-tonal mixing signal φ 2 , where φ 1 *φ 2 has significant power at the frequency of the local oscillator signal being emulated, and neither of said φ 1 nor said φ 2 having significant power at the frequency of said input signal x(t), said LO signal being emulated, or an output signal φ 1 φ 2 x(t);
mixing said input signal x(t) with said multi-tonal mixing signal φ 1 , to generate an output signal φ 1 x(t);
mixing said signal φ 1 x(t) with said mono-tonal mixing signal φ 2 , to generate said output signal φ 1 φ 2 x(t);
measuring the power of said output signal φ 1 φ 2 x(t); and
adjusting the characteristics of said mono-tonal mixing signal φ 2 to minimize the power of said output signal φ 1 φ 2 x(t).
25 . A computer readable memory medium for storing software code executable to perform the method steps of claim 24 .
26 . (canceled)