Method for reducing IM2 noise in a down conversion circuit
The present invention relates generally to communications, and more specifically to a method and apparatus for minimizing DC offset and second-order modulation products (IM2 noise) while demodulating RF signals. The principle of the invention can be applied to differential, down-conversion circuits ( 50 ) consisting of two differential mixers ( 54, 56 ) in series, a follows: a pair of current sources Ia and Ib are used to provide current to positive and negative channels of the first differential mixer ( 54 ). Providing current to the amplifying transistors of the first mixer ( 54 ) reduces the current drawn through the active mixer switches, reducing the noise generated. The current sources 1 a and 1 b are trimmed in a complementary manner where 1 a= I+Delta 1 , and 1 b =Delta 1 . The value of Δ 1 can be determined in a number of manners; for example, it could be established by testing after the circuit has been fabricated, and the value stored on-chip, for future use.
1. A method of signal demodulation for a circuit having a differential transconductance input cell consisting of separate positive and negative channels for receiving positive and negative channels of said input signal x(t) and amplifying said positive and negative channels of said input signal x(t); a first differential mixer for receiving said amplified input signal x(t), and mixing said input signal x(t) with a first mixing signal φ 1 , to generate an output signal φ 1 x(t); a second differential mixer for receiving said signal φ 1 x(t) as an input, and mixing said signal φ 1 x(t) with a second mixing signal φ 2 , to generate an output signal φ 1 φ 2 x(t); a pair of current sources Ia and Ib for providing current to respective ones of said positive and negative channels of said differential transconductance input cell, to reduce the current drawn from said first differential mixer; said current sources Ia and Ib being trimmed in a complementary manner where Ia=I+ΔI, and Ib=I−ΔI; said method comprising the steps of:
injecting a two-tone signal at said input;
measuring IM2 at the baseband output of said circuit;
determining the level of ΔI which minimizes IM2;
recording the level of ΔI which minimizes IM2; and
using said recorded level of ΔI during normal operation of said down-convertor.
2. A computer readable memory medium for storing software code executable to perform the method steps of claim 1 .
3. The method of claim 1 , including operating a means for manipulating ΔI to reduce the IM2 and DC offset in the output signal φ 1 φ 2 x(t), whereby matching parameters for said mixers can be relaxed.
4. The method of claim 1 , including operating a means for setting a level of ΔI.
5. The method of claim 1 , wherein the current sources Ia and lb each include parallel arrays of transistors, and the step of using includes selectively driving the parallel arrays of transistors with a level of ΔI.
6. A method of down-converting a differential input signal x(t) comprising the steps of:
amplifying positive and negative channels of said input signal x(t) using a differential transconductance input cell consisting of separate positive and negative channels;
mixing said amplified input signal x(t) with a first mixing signal φ 1 , to generate an output signal φ 1 x(t), using a first differential mixer;
mixing said signal φ 1 x(t) with a second mixing signal φ 2 , to generate an output signal φ 1 φ 2 x(t), using a second differential mixer; and
providing current to respective ones of said positive and negative channels of said differential transconductance input cell, using a pair of current sources Ia and Ib, reducing the current drawn from said first differential mixer; and trimming said current sources Ia and Ib in a complementary manner where Ia=I+ΔI, and Ib=I−ΔI; wherein ΔI can be manipulated to reduce the IM2 and DC offset in the output signal φ 1 φ 2 x(t), and wherein matching parameters for said mixers can be relaxed.
7. The method of claim 6 , wherein ΔI is determined using a two-tone test, Al being the current level which minimizes IM2 output at baseband.
8. The method of claim 7 , wherein the two-tone test includes injecting a two-tone signal as the input signal x(t);
measuring IM2 of the output signal φ 1 φ 2 x(t);
determining the level of ΔI which minimizes IM2;
recording the level of ΔI which minimizes IM2; and
using said recorded level of ΔI during the step of providing current.
9. The method of claim 6 , wherein the current sources Ia and Ib each include parallel arrays of transistors, and the step of providing current includes selectively driving the parallel arrays of transistors with a level of ΔI.