Dc trimming circuit for radio frequency (rf) down-conversion

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 φ1, to generate an output signal φ1 x(t);
• a second differential mixer for receiving said signal φ1 x(t) as an input, and mixing said signal φ1 x(t) with a second mixing signal φ2, to generate an output signal φ1 φ2 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 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.

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 φ1, to generate an output signal φ1 x(t), using a first differential mixer;.
• mixing said signal φ1 x(t) with a second mixing signal φ2, to generate an output signal φ1 φ2 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 A1 can be manipulated to reduce the IM2 and DC offset in the output signal φ1 φ2 x(t), and wherein matching parameters for said mixers can be relaxed.