A simple switching mixer circuit is given below: RD IF LO 1) Unbalanced and bala

Question

A simple switching mixer circuit is given below: RD IF LO 1) Unbalanced and balanced mixer: a) What is the advantage of a balanced mixer architecture over an unbalanced one? b) Is this circuit a single-balanced or double-balanced mixer? Explain c) What is the advantage of a double-balanced over a single-balanced mixer architecture? 2) What is the role of the transistors M1, and M2 & M3? 3) What is the role of the output resistances Rp? 4) What makes a Gilbert cell (double-balanced active mixer) so attractive?

Explanation / Answer

1) a) Advantages of a balanced mmixer architecture over an unbalanced one are

i)Rejection of spurious responses and intermodulation products.

ii) Better LO to RF, RF to IF and LO to IF isolation.

iii) Rejection of AM noise in the LO.

b) This architecture is a single balanced mixer.

The transistor of RF port always works in saturation. The RF signal is applied at the gate of device of RF port to get it converted to current signal. The DC voltage of LO+ & LO- are set around threshold level. When LO signal is applied LO+ & LO- transistor will turn ON & OFF in turn. While one LO transistor is ON the other is OFF. It require large LO signal to drive complementary switch. Here, RF signal is multiplied by LO signal.

c) Advantages of a double balanced over a single balanced mixer architecture

i) Increased linearity compared to single balanced architecture.

ii) LO & RF are both balanced and hence provide both LO & RF rejection at the IF output instead of just one LO or RF in single balanced.

iii) Improved supression of spurious products than single balanced.

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