The goal of this project is to design a Gilbert cell down-conversion mixer that can be used in wireless receivers to down-convert the RF signal to a lower frequency in order to facilitate filtering and further processing in analog and digital domains.
The mixer is a 3-port linear time-varying circuit that ideally multiplies the input signal by the carrier in the time domain, which corresponds to convolution in the frequency domain, which means that the signal is now centered around the sum and difference frequencies (up and down conversion components).
The ratio of voltage (power) at output frequency to voltage (power) at input frequency. Active mixers have CG>1, while passive mixers have CG<1.
- Noise Figure:
NF = SNR(input)/SNR(output). It indicates the amount of noise added by a circuit. DSB NF of a noiseless mixer is 0 dB (Direct Conversion Receivers), while SSB NF of a noiseless mixer is 3 dB due to noise in the image band (Heterodyne Receivers).
- Feedthrough:
Mixers suffer from unwanted coupling (feedthrough) from one port to another which leads to several undesirable effects.
LO-to-RF leakage, which will mix with LO again, causes a self-mixing (DC Offset) problem in direct conversion. Due to the nonzero reverse gain of LNA, the LO leakage may even reach the antenna through the LNA.
LO-to-IF leakage may cause saturation of the blocks following the mixer due to the large amplitude of LO.
RF-to-LO leakage allows interferers and spurs present in the RF signal to interact with the LO which causes frequency pulling.
A double-balanced Gilbert cell mixer is proposed to achieve good port-to-port isolation.
- Linearity:
The linearity of a circuit can be characterized by IP3 for Heterodyne Receivers or IP2 for Direct Conversion Receivers because IM2 leads to a DC component that falls on top of our signal. Larger IP3 or IP2 indicates that our circuit can tolerate larger signals while still having acceptable linear performance.
Notes: 1- The specs can be changed if the circuit was integrated with a system that requires different specs and the specs are meant to be achieved at 915 MHz frequency which can also be changed.
2- Linearity requirement is shown as an IP3 spec assuming we have a heterodyne receiver and that can be changed if it's used in a direct conversion receiver.
| Gain | 10 dB |
| Noise Figure | 12 dB |
| IP3 | 2.5 dB |
| Current Consumption | 2.5 mA |
- Q3 operates as a tail current source to bias the circuit and improves the CMRR. It can be removed or replaced with an inductor for low supply voltages.
- Q1,2 convert RF voltage to current (transconductance stage).
- Q4-7 operate as switches to multiply the RF signal by a square wave where it's fundamental frequency is the required LO frequency.
- R1,2 are equal and act as the load to convert the current back to voltage.
[1] B. Gilbert, "A precise four-quadrant multiplier with subnanosecond response," in IEEE Journal of Solid-State Circuits, vol. 3, no. 4, pp. 365-373, Dec. 1968, doi: 10.1109/JSSC.1968.1049925.
[2] RF microelectronics, 2nd Edition, Behzad Razavi. Pearson Education, Inc., 2012.
Design of Gilbert cell down-conversion mixer that can be used in wireless receivers to down-convert the RF signal.