Analog Signal Chain · Discrete Build

SDR Quadrature Mixer

An HF receiver that takes 8–16 MHz off an antenna and hands a sound card two baseband channels, I and Q. The mixer is not a chip, it is a Gilbert cell built out of individual transistors.

Tools

LTspice · Altium · PyVISA

Board

2-layer, full SMD

Conversion gain

37.5 dB, I and Q

Phase balance

within ±10°

3D render of the two-layer SDR quadrature receiver board, RF input at the left and baseband output at the right

Schematic of the LC input filter, back-to-back diode clamp and MABAES0060 balun with bias resistors — Input filter, diode clamp and balun

Schematic of the local-oscillator balun: the LO arrives on a pin from off board and the transformer splits it into a differential pair biased to 2.5 V — LO balun, off-board LO in on a pin

Schematic of a discrete Gilbert cell mixer built from MMBT3904 transistors with a current-mirror tail — Gilbert cell, 8 transistors per phase

Schematic of two op-amp unity-gain buffers generating the 2.5 V and 1.5 V bias rails from resistor dividers — Buffered 2.5 V and 1.5 V bias rails

Schematic of a TLV9062 difference amplifier with 10k input resistors and 100k feedback — Difference amplifier, gain of ten

Schematic of a two-pole Sallen-Key low-pass filter built around a TLV9062 — Sallen-Key low-pass, 96.5 kHz

RF input band	8 – 16 MHz
Baseband output	0 – 96 kHz, quadrature I/Q
Input subsystem gain	≈ 43 dB peak, 7 – 12 MHz
Conversion gain, I and Q	≈ 37.5 dB, flat to 50 kHz
Amplitude balance	0.4 – 0.65 dB in band
Phase balance	within ±10° of quadrature · spec ±12.5° below 96 kHz
Low-pass corner	96.5 kHz
Mixer	2 × discrete Gilbert cell, 16 × MMBT3904, current-mirror tails

All projects Next: 137 MHz LNA