How to build an Analog Mixer - Additive Mixer and Multiplicative Mixer

An analog mixer combines multiple input signals into a single output, and this project explores two fundamental mathematical operations: addition and multiplication. The additive mixer uses a network of resistors, specifically utilizing two 10K resistors and four 3.3K resistors, to sum incoming voltage signals together into a single node. This technique is commonly found in passive or active summing circuits where signals are blended linearly without altering their fundamental frequency components. In contrast, the multiplicative mixer leverages six NPN transistors (such as the 2N2222 or BC547) configured to achieve signal modulation. By biasing these transistors into their active regions, the circuit multiplies two input signals together, which is the core mechanism behind frequency mixing, amplitude modulation, and demodulation. To visualize and verify how these circuits manipulate waveforms, builders use a two-channel signal source to feed inputs into the system and an oscilloscope to observe the combined or modulated output waveforms in real time. Additionally, an Arduino board can be integrated into the setup to provide control voltages, reference signals, or digital monitoring, while the Arduino IDE is used for writing any necessary sketches to support the testing process.