RF mixers are essential components in electronic circuits, designed to produce multiplicative, additive, or subtractive outputs from two or more input signals. These versatile devices find applications across various fields, but their primary role is in radio frequency (RF) circuits , where they perform critical functions such as signal modulation and demodulation .
In wireless communication systems, modulators and demodulators are indispensable. They enable frequency conversion , allowing a low-frequency signal to be combined with a high-frequency carrier signal to create a modulated signal at a higher frequency—or vice versa, demodulating a high-frequency signal into a lower-frequency one. This process is fundamental to the operation of wireless transmitters and receivers .
Types of RF Mixers and Their Construction
There are multiple ways to construct an RF mixer or modulator, but all designs rely on non-linear devices such as diodes, bipolar junction transistors (BJTs), or field-effect transistors (FETs). Depending on the desired output signals, mixers can be classified into different categories:
- Single-Balanced Mixers/Modulators : These suppress certain signal components, such as the RF signal or local oscillator (LO) signal.
- Double-Balanced Mixers/Modulators : These provide superior suppression of unwanted signal components, making them ideal for high-performance applications.
Common mixer designs include:
Here, we will focus on designing a two-diode balanced RF mixer , a simple yet effective solution for amplitude modulation (AM) applications.
Two-Diode Balanced RF Mixer: Design and Working Principle
The two-diode balanced mixer is a popular choice for RF applications due to its simplicity and effectiveness. Below is a detailed explanation of its design and functionality.
Schematic Diagram
The schematic diagram of a basic two-diode balanced mixer is shown below:
- Two 1N4148 diodes (D1 and D2) are connected end-to-end.
- The carrier signal (Vc) alternately switches these diodes on and off.
- The modulation signal (Vm) —also known as the information signal—enters the mixer via a center-tapped transformer (TR1) .
- The center terminal of the transformer is connected to the carrier signal, which has a higher frequency than the modulation signal.
During each half-cycle of the carrier signal, the sum of the modulating and carrier signals passes through one diode at a time. The output is taken from the junction between the two diodes, resulting in an amplitude-modulated (AM) signal .
LC Tank Circuit for Filtering
To refine the output, an LC tank circuit is used as a bandpass filter. In this example, the LC tank is tuned to resonate at 100 kHz , matching the carrier signal's frequency. This ensures that only the desired frequency components pass through while rejecting out-of-band harmonics.
For improved performance, you can replace the direct output connection with another center-tapped transformer . This alternative design is explained in detail in the guide how two diode single balanced mixer design works.
Animation of the Two-Diode Balanced Mixer
The following animation illustrates how the carrier signal (Vc) and message signal (Vm) flow through the diodes during the positive and negative half-cycles in the balanced diode mixer:
Frequency Spectrum Analysis
To analyze the output signal, you can plot its frequency spectrum . The graph below shows the frequency components of the output modulated signal:
- Message Signal (Vm) : Low-frequency input.
- Carrier Signal (Vc) : High-frequency input.
- Double Sideband AM Signal : Output modulated signal.
This visualization helps identify which signal components pass through the mixer and LC tank filter.
The following shows the message signal, carrier signal and the double sideband AM signal waveform on the virtual oscilloscope.
Advantages of Using an LC Bandpass Filter
While the basic design includes an RF choke , replacing it with an LC tuned bandpass filter enhances filtering efficiency. The LC resonant tank rejects out-of-band frequencies, ensuring a cleaner output signal. For instance, if the carrier frequency is 10 kHz , you can calculate the values of inductor (L1) and capacitor (C1) using the LC Parallel Resonant Circuit Online Calculator. In this way balanced mixer can be designed using two diodes.
Video demonstration
The following video shows animation of how balanced RF mixer/modulator designed with two diodes works.
