TECHCENTER

At the heart of every superheterodyne RF receiver, transmitter, or transceiver system is an RF/microwave mixer — the essential element for converting baseband signals into RF (in a transmitter) or RF into baseband signals (in a receiver).

All mixers have three ports:

IF: Intermediate frequency port (also called the baseband port)

LO: Local oscillator port (also called the carrier port)

RF: Radio frequency port

When used in a receiver, signals are inputted into a mixer’s LO and RF ports to produce an output at the IF port in a mixing process called downconversion.

Conversely, when used in a transmitter, signals are inputted into a mixer’s LO and IF ports to produce an output at the RF port in a mixing process called upconversion.

Any passive mixer can be used in either fashion. In both cases, the two input signals are being “mixed” to create two new signals at the output port: one at the sum frequency of the inputs (i.e. LO + (RF or IF)) and one at the difference frequency of the inputs (i.e. LO − (RF or IF)). In most applications, only one of these two products is desired, and the other will need to be suppressed.

A basic block diagram for an RF mixer as well as idealized downconversion and upconversion examples are shown in Figure 1:

Figure 1: Simplified schematic of an RF mixer and idealized downconversion and upconversion examples

Mixers started as the bread and butter of Mini-Circuits back when our founder, Harvey Kaylie, created the blockbuster SRA-1, the product represented in our logo and which we still produce to this day. Over the decades, Mini-Circuits has leveraged its mixer expertise and grown to offer hundreds of unique mixer models over six different circuit topologies. This variety gives designers options to suit just about every application requirement, even up to millimeter wave! With all these options, though, understanding the differences between all these mixer designs can complicate the component selection process — and that’s where this article comes in.

Today, we’ll provide a broad overview of the different mixer topologies, including both balanced and unbalanced architectures. It should be noted that, in theory, any nonlinear device can be used to make a mixer, but Schottky diodes and field effect transistors (FETs) are the most common. Mini-Circuits designs both diode- and FET-based mixers, but the topologies here will be presented using diode mixers for simplicity. However, the same principles can be applied to other technologies as well.

Unbalanced (Single Diode) Mixers

A single diode, or unbalanced, mixer is the simplest and oldest mixer topology. A single diode mixer is fundamentally a two-port device, with the RF and LO combined and fed into the diode, and the IF delivered on the other side of the diode. The schematic and time domain response of this topology is shown in Figure 2.

Figure 2: Unbalanced / single-diode mixer schematic and time domain response.

One of the limitations of the unbalanced mixer is that in addition to the desired IF frequency (sum or difference), the output frequency spectrum also includes RF and LO signal content, and therefore requires a narrowband IF filter to reject the RF and LO frequency components of the output signal. The output RLC tank in Figure 2 is tuned to match the IF frequency. This means the single device mixer has a rather narrow IF bandwidth because it has no port isolation. Single diode mixers are used in economical receiver front-ends, and bandpass filters can be used at the input and output to separate the LO, RF, and IF signals. They can, however, be problematic if the RF and LO frequencies overlap and the filtering requirement becomes too difficult.

Advantages and Disadvantages of the Unbalanced Diode Mixer

Advantages

Very useful in millimeter wave band

Economical

Lowest LO power requirement

Disadvantages

No isolation

Filtering results in narrow operation band

No rejection of LO AM noise or intermodulation products