What are Mixer Spurs?

Mixer Spurs, also known as spurious responses, are unwanted frequency components generated during the frequency conversion process inside an RF mixer. They are an unavoidable result of a mixer's nonlinear behavior and can interfere with desired signals, reduce receiver sensitivity, and cause systems to fail regulatory compliance tests.

Although a mixer is intended to produce only the desired sum or difference of the RF and Local Oscillator (LO) signals, it also generates additional frequencies formed from combinations of harmonics of both signals. These unwanted frequencies are known as mixer spurs.

Understanding where these spurs come from, which ones are important, and how to reduce their impact is an essential part of RF system design.

How are Mixer Spurs Generated?

An ideal mixer produces only two output frequencies:

• The sum of the RF and LO frequencies
• The difference between the RF and LO frequencies

For example, if:

• RF = 2400 MHz
• LO = 2300 MHz

The desired outputs are:

• RF − LO = 100 MHz
• RF + LO = 4700 MHz

Real mixers, however, are nonlinear devices. This nonlinearity causes harmonics of both the RF and LO signals to mix together, creating many additional frequency components.

The frequency of these spurious products is given by:

f_spur = |m × f_RF ± n × f_LO|

where:

• m is the RF harmonic order
• n is the LO harmonic order

In theory, an infinite number of mixing products can be generated, although only the lower-order products are usually significant.

Common Mixer Spur Frequencies

Some of the most common mixing products are shown below.

As the harmonic order increases, the amplitude of the spur generally decreases because higher-order harmonics are weaker. However, under high-power conditions, higher-order spurs can still become significant.

Example of Mixer Spurs

Consider a mixer with:

• RF = 2.45 GHz
• LO = 2.30 GHz

The desired downconverted signal is:

RF − LO = 150 MHz

However, the mixer also generates several unwanted frequencies:

Only the 150 MHz output is desired. Every other frequency is a mixer spur that may need to be filtered or prevented from entering subsequent stages of the receiver or transmitter.

Which Mixer Spurs Matter Most?

Not every spur causes problems. Engineers primarily focus on spurs that meet one or more of the following conditions.

Spurs Inside the IF Band: The most critical spurs are those that fall within the receiver's intermediate frequency (IF) bandwidth. Since they occupy the same frequency range as the desired signal, they cannot be removed using IF filtering and may appear as legitimate signals.

Spurs Inside the Receiver Passband: Even if a spur lies outside the IF frequency, it can still degrade receiver performance if it falls within the bandwidth of later amplifier or filter stages. This can reduce dynamic range or mask weak signals.

Low-Order Spurs: Products such as RF ± LO, 2RF − LO, and 2LO − RF are usually the strongest because they originate from the lowest RF and LO harmonics. These are typically the first products analyzed during receiver design.

Spurs in Regulated Frequency Bands: For transmitters, spurs that fall within licensed or protected frequency bands can violate regulatory emission limits, causing the equipment to fail compliance testing.

Spur Order

Engineers often classify mixer spurs by their order, which is simply the sum of the harmonic coefficients:

Order = m + n

Higher-order spurs are generally much weaker, which is why most spur analyses consider products only up to the 5th, 7th, or 9th order.

How are Mixer Spurs Predicted?

Before building hardware, engineers use mixer spur calculators or spur charts to predict where unwanted mixing products will appear.

A spur calculator typically requires inputs such as:

• RF frequency
• LO frequency
• Maximum spur order
• IF frequency or bandwidth, if the goal is to identify problematic spurs

The calculator then computes every significant mixing product using:

f_spur = |m × f_RF ± n × f_LO|

The results are usually presented as:

• A table of spur frequencies
• A spur chart showing how spurs move as the LO frequency changes
• Warnings when a spur falls inside the IF bandwidth or another critical frequency band

It is important to note that most spur calculators predict where a spur will occur, not how strong it will be. Spur amplitude depends on the mixer's nonlinear characteristics, RF and LO power levels, harmonic content, impedance matching, and circuit design.

What Determines the Amplitude of a Mixer Spur?

The amplitude of a mixer spur depends on several factors, including:

• Mixer nonlinearity: More nonlinear behavior produces stronger spurious products.
• Spur order: Low-order spurs are usually stronger than high-order spurs.
• RF input power: Higher input power can increase spur levels, especially if the mixer is driven close to compression.
• LO drive level: Mixers are designed for a specific LO drive level. Too little or too much LO power can degrade spur performance.
• LO harmonic content: A clean LO signal helps reduce unwanted mixing products.
• Port isolation and impedance matching: Poor isolation or mismatch can increase unwanted leakage and spurious responses.

This is why two mixers operating at the same RF and LO frequencies can produce very different spur levels.

How Can Mixer Spurs Be Reduced?

Although mixer spurs cannot be completely eliminated, their impact can often be minimized through careful system design.

Common techniques include:

• Selecting an LO frequency that moves critical spurs away from the IF band.
• Using high-linearity mixers to reduce the generation of higher-order products.
• Filtering the RF, LO, or IF paths to suppress unwanted frequencies.
• Operating the mixer within its recommended RF and LO drive levels.
• Using a clean LO source with low harmonic content.
• Carefully planning the receiver frequency architecture before hardware development.

In many cases, changing the LO frequency by only a few megahertz is enough to move a troublesome spur outside the receiver bandwidth.

Image Frequencies vs. Mixer Spurs

Image frequencies and mixer spurs are often confused because both create unwanted signals after frequency conversion. However, they originate from different mechanisms.

A mixer spur is an unwanted output frequency generated inside the mixer due to nonlinear mixing between harmonics of the RF and LO signals.

An image frequency is an unwanted input signal that also converts to the desired intermediate frequency. Since both the desired RF signal and the image frequency produce the same IF, the mixer cannot distinguish between them.

Example

Suppose a receiver uses:

• LO = 2.30 GHz
• Desired RF = 2.45 GHz

The desired IF is: 2.45 GHz − 2.30 GHz = 150 MHz

Now consider another signal at 2.15 GHz. This signal also produces: 2.30 GHz − 2.15 GHz = 150 MHz

Although the receiver is tuned to 2.45 GHz, the 2.15 GHz signal appears at exactly the same IF output. This unwanted input is known as the image frequency.

Key Differences Between Image Frequencies and Mixer Spurs

Although both reduce receiver performance, they are solved using different techniques. Image frequencies are rejected before they reach the mixer, while mixer spurs are minimized through careful frequency planning and mixer selection.

Conclusion

Mixer spurs are an unavoidable consequence of frequency conversion, but they do not have to limit RF system performance. By understanding how spurs are generated, identifying the low-order products most likely to cause interference, and using spur analysis during the design stage, engineers can optimize receiver architectures, improve sensitivity, and ensure regulatory compliance.

Whether designing a radar, satellite transceiver, wireless base station, or test instrument, analyzing mixer spurs is an essential step in developing high-performance RF systems.