Band-Edge Emissions Testing: How the Right Preamplifier Can Reduce Test Time and Improve Measurement Confidence

Band-Edge Emissions Testing: How the Right Preamplifier Can Reduce Test Time and Improve Measurement Confidence

Introduction

Wireless products continue to expand into more crowded RF environments. Devices using Bluetooth, Wi-Fi, LTE, 5G NR, UWB, and other wireless technologies must demonstrate that they operate within their assigned frequency bands and do not create excessive interference outside those bands. These requirements are addressed through intentional radiator testing, including band-edge emissions measurements. FCC Part 15 regulations include requirements for controlling emissions outside authorized operating bands, with limits defined relative to the transmitter and applicable emission limits.

For EMC laboratories, band-edge testing presents a unique challenge. The measurement system must detect extremely low-level unwanted emissions while simultaneously handling the much stronger fundamental transmitter signal. A poorly selected preamplifier can become the limiting factor, creating distortion, requiring additional filters, increasing setup time, and potentially producing inaccurate results.

The correct high-dynamic-range preamplifier can simplify the measurement process by reducing overload concerns, minimizing filter changes, and improving confidence in the final measurement data.

What Is Band-Edge Emissions Testing?

A wireless transmitter is designed to operate within a specific frequency allocation.

For example:

2.4 GHz Wi-Fi / Bluetooth

The transmitter energy should remain inside this allocated band.

However, real transmitters produce unwanted emissions caused by:

• Modulation
• Power amplifier nonlinearity
• Harmonics
• Clock frequencies
• Digital processing
• Filtering limitations

These unwanted signals may appear immediately outside the allowed operating range.

Example:

The purpose of band-edge testing is to verify that these emissions are sufficiently attenuated.

Why Band-Edge Measurements Are Difficult

A typical radiated emissions measurement might involve detecting signals close to the receiver noise floor.

A wireless band-edge measurement is different.

The system may need to measure:

Strong transmitter signal

Example:

+20 dBm equivalent signal level

while also measuring:

Weak band-edge emission

Example:

-50 dBm

The measurement system must handle:

70 dB of signal difference.

This creates a challenge for every component in the RF path:

• Antenna
• Cable
• Filters
• Preamplifier
• Spectrum analyzer

The Preamplifier Is Often the Limiting Component

A common EMC measurement approach is:

The purpose of the preamplifier is to:

• Reduce the effective system noise floor
• Improve receiver sensitivity
• Compensate for cable loss

Preamplifiers are extremely valuable for radiated emissions testing because they amplify weak signals before they are degraded by receiver noise and cable losses.

However, gain alone is not enough.

A preamplifier must also maintain linear operation when strong RF signals are present.

The Problem With Traditional High-Gain Preamplifiers

Many preamplifiers are optimized primarily for:

• Low noise figure
• High gain
• Improved sensitivity

These are important specifications, but they do not tell the entire story.

The missing specification is:

Output 1 dB Compression Point (P1dB)

P1dB defines the output power level where the amplifier begins to leave its linear operating range.

When a preamplifier compresses:

• Gain is reduced
• Harmonics may be generated
• Intermodulation products may appear
• Sidebands can be distorted
• False emissions may be created

The result:

The measurement system may report emissions that are not actually produced by the device under test.

Why Notch Filters Are Commonly Used

A traditional solution is adding a notch filter before the preamplifier.

Example:

Without Filter

With Notch Filter

The filter removes the strong transmitter frequency while allowing the measurement system to examine the weaker emissions near the band edge.

However, filters introduce additional challenges:

• Additional insertion loss
• Additional calibration data
• More cables and connectors
• More setup complexity
• Filter selection for each wireless technology
• Additional measurement uncertainty

For laboratories performing many wireless tests, constantly changing filters can significantly increase test time.

The Advantage of a High-Dynamic-Range Preamplifier

A high-performance EMC preamplifier changes the measurement approach.

Instead of immediately filtering the transmitter, the system can often maintain linear operation without additional filtering.

The ideal measurement chain becomes:

Benefits:

• Faster measurements
• Fewer setup changes
• Reduced filter dependency
• Better signal integrity
• Improved repeatability

EMC118A45SE: Designed for Modern Wireless EMC Testing

The EMC118A45SE 1–18 GHz preamplifier was developed specifically for demanding EMC measurement applications where sensitivity and dynamic range are both important.

Key features:

1. Wide Frequency Coverage

One preamplifier covers:

• Bluetooth
• 2.4 GHz Wi-Fi
• 5 GHz Wi-Fi
• Wi-Fi 6E
• LTE
• 5G NR
• Other intentional radiator applications

Frequency range:

1 GHz to 18 GHz

This eliminates the need for multiple narrow-band amplifiers in many laboratories.

2. High Dynamic Range

The major advantage is not simply gain.

The EMC118A45SE provides:

• High gain
• High output compression capability
• Improved tolerance to strong RF signals

This is especially important for:

• Band-edge measurements
• Radiated spurious emissions
• Wireless certification testing

3. Reduced Dependence on Notch Filters

With a conventional lower-linearity amplifier, filters may be required to protect the input stage.

With a higher dynamic range amplifier:

• Some measurements can be performed without filters
• Fewer filter changes are required
• The RF path remains simpler

4. Better Preservation of the Actual Spectrum

A linear measurement system should reproduce the transmitter spectrum accurately.

A compressed amplifier can create:

• False harmonics
• Artificial spurs
• Distorted sidebands

Maintaining amplifier linearity helps ensure the measurement represents the actual EUT performance.

The Hidden Cost of the Wrong Preamplifier

A lower-cost preamplifier may appear attractive initially.

However, the laboratory may spend more time:

• Troubleshooting unexplained emissions
• Adding notch filters
• Repeating measurements
• Changing RF configurations
• Validating questionable results

The real cost is not the amplifier purchase price.

The real cost is:

operator time, test delays, and measurement uncertainty.

Selecting the Right Preamplifier for Band-Edge Testing

When evaluating a preamplifier, consider:

Conclusion

Modern EMC laboratories are increasingly performing measurements on products that intentionally transmit RF energy. These measurements require more than simply lowering the noise floor.

The best measurement system must:

• Detect weak emissions
• Handle strong transmitters
• Maintain linear operation
• Minimize setup changes

A high-dynamic-range preamplifier such as the EMC118A45SE allows EMC laboratories to perform challenging band-edge measurements faster and with greater confidence.

The correct preamplifier does not just improve sensitivity.

It improves the entire measurement process.