ACLR Measurement
Understanding ACLR Measurement
If AT&T spends $5 Billion to buy a block of 5G spectrum, and Verizon owns the block right next to it, AT&T's cell towers are legally forbidden from transmitting a single watt of power into Verizon's block. To legally prove they are obeying the law, AT&T's hardware must pass a strict ACLR Measurement.
The Bleeding Skirts
In a perfect mathematical world, a 5G radio signal looks like a perfect, sharp rectangle on a Spectrum Analyzer. It stays exactly inside its lane.
In the real world, the silicon transistors inside the massive Power Amplifier naturally distort the signal. The sharp edges of the rectangle sag outward, creating "skirts" that violently bleed into the left and right adjacent channels. This is called Spectral Regrowth.
The Ratio Calculation
To measure the ACLR, an engineer connects a $100,000 Vector Signal Analyzer (VSA) to the amplifier.
- The VSA mathematically measures the total raw power sitting perfectly inside the "Main Channel" (e.g., 100 Watts).
- The VSA then measures the total rogue power that has bled into the "Adjacent Channel" (e.g., 0.001 Watts).
- The software calculates the strict mathematical ratio between the two, displaying it in decibels relative to the carrier (dBc). An ACLR of -50 dBc means the noise bleeding into the neighbor's channel is 100,000 times weaker than the main signal, legally passing the FCC test.
Key Equations
ACLR Measurement (Adjacent Channel Leakage Ratio) is an absolutely foundational, legally mandated RF test metric used to quantify the spectral purity of a transmitter. Because...
Key specifications:
-50 dB | 000 V | 100 Watts | 0.001 Watts | 0 dB
Uncertainty: U = k×√(Σui²), k=2 (95%)
Comparison
| Aspect | ACLR Measurement Spec | Typical Range | Impact | Design Note |
|---|---|---|---|---|
| Primary function | ACLR Measurement (Adjacent Channel Leaka... | Application-dep. | Critical | Verify in sim |
| Operating range | Because real-world RF amplifiers are mat... | Application-dep. | Critical | Verify in sim |
| Performance | This distortion forces the transmitter's... | Application-dep. | Critical | Verify in sim |
| Integration | To legally prove they are obeying the la... | Application-dep. | Critical | Verify in sim |
| Trade-off | The Bleeding Skirts In a perfect mathema... | Application-dep. | Critical | Verify in sim |
Frequently Asked Questions
What causes an amplifier to fail an ACLR test?
Greed. If an engineer tries to push 200 Watts of power out of an amplifier that was only designed for 100 Watts, the amplifier goes into 'Compression.' The silicon physically runs out of electricity, violently clipping the top off the radio wave. This clipping instantly triggers massive intermodulation distortion, completely ruining the ACLR and failing the FCC test.
What is the difference between ACLR and ACPR?
Nothing. They are the exact same mathematical measurement. ACLR (Adjacent Channel Leakage Ratio) is the strict terminology used by the 3GPP standards body for 3G, 4G, and 5G cellular networks. ACPR (Adjacent Channel Power Ratio) is the older, generalized terminology used in Wi-Fi, military radar, and legacy radio systems.
Can you fix bad ACLR with an RF filter?
Yes, but it is a massive, heavy, and expensive solution. You can bolt a massive metal Cavity Filter to the output of the amplifier. The filter acts like a physical brick wall, violently chopping off the bleeding skirts. However, modern telecom engineers prefer to fix ACLR using software (Digital Pre-Distortion - DPD) because it requires zero extra physical hardware on the tower.