Troubleshooting Modulation Spectrum Failures in Wireless Device Testing

During my career in RF and Wireless baseband testing, I’ve frequently encountered issues with modulation spectrum failures when testing the transmitter part of various wireless devices like WLAN, WiMAX, GSM, CDMA, and LTE.

The modulation spectrum represents the modulated output of the transmitter. Regulatory bodies like the FCC mandate that the spectrum meet specific power level requirements at various frequency spacings from the RF center frequency. This ensures compliance with interference specifications defined by countrywide regulations. To facilitate this testing, standards organizations define power spectrum masks, which serve as reference points. The device under test (DUT) must exhibit a power spectrum that falls within the mask to pass the test.

The test fails when the modulated spectrum exceeds the boundaries of the defined mask. There are primarily two reasons why a modulated spectrum might fail this requirement:

1. Inadequate RF and Baseband Filters: If the RF and baseband filters don’t meet the desired specifications, it can lead to unequal power distribution across different frequency points in the spectrum.
2. LO Leakage: Local oscillator (LO) leakage can introduce a pure carrier signal at the output, alongside the intended modulated spectrum. Refer to a discussion on pure carrier vs. modulated carrier signals for further details.

Troubleshooting Steps

Here’s a breakdown of troubleshooting steps for each of the potential causes:

1. Addressing Filter Issues:

• RF Filter Tuning: The RF filters may require careful tuning to ensure they meet the required performance characteristics.
• Baseband Filter Redesign: The baseband filter design might need a complete overhaul if it’s contributing to the spectral issues.
• Baseband IQ Output Verification: Verify the magnitude response of the baseband IQ output to ensure it’s within acceptable limits.

2. Resolving LO Leakage:

• Baseband Input Analysis: Examine the input to the baseband chain. Ensure it doesn’t consist of all ones or all zeros. Continuous ones or zeros at the input can produce a pure or unmodulated carrier at the output. The ideal scenario is to have a randomized binary pattern as input to the baseband chain.
• RF Component Check:
  • Data Randomization and Persistent Carrier: If the input data is randomized, but a pure carrier is still observed at the output, shift the focus to the RF section.
  • Mixer Input Power Levels: Verify that the RF mixers in the transceiver chain are receiving the correct inputs with appropriate power levels. Refer to documentation on RF mixers for rule-of-thumb guidelines for RF circuit design.
  • RF Filter Implementation: Confirm that the RF filters are implemented according to specifications.