Modulation spectrum failed

During my career in RF and Wireless baseband testing, I have faced with this issue so often with testing transmitter part of various wireless compliant devices such as WLAN,WiMAX,GSM,CDMA or LTE.

Modulation spectrum is the modulated output of transmitter. As per FCC the spectrum should meet power level requirement at various frequency spacing from the RF center frequency to meet interference specifications as per countrywide regulatory requirements.

To help accomplice the testing, standard bodies specify power spectrum mask which need to be taken as reference. Device under test should have power spectrum fall within the mask in order to pass the test. The test fails when the modulated spectrum goes out of the mask.

There are two main reasons of modulated spectrum to be failed this test requirement.
1. RF and baseband filters are not upto the mark as per desired specifications, leading to unequal power at different frequency points.
2. LO leakage leading to pure carrier at the output along with modulated spectrum. Refer pure carrier vs modulated carrier to know more.

Troubleshooting

1. For the first point, RF filters need to be tuned and baseband filter need to be redesigned and baseband IQ output magnitude response to be verified.

2. For the second point,
check whether baseband chain input should not contain all ones or all zeroes. As continuous ones or zeroes at the input of baseband chain produce pure carrier or un-modulated carrier at the output. The baseband chain must have randomized binary pattern as input. •  If the data is randomized and evenif the pure carrier is observed at the output then check for RF part. Check whether proper inputs with appropriate power levels are provided as input to the RF mixers in the RF transceiver chain. Refer RF mixer page to know rule of thumbs for the RF circuit design using RF mixers.
•  Check whether RF filters are employed as desired as per specifications.

Test and Measurement Equipments

RF and Wireless Terminologies