Federal Realty Investment Trust, a Chevy Chase real estate trust that concentrates on upgrading older shopping centers, said its first-quarter net income increased to $2.4 million (32 cents per share), compared with $1.7 million (28 cents) during the same quarter of 1984.

Steven J. Guttman, president and chief executive officer of Federal Realty, attributed the improved earnings to higher income from trust properties, contributions from two recently acquired properties and increased interest income from short-term investments.

The trust's revenue during the quarter ended March 31 increased 20 percent to $7.9 million, compared with $6.6 million during the first quarter of 1984.

* Cerberonics Inc., a Bailey's Crossroads firm that provides defense-related engineering and technical support services to the federal government, said that its third-quarter earnings increased 9 percent to $278,000 (20 cents), compared with $255,000 (16 cents) during the same period in 1984.

Revenue increased to $10.7 million, up from $10.1 million during the company's third-quarter a year ago.

Cerberonics attributed the improvement mainly to close-out operations of two of its major contracts.

For the first nine months, earnings climbed to $1.1 million (79 cents) on revenue of $29.8 million, compared with profits of $989,000 (60 cents) on revenue of $30.7 million for the first nine months of the previous year.