Upon the filing of a referendum petition, which must contain valid signatures of 15 percent of the total number of names of persons listed as registered voters within the city on the date of the last preceding city general election, within 30 days of the passage of an ordinance petitioning therein that such ordinance be submitted to the electorate, the council shall reconsider an ordinance subject to referendum and upon reconsideration shall defeat it in its entirety or shall submit it to a vote of the people. The operation of an ordinance so protested against shall be suspended until the referendum petition is finally found insufficient or until the ordinance protested against has received a majority of the votes cast thereon at the election. (Ord. 2230 § 24, 1975.)