Source Locking Microwave Frequency Counter with Selective Power Measurement. This family of Phase Matrix/EIP microwave frequency counters provides fully automatic source locking of virtually any electronically tunable source to the same accuracy and long term stability as the timebase oscillator in the counter. The ability of the 575B and the 578B to accurately set and stabilize the frequency of a source generator often eliminates the need for an expensive, synthesized signal generator