The instantaneous frequency-measuring receiver (IFR) is becoming widely known to microwaye systems engineers, but as a consequence of the defense applications its origins are obscure. The pioneering work on the phase-discriminator technique, which is the basis of most modern IFR systems, was done in the Mullard Research Laboratories, Red Hill, Surrey, England, UK. A brief account of the work and a record of one of the lirst signal observations with this type of receiver is provided. In concluding the author illustrates a signal display recorded in 1958 which is believed to be the first observation of signals with a balanced phase discriminator IFR. The receiver operated over an octave band from 2 to 4 GHz with one discriminator circuit. It is measuring five pulsed signals, one of which is a multifrequency transmission.