The Hewlett-Packard Model 5254C Frequency Converter is a plug-in unit which extends the frequency measuring capability of an HP Electronic Counter from . 15 to 3. 0 GHz (150 MHz to 3000 MHz). The stability and accuracy of the basic counter are retained by multiplying a 10-MHz signal, derived from the 1-MHz internal tizne base of the counter, to 50 MHz and selecting a harmonic frequency between 200 and 2950 MHz. This known harmonic of 50 MHz is then heterodyned with the INPUT signal. The resulting difference frequency, if between 1 MHz and 53 MHz (bandwidth of amplifier in plug-in) is counted and displayed by the counter. The frequency of the INPUT signal is then indicated by the combination of the MIXING FREQUENCY control (in gigahertz; front panel of plug-in) and the digital display of the counter (in megahertz).