Measurement of elapsed time for a predetermined frequency shift

摘要

The circuit arrangement described measures the time when a frequency Fo, changing towards a desired frequency value, differs by not more than a predetermined amount (e.g. 100 Hz) from the desired value. Initially Fo is mixed with the sum of 10 MHz and the new desired value for Fo. The lower sideband is selected and thus produces a frequency F1 which is thus always 10 MHz when Fo reaches its desired value (which may be variable). The circuit therefore has to measure when F1 comes within 100 Hz of 10 MHz. Initially F1 is multiplied by a predetermined multiplication factor (x100) and the product subtracted from a reference frequency. The latter is the sum of a fixed frequency of 300 KHz and of 10 GHz (i.e. 10 MHz multiplied by 100). This therefore produces an output frequency F2 whose value differs from 300 KHz by 100 Hzx100 (or 10 KHz) when F1 differs from 10 MHz by 100 Hz. It is therefore a relatively simple matter to determine a 10 KHz difference in 300 KHz, by detecting the output of a filter and comparing this with a reference.