GB1515051A - Frequency counters and to methods of determining the unknown frequency of an input signal - Google Patents

Abstract

1515051 Digital frequency measurement JOHN FLUKE MFG CO Inc 12 April 1976 [23 April 1975 9 April 1976] 14850/76 Heading H3H In measuring frequencies up to a few GHz, an input signal is mixed 16 with a reference frequency, stepped in integral multiples of a fundamental frequency F R until the resulting difference frequency signal, selected by filter 28, exceeds the threshold level of a detector 38 whereupon a gate 36 is enabled for a short time to derive a first rough count, 30, related to the difference frequency; the reference frequency is then offset by F o and if the resulting difference frequency signal still exceeds the threshold a second rough count is made, if the threshold is not exceeded, the next harmonic, still offset, is selected as reference frequency and this should produce a difference frequency signal that exceeds the threshold whereupon the second rough count is made; a digital processor 40 then selects one of the two "accepted" reference frequencies (usually the last, except when the associated difference frequency is too low) and determines whether it is below or above the input frequency; and finally gate 36 is enabled for a sufficiently long time (e.g. I second) to obtain an accurate count for the difference frequency which is then added to or subtracted from the reference frequency and displayed at 58. Jitter may be added to the reference frequency for the first two (rough) counts and removed for the final (accurate) count. The offset may be constant irrespective of which harmonic of F R is selected, or may depend on F R . Two forms of generator 64 are described, Figs. 2 and 3 (not shown).