For low frequency measurements with one counter, you measure one period of your signal using a known timebase.
You can route the signal to measure (fx) to the Gate of a counter. You can route a known timebase (fk) to the Source of the counter. The known timebase can be an onboard timebase, such as 80 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with a known rate.
You can configure the counter to measure one period of the gate signal. The frequency of fx is the inverse of the period. The following figure illustrates this method.