User Manual

15
Switching between FREQUENCY and PERIOD measurement on the same input, or switching
between WIDTH, RATIO H:L and DUTY (Input A), will immediately convert the present
measurement; otherwise, a change of function (including a change of input) or measurement
time will initiate a new measurement. A new measurement may also be started without a change
of function or measurement time by using RESET, the 2
nd
function of the HOLD key.
Measurement Principles
Frequency and Period
The instrument uses a measurement method generally known as reciprocal counting. After each
measurement interval (gate time) ends, it waits for the completion of the present cycle of the
input signal before capturing the count data. It has therefore measured the time taken by a whole
number of input cycles with a resolution of one cycle of its internal measurement clock. It then
calculates the average period of the input signal by dividing the total time by the number of input
cycles; the frequency is the reciprocal of this period value. This method yields much more
accurate results at low frequencies than the traditional method of counting input cycles for an
exact gate time.
The hardware captures count values without either stopping or resetting the counters. This is
known as “capture and continue” counting, and means there is no dead time at the end of each
gate interval. This allows successive measurements to be concatenated without incurring a one
clock cycle uncertainty at the intermediate points of the measurement. The instrument uses this
capability to give a rolling update in the display more often than the selected gate time. Each of
these updates show the average value of the input frequency over the time interval equal to the
selected gate time immediately preceding it being displayed.
If the signal has frequency modulation the instrument will display the average value across the
gate time; the modulation is almost certainly not synchronous with the gate, so there will be small
random variations in the displayed value.
If the signal has amplitude modulation, its amplitude at the trough of the modulation must exceed
the sensitivity threshold of the input. Counting deeply modulated signals requires both
considerable amplitude and a sensitive adjustment of the trigger threshold.
Width, Duty Cycle and Ratio H:L Measurements
When Width mode is selected, the instrument continues to use the capture and continue method
to measure the signal period. It cannot measure the width of the active part of the signal this way
because, by definition, there are gaps between the measurements while the signal is in the
inactive state. Instead, it measures the width of a sample of individual cycles of the input signal at
a rate up to about 1000 samples per second. It accumulates up to 50 such samples spread
across the selected gate time, computes the average and displays the result. Each sample has a
resolution of 20ns, and the average is displayed with a resolution up to 1ns. The values for duty
cycle and ratio H:L (better thought of as the ratio active:inactive) are computed from the average
width and the accurately known period. The display resolution presented in these modes is a
reasonable representation of the probable measurement accuracy.
Ratio B:A
This mode is entered by a long press of the WIDTH / RATIO key when the B input is selected. It
takes as nearly simultaneous capture and continue measurements of both input signals as
possible. Because each measurement terminates on a transition of its respective signal the two
measurements are not exactly simultaneous unless the signals are synchronously related. This is
not likely to be an issue unless the signals are significantly frequency modulated.
Note that this method is completely different to the previous model (the TF830) which
implemented ratio B:A mode by counting the B input using the A signal as the reference
timebase.