User Manual

Function Selection – B input
With Input B (80MHz – 3GHz) selected only the FREQUENCY and PERIOD functions can be
used; attempts to select WIDTH, COUNT or DUTY will be ignored by the firmware; the
B
annunciator will flash briefly to indicate that this is not a valid selection and the existing setting will
remain unchanged.
Selecting RATIO with Input B selected (by a long press on the PERIOD key) is valid, but sets the
instrument to a RATIO B:A mode (Frequency B : Frequency A), not RATIO H:L as described for
the A input. The ratio B:A is obtained by making simultaneous frequency measurements on the
two inputs and dividing the B result by the A result. The result of the calculation is as accurate as
the measurements. Each signal can be any frequency within the permitted range of the
respective input. If the ratio is so large that the decimal point cannot be shown on the display,
then the result is shown with six digits and an exponent.
Measurement Time
Measurement time is changed using the left ◄ and right ► MEASUREMENT TIME keys, the
selected time being shown by the appropriate annunciator in the display. With a suitable signal
connected to the selected input the Measure annunciator will flash in the display to indicate that
the signal has been detected; the Measure annunciator continues to flash until a true result for
that selected measurement time is displayed, at which point it stays on continuously. Further
display updates then show the running average of the signal behaviour over the last 0.3s (1
update every measurement), 1s (2 updates per second), 10s (1 update per second) or 100s (1
update every 2 seconds) depending on the selected measurement time. Note that if a 1s, 10s or
100s measurement time is selected, starting or restarting a measurement gives a true result with,
generally, 7 digits resolution after 0.3s, 8 digits after 1s, 9 digits after 10s and, finally, 10 digits
after 100s. The units and the decimal point position are automatically adjusted to give the result
in the most convenient units.
Pressing the HOLD key will freeze the displayed measurement and the Hold annunciator will
be shown; Hold is cancelled by a second press of the HOLD key. The measurement continues in
the background while Hold is selected.
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.
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