Measuring Time And Resolution; Special Functions - Hameg Instruments HM8122 Manuel

DC-coupling, trigger slope selection and a continously
variable setting of the trigger level is necessary for setting
the trigger level at any required point of the input signal,
independent of waveform and duty factor. Two identical
inputs are also necessary to minimize the systematic
channel mismatch error.
Autotriggering requiring AC-coupling is also possible.
Pulsewidth measurement
Select the TI A : B or TI AVG function with the function
selection keys.
Apply the measurement signal to channel A. Depress
common. The B input is now disabled, except its signal
conditioning controls. Select the complementary slope of
input A on input B. Select DC coupling and set the same
attenuation for both channels. Set trigger level
potentiometers for correct triggering (or select autotrigger
with AC-coupling).
Remember always that the A signal is the start signal and
the B signal is the stop signal.
The display resolution changes with the number of
measurements taken from the signal. In the single
measurement mode (TI A to B) the resolution is 10 ns,
whereas the resolution in the TI AVG mode may be as
small as 1 ps, depending directly on the measurement
time set with the gate time potentiometer (4) and the
repetition rate of the input signal.
Frequency Ratio measurement
The function RATIO A/B measures the ratio between
signals connected to input A and input B.
A ratio measurement is useful, for instance, when
calibrating oscillators with an odd frequency. For example
say that the frequency should be 27.458934 MHz. This is
difficult to recognize on the display.
By connecting such a reference signal to input B and
measuring the ratio instead, the oscillator is correctly
calibrated when the display shows 1.000000, which is
much easier to read.

Measuring time and resolution

The measuring time can be varied in 199 steps between
1ms and 10 sec.. The gate time may be modified during
a measurement or while the gate time is displayed.
The set measurement time can be displayed by depressing
the READ pushbutton. Pressing READ again returns to
the previous measuring function. In the reciprocal mode
(at all frequencies with HM8122), the counter totalizes the
input cycles until the set measuring time has elapsed and
the synchronisation conditions are met. Hence, the
effective measuring time (also called gate time) is longer
than the set measurment time.
The measurement in the HM8122 is always synchronised
to the input signal. This is called the input synchronized or
reciprocal method.
In this mode, both the opening and closing of the main
gate are synchronized with the input signal, so that only
completed input cycles are counted. This means that a ±1
input cycle error is avoided. During the gate time, the
counter totalizes the number of clock cycles. When the
preselected gate time is over, the counter waits for the
next active transition of the input signal to stop counting.
24 HM8122
If the recurrence of this signal is low, e.g. with long period
times, the stop synchronisation time may be long compared
to the preset gate time. In that case the effective gate time
may be very different from the preset value (If the signal was
removed during measurement, this time becomes infinite
and the measurement finishes never).
The resolution in the input synchronized mode is caused
by truncation of the clock pulses, which results in ±1 clock
pulse error (10 ns). The resolution of the measurement
thus only depends on the measurement time. For example,
the resolution for 1s measuring time is 10
of input frequency.
In conventional counters the gate time is synchronized
with the clock singal. The first and last input cycle can
therefore be truncated, causing a ±1 cycle error. This
results in a good resolution for high frequency
measurements, but a poor resolution for low frequency
measurements (±1 : frequency, for 1sec. measuring
time). For this reason the HM8122 with its very fast
internal clock, uses the reciprocal mode for all frequencies.

Special functions

External Arming
Arming enables the counter to avoid starting on unwanted
signals. The external arming input (40) allows an additional
trigger condition. When input (40) is low (TTL-Level), the
counter is prevented from starting a new measurement.
However, the counter makes all preparations for a
measurement. When input (40) returns to high (>2V), the
measurement is prepared to start with a minimum of
delay (the delay is approx. 50 ns) according to the
synchronisation delay. The counter waits for an active
slope of the input signal.
The measurement will be performed according to the
settings of the instrument (gate time, display hold, offset).
If a new transition arrives on the EXT. ARMING Input
while the counter is busy, this transition will be ignored
and the counter starts with the next active transition after
completion of the measurement cycle.
Arming
Signal
Input A
Measuring
Time
Counted Amount
of Periods
Figure 2
External Gate
The external gate function allows full control of the start
and stop of the measurement. When EXT GATE is selected
(7) and the control input signal (40) is low, the counter
makes all necessary preparations for a measurement.
–8 , independent
Meas. Time
Subject to change without notice