User manual
22
O p e r a t i o n a l c h e c k
Input Triggering
Measuring time and resolution
24
Subject to chance without notice
The HM8021-4 offers a variaty of signal shaping
possibilities to improve triggering, such as AC/
DC coupling and two trigger level ranges (–2... 2V,
–40...+40V). The trigger level can be set in one of
two ways: either by Autotrigger or with the trigger
level potentiometer. In Autotrigger mode the
counter automatically sets the triggering to the
50% level of the input signal. When this mode is
selected AC coupling is necessary. When the
trigger level control has been set in the manual
trigger mode, the functioning of the trigger circuit
can easily be checked on the trigger indicator.
The LED shows the state of triggering.
LED on: the signal is above the trigger level
LED off: the signal is below the trigger level
LED blinking: the signal is crossing the hysteresis
band, correct triggering.
For reliable triggering the trigger level should, in
always most cases, be at 50% of the signal’s
peak-to-peak voltage.
Selecting the correct attenuation is important to
obtain the best results from your instrument. If
the attenuation is too high, the measurement will
be affected by the noise of the input comparator.
This results in an unstable display. With an input
signal too great, the input stage may saturate and
thus producing overshoots which result in a display
which is twice too high e.g. at frequency
measurements. Always try to set the control to
AC-coupling and use as much attenuation as
possible for frequency measurements and DC-
coupling with no attenuation for time measure-
ments. In many cases it is vital to have a good
impedance matching to avoid reflections which
might make the trigger level setting very difficult.
Always use a 50 Ω termination in 50 Ω systems.
The C-input facilitates no input conditioning
controls and needs no trigger level setting. The
input signal is triggered f rom 50 mV up to the
maximum input voltage of 5V. The input f requency
for the C-input must always be in the range f rom
100 MHz to 1000 MHz. For frequencies lower
than 100MHz the measurement result may be
erroneous.
Measuring time and resolution
The measuring time can be varied in 3 steps
between 100ms and 10sec. The gate time may be
modified during a measurement. In the reciprocal
mode (atall frequencies with HM8021-4), the
counter totalizes the input cycles until the set
measuring time has elapsed and the syn-
chronization conditions are met. Hence, the
effective measuring time (also called gate time) is
longer than the set measurement time.The
measurement in the HM8021-4 is always
synchronized 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.
If the recurrence of this signal is low, e.g. with
long period times, the stop synchronization 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 trunctation of the clock pulses, which
results in +1clock pulse error (100ns). The
resolution of the measurement thus only depends
on the measurement time. For example, the
resolution for 1s measuring time is 10
-7
, indepen-
dent of input frequency. In conventional counters
the gate time is synchronized with the clock
signal. The first and last input cycle can therefore
be truncated, causing a ±1 cycle error. This resuits
in a good resolution for high frequency measure-
ments, but a poor resolution for low frequency
measurements (±1: frequency, for 1sec.
measuring time).
Signal inputs
The front panel of the HM 8021-4 has two BNC
input sockets. One (Input A) with an impedance of
1MΩ II 40pF. As the frequency measuring range