Hameg Instruments HM8122 Manuel page 25

With the high level of the gate signal, measurement starts
when the input signal triggers after a synchronisation
delay. Measurement stops on the first trigger after the
gate signal changes from high to low. The external gate
overrides the set measurement time.
The external gate signal must be in the range 50 ns .. 10 sec.
but the effective gate time will never be smaller than 20 µs.
Ext. Gate
Signal
Input A
Actual
Gate Time
No. of
Counts
Figure 3
Either external gate or external arming are selected by
means of the pushbutton (7) and indicated by means of
LEDs. Example applications are multiple burst frequencies
and masked time intervals. Note that if RF bursts are to be
measured using frequency C, the burst should contain at
least 128 cycles.
While external arming is slope sensitive, external gating is
active according to the level applied to the input (40). The
external gate signal may have a duration as short as 50ns.
Nevertheless, the effective gate time is min. 20µs.
External Reset
External Reset (Rear panel input 36) provides an equivalent
function to the front panel reset push button. The counter
is reset when the input is set to high logic level (>2V). A
new measurement can be made when input (36) has
returned to low (<0.5V).
External Reference
If an external reference frequency is applied to the ext. ref.
input (35) at the rear panel of the HM8122, the internal
crystal oscillator is switched off. The external reference
must have a frequency of 10MHz with a max. deviation of
±30ppm. The amplitude required is approx. 2V
RPM (NPR setting)
(Number of pulses per revolution)
Using the RPM function the counter enables the
measurement of revolutions per minute. The default
setting for pulses per revolution at start up is 1. The
HM8122 permits to preselect a number of pulses per
revolution, ranging from 1 to 65535.
NPR-setting:
1 Depressing READ (5) shows 00001 on the display with
the rightmost digit highlighted.
2 Change this digit in incremental steps of 1 by depressing
the ARM/EXT Gate pushbutton (7).
3 When the desired value is reached use the function
keys (17) to shift to the next digit.
4 Returning the NPR setting by pushing READ (5) once
again, stores the value until power is switched off or a
new value is selected.
Subject to change without notice
Exceeding a NPR of 65535 indicates error 5. Returning to
RPM with Error 5 indicated, keeps the previous value of NPR.
Gate View
The HM8122 is equipped with a gate open output for
monitoring the gate time on an oscilloscope. Note that the
gate open signal is longer than the set measuring time due
to the synchronisation time.
Calibration
The calibration mode offers two possibilities:
a) setting a new date without recalibration
Trig. Level
b) setting a new date and carry out a complete recalibration.
When making a recalibration, the internal crystal oscillator
will only be compensated for deviation in frequency due
to aging.
The mean time between recalibration, MTBRC, can be
calculated when the Total Tolerated Error is known. The
total tolerated error is defined as:
TTE = Deviation of intern. ref. frequency : Nominal
The meantime between recalibration is defined as:
MTBRC = (Acceptable error Temperature stability)
If recalibration is necessary it is carried out as follows:
1 Select function FREQ. A
2 Apply a frequency standard of 5, 10 or 100 MHz to input
A and adjust channel A settings for a stable reading.
3 Depress Reset (21) and gate time read (5) together, for
approx. 10 secs.
4 After 10 secs. the display shows C ... during the
pushbuttons are depressed.
5 When the switches are released the display shows C
00 00 00 to set the date of the last (actual) recalibration.
(Same procedure as RPM, 2-3).
6 You may leave the calibration mode without changing
anything by depressing reset (21).
For changing only the date you have to leave the
calibration mode when the right most digit is highlighted.
Firstly now depress the right shift key (17) and then
depress the reset key (21).
7 To start the recalibration, depress only the right function
shift key.
8 The actual calibration frequency is displayed.
.
9 During the calibration procedure which lasts at least for
pp
approx. 10 secs., the display shows the following
informations:
C ... if the standard frequency applied to the A input is
not accepted.
C ... 5x10E6 if the counter has recognized a 5 MHz
standard.
C ... 10x10E6 if the counter has recognized a 10 MHz
standard.
C ... 100x10E6 if the counter has recognized a 100 MHz
standard.
In the first case the counter cannot be recalibrated either
because no signal is connected, or the input signal deviates
to much from one of the 3 accepted frequency standards.
The result of the calibration is tested at the end of the
calibration routine. If the test fails, the counter starts a
new calibration cycle.
(Output 39)
intern. = ref. freq.
: Ageing
HM8122 25