34
Subject to change without notice
enables the user to select the most suitable mode for the
specific measurement. The
HM407
contains two 8 bit flash A/
D converters with a maximum sampling rate of 50MS/s each.
Except in the combination of
DUAL
mode and
SINGLE
event
recording with a maximum of 40MS/s (time coefficient 5µs/
div) for each channel, in all other modes 100MS/s is available
if the lowest time coefficient is chosen. Apart from the factors
above, there is no principle difference between capturing
repetitive signals and one-time events. The signal is always
displayed with a linear connection between the dots (Dot Join
function).
Signals captured and stored in storage mode can be called via
the built-in RS232 interface for documentation purposes. For
further information please note section “RS232 Interface -
Remote Control”.
Signal recording modes
Signals can be recorded and displayed in six different modes:
REFRESH
mode (RFR LED lit, readout indicates RFR)
ENVELOPE
mode (ENV LED lit, readout indicates ENV)
AVERAGE
mode (AVM LED lit, readout indicates AV...)
SINGLE
mode (SGL LED lit, readout indicates SGL)
ROLL
mode (ROL LED lit, readout indicates ROL)
XY
mode (RFR LED lit, readout indicates XY and the sampling rate)
Except
ROLL
and
XY
mode, a signal recording in all other
modes requires a trigger signal.
In
REFRESH, ENVELOPE
and
AVERAGE
modes the
instruments behaves like an analog oscilloscope. The trigger
circuit starts a recording, overwriting the previous recording
from the left to the right side of the screen. After the recording
has been finished, the next trigger event starts the same
procedure. This can also be caused in automatic trigger mode
without an applied signal by the automatic circuitry. Then only
the trace (
Y-POS.
setting) is recorded.
In contrast to automatic trigger mode, in normal trigger mode
the automatic system is switched off and consequently only
a trigger signal can start a recording. Unlike analog mode
where the screen is dark until a trigger signal starts the time
base, in store mode the last recorded signal remains visible as
long as no new recording is triggered by an input signal.
AVERAGE
and
ENVELOPE
are
REFRESH
sub-modes and
described in section “Controls and readout” under item
(9)
.
SINGLE
mode
(SGL)
enables the capture of one-time events,
started by a suitable trigger signal. It is recommended, to
select input ground
(GD)
condition and set the trace on a
graticule line which is then used as the 0 Volt position
(reference) line. In
SINGLE
mode the instrument is
automatically set to normal trigger mode. After
SINGLE (SGL)
mode has been selected, the trigger point symbol should be
set above or below the 0 Volt position line, according to the
expected voltage of the event to be captured. Whether the
slope selection is set for a rising or falling slope depends on the
measurement task. After this procedure AC or DC input
coupling must be selected and the signal capture is started
after activating the RESET function. The following example is
for additional explanation.
If a voltage drop on a +5 Volt line is expected, the 0 Volt line
may be set to the horizontal center graticule line by using the
Y-POS.
control. Then the trigger point symbol may be set 2
division above the 0 Volt position using the
LEVEL
control. If
the deflection coefficient is set to 200mV and
DC
input
coupling selected, using a x10 probe, the trigger point is at +4
Volts. A voltage drop from +5 Volts below +4 Volts then is used
for triggering, if negative slope setting is selected. If the slope
selection is positive the end of the event (voltage drop ended)
is used for triggering.
For explanations regarding ROLL mode, please note this item
(9)
in section “Readout and Controls”.
Vertical resolution
The dot density in each operation mode is 8 bits = 2
8
= 256 dots
displayed over a height of roughly 10 divisions. The instrument
is adjusted for 25 dots per division.
This eases processing and cursor measurement. Insignificant
differences between the (analog) screen display and the
(digital) data are unavoidable.
This concerns signal height as well as the position. The trace
position is defined in respect to the following horizontal
graticule lines:
Center line
= 10000000 (binary) = 80 (hex)
= 128 (dec).
Top line
= 11100100 (binary) = E4 (hex)
= 228 (dec).
Bottom line = 00011100 (binary) = 1C (hex) = 28 (dec).
In contrast to analog mode with its theoretically unlimited
resolution, the vertical resolution has 25 possible trace positions
per division.
If the signal is superimposed by noise or a critical Y-POS.
setting is used, the least significant bit (LSB) may change
continuously. This additionally reduces the vertical resolution
in storage mode, but is unavoidable. In contrast to the expen-
sive flash A/D converters used in this instrument, other
converters such as CCD cause more noise.
Horizontal resolution
The maximum number of signals to be displayed simultaneously
is three. Each signal consists of 2048 (2
11
) byte (samples).
Referred to the horizontal raster, the resolution is 200 samples
per division.
Pure (only) digital oscilloscope with VGA monitor type CRTs
offer only 50 samples per division. If LCD displays are used the
current resolution is 25 samples per division. For a given time
base setting the
HAMEG
instrument samples at a 4 (compared
to VGA) or 8 ( referred to LCD) times higher sampling rate. The
higher number of samples/div results in a shorter sampling
interval. For the following example it must be kept in mind, that
the time base setting is related to the signal period duration and
consequently should enable the display of one complete signal
period. If e.g. a 50Hz signal has to be displayed the time base
should be set to 2ms/div. The maximum signal frequency of a
superimposed sinewave signal, which must be sampled with at
least 10 samples per period, depends on the horizontal resolution:
samples/div
sampling interval
sampling rate max frequency
200
2ms : 200 = 10µs
100kS/s
10kHz
50
2ms : 50 = 40µs
25kS/s
2.5kHz
25
2ms : 25 = 80µs
2.5kS/s
1.25kHz
During X magnifier mode the record length is still 2048 byte.
The CRT displays a tenth of the recording (20 byte/division)
plus 180 byte/div calculated in linear interpolation operation by
a RISC processor. The smallest available time coefficient then
is 50ns/div instead of 500ns/div without the X magnifier.
Maximum signal frequency in storage mode
The highest capturable signal frequency cannot be exactly
defined, since it depends to a large extent on the waveform.
Storage Mode
