Average-Mode - Philips PM3320 Operating Manual

4.3.12
4-107E
Glitch detection
Glitch detection in a digital storage oscilloscope is usually not possible. By means of the
MIN/MAX mode, even glitches with a pulse width of 3 ns are still displayed with an amplitude of
about 50%.
lf a glitch occurs in the reset time of a PEAK DETECTOR it is not seen.
Glitches longer than 20 nsec are always seen, The amplitudes depend on the part of the glitch
that is outside the reset time of the PEAK DETECTOR.
The chance of not catching a glitch can be decreased by the selection
of a lower time base frequency,
Envelope measurements
Because the minimum and maximum value of the input signal is measured between every two
adjacent samples, the MIN/MAX mode is perfectly suitable to measure and to display the
envelope of an Amplitude Modulated rf. signal.
Detection of the aliasing
In a digital storage oscilloscope the input signal is sampled at a high frequency, to convert the
analog signals into digital signals. This sampling frequency is determined by the time-base
setting or by the EXT CLOCK input frequency.
Ifthe frequency of the input signai is almost the same (or a multiple higher) as the frequency ofthe
sampling clock, a low frequency interference signal will be displayed, which seems to be not
triggered on the screen, but the NOT TRIG'D pilot lamp does not light.
This phenomena is called aliasing.
Usually this is indicated by the ALIASED pilot lamp.
This lamp lights if the sampling frequency is smaller than twice the trigger pulse frequency. The
latter is usually twice the selected input frequency.
Sometimes aliasing might still happen, but is not indicated.(E.g. an AM-signal, while the
oscilloscope is EXT triggered on the
LF-component).
To determine if the displayed signal is correct, the MIN/MAX mode can be selected. If the
envelope of the input signal is now displayed, there was aliasing.
The chance of aliasing can be reduced by selecting an as fast as possible time base setting.
AVERAGE mode
AVERAGING is a way to suppress noise without losing bandwidth and can only be used in
RECURRENT made.
Every dot is calculated after every sweep in the following way:
measured - previous
new = previous + —-—---------_--~ =
Cc
In this formula "previous" is a sample on-the same position of the previous sweep.
lf C=1 every new dot is the measured dot; AVERAGE is OFF. If C>1, the dot positions change
slower.
The bigger Cis, the slower the dot positions change. The following values for C can be selected:
C = 2, 4, 8, 16, 32 or 64.
In ROLL mode it is also possible to use AVERAGE.
Between two displayed samples, more samples are taken. These intermediate samples are used
to perform the AVERAGE calculation with a fixed value for C of C = 32.
AVERAGE cannot be used together with MIN / MAX.
NOTE: ff SINGLE or. MULTIPLE SHOT is selected AVERAGE is not effective.