Philips PM3320 Operating Manual page 122

4-102E
ln dual channel mode there are four possibilities to place the sweeps in register RO.
Once again the time that elapses between the trigger pulse and the first sample pulse is used to
place the samples at the right locations.
lf AVERAGE is selected in the time base range 500 us/div ... 200 ns/div, the instrument
automatically selects the MAXIMUM RESOLUTION mode.
When MIN / MAX is selected, the MAXIMUM RESOLUTION mode is automatically switched off.
NOTE: In maximum resolution mode at least 8 sweeps (or 4 in DUAL channel mode) are
needed to make a complete picture. In this way, a high resolution display with low
jitter is realised.
This also means that, if the input signal changes, there is temporarily a situation of
change over, which looks quite strange. This effect becomes more strongly visible
when the AVERAGE mode is turned on. Turning SHIFT, VARIABLE or LEVEL, has a
similar effect.
After a change of an AMPL/DIV or TIME/DIV setting the register is cleared anda
new picture is made. So there are no strange effects visible.
Random sampling mode
In the time base range 100 ns/div...5 ns/div, the random sampling mode is used. This mode
allows the digitizing of input signals with high frequencies.
These frequencies may be even higher than the sampling and conversion rate. It is possible to
digitize a 200 MHz signal with the sample and conversion rate of 50 Msamples/s.
To get a complete picture one condition should be met: the signal has to be repetitive.
The principle is a follows (see fig. 4.48):
After a trigger occurs, the time which elapses between the trigger pulse and the first sample pulse
is measured by the delta-t circuit. Afterwards, a number of samples, depending on the time base
setting, is taken and converted.
E.g. with the sampling rate of 50 Ms/s every 20 nsec a sample is taken. With a time base setting of
10 nsec/div a sweep takes 100 nsec. So 100/20 = 5 samples can be taken during one sweep,
The locations in the register, where these samples have to be stored depend on the measured
time between the trigger pulse and the first sample pulse.
The next sweep another 5 samples are taken and placed in register RO and on the C.R.T. screen.
Because there is probably another time difference between the trigger pulse and the first sample
pulse, the samples are placed on.other places in the register and so they make
the picture more complete. To make a complete picture of 500 dots, at least 100 sweeps (in this
example) are needed, but probably more.The more dots that there are already filled in, the bigger
the chance is that a new sweep overwrites a previous sweep.
Acomplete picture consists of 500 dots over 10 divisions (512 samples in register RO) in single
channel mode and of 2x500 dots over 10 divisions (2x512 samples in register RO) in dual channel
mode.
If DOTS is off intermediate dots are calculated and displayed in the same way as in the direct
mode with the MAXIMUM RESOLUTION mode off.
Because there are samples taken and stored in the oscilloscope before
the trigger pulse occurs, pretrigger is also possible.
The urgency of a repetitive signal as input signal wiil be clear now.
NOTE: When the input signai changes, the picture on the screen changes ina random way
from the old situation to the new one. This looks quite strange for a while, but it is
normal because of the random sampling principle.
Because there is no time relation between the trigger pulses, which are derived from the input
signal, and the sampling pulses, the time between a trigger pulse and the first sample pulse ts
random. This is why this principle is called random sampling.