Specifications

Oscilloscope Settings
The Preamble Instruments 1820/1822 output is intended
to connect directly to the input of an oscilloscope,
spectrum analyzer or digitizer, but it is important to
observe some rules so that the 1820/1822 delivers its
specified performance.
Oscilloscope Input Impedance
The 1820/1822 output impedance is 50 f~ and the
intended load impedance is also 50 ~. Nominal gain
(amplification) is obtained only when the oscilloscope (or
digitizer) input impedance is set to 50fL The
EFFECTIVE GAIN display is correct only when the
1822 is properly terminated into 50 f~.
A factor of two additional gain is achieved by setting the
oscilloscope input impedance to 1 M fL However, the
operator needs to be aware that all the 1820/1822 gain
indicators will be off by a factor of two.
Sensitivity, Position, and Offset
Oscilloscopes are designed to maintain their accuracy for
that portion of a signal that is displayed on-screen. When
the signal is large enough to drive the display off-screen,
the oscilloscope’s amplifier must limit the signal in a non-
linear mode. Oscilloscopes are designed so that no matter
how the sensitivity, position and offset controls are set,
the operator cannot view this distorted portion of the
signal.
The maximum 1820/! 822 output is carefully controlled so
it will not exceed +5V when the output is properly
terminated into a 50 ~ load.
The oscilloscope’s gain and position controls should be
properly set to avoid displaying the non-linear portion of
the 1820/1822’s output signal when it is in overdrive.
This can be accomplished by observing the two following
rules:
Turn the oscilloscope input coupling to "OFF" or
"GND", set the oscilloscope position control to center
screen, and do not change it! If the oscilloscope has an
offset control, it too should be set to zero. Return the
oscilloscope’s input coupling to "DC". Subsequently
adjust the trace position on the oscilloscope screen using
the 1822 PVG (an external source for the 1820) and
VDIFF mode or VCOMP input. This assures that the
oscilloscope is looking at the center of the 1820/1822’s
dynamic range.
182011822 Operator’s Manual
Set the oscilloscope vertical sensitivity to no less than
500mV/div. The most useful range for the oscilloscope
deflection factors will be between ! mV/div and
500mV/div. Using a deflection factor of 2V/Div will
bring the nonlinear portion of the 1820/1822’s output on
screen. Digitizers should not expect accurate
measurements for high frequency signals from the
1820/1822 exceeding +2.5V into a 50 ~ load. This is
equivalent to +5 divisions of deflection at 500mV/div in
an oscilloscope.
More sensitive settings (e.g. 100~tV/div) available on
some oscilloscopes are perfectly acceptable, but their
usefulness may be limited by noise, particularly with the
full bandwidth limit selection and without averaging.
With the oscilloscope set to lmV/div and the 1820/1822
in the XI000 gain mode, the over all deflection factor will
be 1 laV/div.
In its Xl0 gain mode, the 1820/1822 is somewhat quieter
than most oscilloscopes, so it is preferable to use the
1820/1822 X10 gain mode and a lower oscilloscope
deflection factor rather than the other way around. For
example, to obtain the best noise performance at
lmV/div, set the 1820/1822 to XI0 mode and the
oscilloscope to 10mV/div rather than the use XI mode
and lmV/div. Other oscilloscopes give up bits of
resolution to obtain ImV or 2 mV/div sensitivity. The
1820/1822 is very quiet in its XI0 or greater gain mode,
but no better than most oscilloscopes in the X1 mode.
Any oscilloscope bandwidth limit setting may be used so
long as the unlimited signal does not exceed full screen
before invoking bandwidth limit. This is a good rule to
follow in using oscilloscopes with or without the
1820/1822.
2-5