The Model 122A is a dc to 200 kc dual-trace cathode-ray
oscilloscope. It has accurately calibrated input sensitivity
and sweep-speed ranges. This oscilloscope features
dc-coupled differential amplifiers and dual-trace operation.
Differential amplifiers are useful since they reject the
common-mode (in-phase) part of the input while amplifying
the differential (out-of-phase) part. For instance, they
will reject hum pick-up while amplifying the desired signal.
In addition, using differential amplifiers has other
advantages. Normally dc amplifiers are difficult to keep
stable. Changes such as aging and changes of potentials
cause this drift. However, these changes are of the
common-mode type and are rejected by differential
amplifiers. Thus differential amplifiers are also used to
stabilize dc amplifiers.
Dual-trace operation is obtained with an electronic switch.
This permits observation of two signals at the same time.
The two signals may be viewed either alternately during
consecutive sweeps or chopped on each sweep. Chopped
operation involves switching rapidly between the two signals
so that both appear to be traced simultaneously. The most
useful type of presentation in each case will depend upon
the frequencies involved. Generally, chopped operation is
used with low frequency signals. These signals may also be
viewed separately or with their difference (B-A) displayed
on the screen.