Tektronix Type Z Instruction Manual page 10

Operating nstructions — Type Z
matching between A and B input circuits is necessary, one
input d c blocking capacitor may be " padded" with a small
additional capacitance, generally less than 0.001 /ifd.
The PU H TO DI CONNECT
signal to be momentarily removed from the input without
the bother of disconnecting the probe or coaxial input con ­
nector. This provides an easy method for finding the base ­
line of zero voltage level on the crt. (When utmost ac ­
curacy in measurement is required, trace deviation from
exact zero due to gas or grid current must be considered.)
The PO ITION control may then be adjusted to make the
zero level lie at any graticule mark desired.
Differential Preamplifier Operation
The primary purpose of differential operation is to elim ­
inate undesirable common-mode signals. The term " common ­
mode signal" is defined as that signal which is common
to both inputs of a differential amplifier. It most commonly,
but not necessarily, represents unwanted hum or noise.
This feature can be used, for example, to observe the
signal across one circuit element while effectively eliminat ­
ing the remainder of the circuit from the observations. T h is
is accomplished by connecting the signal at one end of
the element to one input of the Z Unit and the signal at
the other end of the element to the other input of the unit
(c) 100-volt 60 ~ hum only.
to (a) except Mode is — B ONLY.
2-4
IGNAL buttons allow the
Control s etting s si mil a r
Fig. 2-4. Common-mode rejection by the Z Unit.
Differential operation between the two inputs is obtained
when the Mode switch is in the A-B DIFF. position. Maximum
common-mode rejection is obtained when both input at ­
tenuators are set to XI. Common-mode rejection is a func ­
tion of frequency in practical amplifiers. It is 40,000 to 1
for de common-mode signals in the Z Unit and remains
near that value through audio frequencies, decreasing as
the frequency increases.
The differential or common-mode rejection ratio of the
Z Unit describes the ability of the unit to reject common ­
mode signals. Common-mode rejection ratio is best defined
as the ratio of amplifier response to that part of the input
signal not common to both, with respect to the response
of the amplifier to any input signal which is common to
both inputs. It is defined numerically in the following ex ­
ample.
If an input signal consists of 100 volts (p-p) of 60-cps hum
and 0.1 volt of desired signal, the 100-volt hum would cause
, — 1 --------------- times 100 volts or 2000 cm of deflection, and
. 05 volts/cm
the signal would cause an additional 2 cm of deflection.
If conventional preamplifier operation were used, the de ­
sired signal would be deflected off the screen and could
not be observed. However, if differential operation is
used and common-mode rejection were 2000 to 2 or
1000 to 1, then the hum and desired signal would each
produce 2 cm of deflection. The resulting combined wave ­
0.05 v/cm. Mode: A-B D FF., dc-coupled. Sweep Rato:
0.5 mill se c/cm.