Tektronix 1A5 Instruction Manual page 23

the coupling capacitor
to DC.
A charged coupling capacitor can
ponents connected to the next point tested.
also be detrimental to the Type 1A5 input
cuitry if not discharged properly. Step 4 will
discharge the coupling
capacitor precharge-discharge procedure
become a habit in
ments.
It may be noted that
cedure is followed faithfully, the Input Signal Range for the
most sensitive Type 1A5 settings can be extended to ±500
volts, providing the
Display Polarity
Signals applied to the A INPUT connector will produce a
vertical deflection in phase with the input signal. Signals
applied to the В INPUT connector will be inverted.
Determining Signal Amplitude
In single-ended operation the signal amplitude is deter ­
mined by multiplying the number of centimeters of trace
deflection by the deflection factor.
of the Type 1A5 and oscilloscope combination is the prod ­
uct of the probe attenuation and the setting of the VOLTS/
CM control.)
Example: Operational setup 2 from Table 2-2 has been
selected, a 10 ×
and there are 3 cm of deflection on the CRT. Deflection
factor (10 × 5 mV) times deflection (3 cm) equals 150 mV
signal amplitude.
In differential amplifier operation, the deflection factor
multiplied by the amount of trace deflection indicates the
difference in amplitude between the two signals under test.
Example: Operational setup 1 from Table 2-2 has been
selected, 1 × probes are used, DC coupling selected for
both inputs, VOLTS/CM set to 2 mV, and 4.5 cm deflection
occurs. Deflection factor (1
(4.5 cm) equals 9 mV signal difference. (If the DC + AC
amplitude of either input were desired to be known, it
should be determined under single ended operation as in
the first example.)
Differential comparator operation can also be used to
determine signal amplitude, and its accuracy can be relied
upon to three significant digits. Under setup 4 from Table
2-2, the deflection on the CRT will be the difference between
the signal amplitude and the amount of comparison volt ­
age introduced to the other channel. The signal amplitude
can be determined by multiplying the deflection factor by
the amount of deflection and adding Vc (from the AMPLI ­
TUDE control) to the product.
Example: Operational setup 4 from Table 2-2 selected, 1
probe coupled into
Vc POLARITY at -, Vc AMPLITUDE at 240 V (2-4-0 posi ­
http://manoman.sqhill.com
by placing the AC-GND-DC switch
CAUTION
damage com ­
capacitor. The coupling
all AC input coupled measure ­
if the precharge-discharge pro ­
AC component does not exceed 5 volts.
(The deflection factor
probe is in use, VOLTS /СМ is at 5 mV
× 2 mV) times deflection
A INPUT, AC-GND-DC switch at DC,
tion), VOLTS /СМ at
Deflection factor (1
— 6 V. Adding the Vc — 240 V (which removed 240 V from
the trace deflection by common-mode rejection) results
— 246 volt total signal input.
The above example is characteristic of the measurement of
It can
AC riding on a DC level. If the entire voltage had been
cir ­
the measurement would have been simplified by introducing
safely
enough Vc to put the trace at the DC reference position.
Then the deflection factor could have been ignored
should
total signal voltage read directly from the Vc AMPLITUDE
dial. In either case, notice that the Vc feature allowed
determination of a voltage which, in single-ended
would have deflected the trace off of the face
even with the Type 1A5 set up in its least sensitive calibrated
position.
The explanation concerning the deflection factor
is true only when the VARIABLE control is
to the CAL position
The VARIABLE control can increase the deflection factor
to more than 2.5 times the value indicated by the VOLTS/CM
control setting. This provides uncalibrated vertical deflection
factors between the calibrated settings of the
switch, and when in the 20 V/cm position extends the max ­
imum total waveform display capability to at least 300 volts
peak to peak.
Vc MONITOR
COMPARISON VOLTAGE (Vc) can be read on
or oscilloscope connected to the MONITOR
AMPLITUDE and POLARITY controls make 0 to ±5 V
able in all of the VOLTS/CM control settings except the
5-V, 10-V and 20-V positions, where it is
±0.5 V maximum value.
(one which draws little or no current from the measured
voltage) is used, the value will be within ±0.5% or
(whichever is greater) of the Vc AMPLITUDE dial setting.
If an accurate, low impedance measuring device is used,
the value read will be the value applied to
put channel, but it will disagree with the
control setting. The amount of difference will
by the degree of loading caused by the measuring
When the load is removed by disconnecting
device, the voltage will return to that indicated by
PLITUDE control.
The Vc MONITOR output can be used as a
but the accurracy and value of Vc will vary inversely with
loading effect, just as it does with a measuring
nected to it. This is illustrated in Fig. 2-12.
Use sheilded leads whenever connecting to
MONITOR JACK while waveform observations
being made. Stray voltages of 10 mV or more can
be picked up by unshielded cables connected to
MONITOR jack This induced noise will affect
×
CRT presentation whenever the DISPLAY
in the A-Vc or Vc-B position.
Operating Instructions — Type 1A5
2 V, — 3 cm trace deflection on CRT.
× 2 V) times deflection ( — 3 cm)
operation,
of
NOTE
VOLTS/CM
a
pin-jack.
reduced
If a high impedance
the selected in ­
Vc AMPLITUDE
be determined
the measuring
voltage source,
device con ­
NOTE
switch
equals
in a
DC,
and the
the CRT,
set
voltmeter
The
avail ­
to a
voltmeter
±5 mV
device.
the AM ­
the
the
are
the
the
is
2-15