Tektronix 1A5 Instruction Manual page 22

Operating Instructions — Type 1A5
Operating Mode
Passive Probe
1. Differential Amplifier
2. Conventional Amplifier
(Method a)
3. Conventional Amplifier
(Method b)
4. Differential Comparator
Active Probe
5. Differential Amplifier
6. Conventional Amplifier
ZB INPUT inverts waveform presentation.
Consider the signal source loading and the resulting
change in the source operating characteristics due to the
input circuit of the Type 1A5 and the signal cables. The
circuit at the input connectors can normally be represented
by a 1 MΩ resistance to ground paralleled by 20 pF.
few feet of shielded cable may increase the capacitance
to 60 pF or more, which could be excessive in many stua-
tions. To minimize these effects, it may be advisable to
use an attenuator probe.
Attenuator probes decrease the resistive and capacitive
loading of the signal source, and extend the measurement
range of the Type 1A5. Attenuator probes and their effect
were discussed previously in this section.
Input Coupling
To display both the AC and DC components of an ap ­
plied sgnal, set the appropriate A Input or B Input AC-GND-
DC switch to DC. To display only the AC component of a
sgnal, set the Input AC-GND-DC switch to AC.
Since a coupling capacitor is used to block the DC compo ­
nent in AC-coupled operation, the AC low frequency half
power point occurs at 2 Hz, and additional attenuation
must be expected below this frequency.
10 × 10-MΩ probe, the AC-coupled low frequency response
is about 0.2 Hz. If a 100 × 10-MΩ probe is used, AC-
coupled low frequency response is again 2 Hz.
The ground positions of the AC-GND-DC switches pro ­
vide a reference for DC zeroing of the CRT trace. However,
this method does not take into consideration ground loop
currents that may be present between the oscillsocope and
the equipment under test. For single ended operation un-
2-14
TABLE 2-2
Operational Set-Ups
Diff. Probe
PUSH
ON/OFF DISPLAY Connector
OFF A-B A INPUT and
OFF A INPUT or
A-B
B INPUT 2
OFF A-Vc As selected
by DISPLAY 2
or Vc-B
OFF A-Vc As selected
or Vc-B by DISPLAY 2
ON N.A. DIFFEREN ­
TIAL PROBE
ON N.A. DIFFEREN-
TIAL PROBE
A
When using a
Sgnal
INPUT
AC-GND - D C
Both to either
B INPUT AC or DC
Selected input
to AC or DC.
Other input to
GND.
Selected input
to AC or DC.
Selected input
to AC or DC.
N.A.
N.A.
der small signal conditions, and whenever extreme accuracy
is necessary, it is recommended that DC zeroing be done with
the AC-GND-DC switch in DC position and the signal probe
and shield grounded at the equipment under test.
When operating in A-Vc or Vc-B mode, the PO ­
LARITY switch must be set at 0 to perform DC
zeroing.
In differential operation, DC zeroing should be done with
the probe tips connected together, and the AC-GND-DC
switches in DC position.
Coupling Capacitor Precharge-Discharge Circuit
A coupling capacitor Precharge-Discharge Circuit is built
into the Type 1A5 input. It allows a small AC sgnal riding
on a larger DC voltage to be capacitively coupled in with ­
out having the amplifier circuitry overloaded by the capaci ­
tor's DC charging current. The circuit is not automatic, but
can be used easily as follows:
1. Place selected sgnal
GND.
2. Connect a sgnal source to the Type 1A5. The cou ­
pling capacitor will charge rapidly to the DC level of the
sgnal source.
3. Change the Input AC-GND-DC switch to AC and pro ­
ceed with waveform measurement.
4. When the Waveform measurement is completed, dis ­
connect the Type 1A5 from the sgnal source and discharge
VOLTS/CM Vc
Any N.A.
Any N.A.
Any 0
Any POLARITY and
AMPLITUDE po ­
sitioned as nec ­
essary to offset
signal deflection.
1 mV through N.A.
. 2 V
1 mV through N.A.
. 2 V
NOTE
Input AC-GND-DC switch to