Figure 12: Setup For Dc Cmrr And Ac Cmrr Adjustments - Tektronix P6248 Service Manual

Adjustment Procedures
24
c. Connect a second BNC cable from the function generator Trigger or
Sync Output to the External Trigger or Channel 2 Input of the oscillo-
scope.
Function
generator
Output
BNC-to-binding-
Red Black
post adapter
with square pins
Red (+) Black (- - )
Y-lead adapter

Figure 12: Setup for DC CMRR and AC CMRR adjustments

÷
2. Set the probe to 1 attenuation.
3. Set the function generator to square wave, 1 ms period (1 kHz frequency),
approximately 10 V
4. Set the oscilloscope to display channel 1. Set channel 1 to DC and 50 Ω
input impedance (or use external terminator), 2 mV/div. Set the time/division
to 200 s/div. Set the trigger source to external (or Channel 2). Set the
acquisition mode to average 8 to 10 acquisitions. Apply vertical bandwidth
limiting, if available, to reduce noise.
5. Adjust the trigger level for a stable trigger. (If the trigger is obtained through
channel 2, it may be necessary to change the volts/div setting.)
6. The displayed square wave is the common mode feedthrough. If the probe is
severely misadjusted the waveform may be off-screen. If necessary, increase
the channel 1 volts/div to keep the waveform on screen.
÷
7. Adjust the 1 DC CMRR for minimum amplitude in the flat portions of the
displayed waveform. This adjustment does not affect the leading edge
transitions. Increase the vertical sensitivity as the amplitude decreases.
Test oscilloscope
Sync
Ch 1
Ground
lead
P6248
, (5 V ).
pk-pk pk
P6248 1.7 GHz Differential Probe Service Manual
Ext trigger
50 Ω terminator
CH 1 CH 1
output input
1103