Sencore PS163 - RISE TIME MEASUREMENT

1. Connect a square wave generator to the input of
the amplifier to be tested. Connect CHANNEL A at
this point as a reference for comparison.
2. Connect CHANNEL B to the output of the
amplifier. It can be connected across the speaker or
across a resistor in place of the speaker. The resistor
is preferred as the noise from the speaker can be annoy-
ing.
3. Compare the resultant waveform with the input
and the ones shown in Fig. 17
GOOD SQUARE WAVE POOR HIGH
FREQ. RESPONSE
EE
aa
POOR LOW FREQ EXCESSIVE HIGH
RESPONSE FREQ. PEAKING
ee
RINGING HIGH FREQ. LOW FREQ.
À PHASE SHIFT PHASE SHIFT
Fig. 17. Distorted square wave resulting
from poor frequency response.
The best check on audio amplifiers is to use a low fre-
quency, a mid frequency and a high frequency. Fre-
quencies of 60Hz, 400Hz and 2KHz will check the
amplifier at all extremes that it will encounter in
normal use. If the amplifier passes a clean square
wave, the response is about 10 times the frequency
used. For example, if the 2KHz square is good, then
the frequency response is good to at least 20,000
Hertz.
If the output square wave is not clean or like the input,
then use the Signal Comparison method described
earlier to locate the trouble.
RISE TIME MEASUREMENT
Rise time is that time between the 10% and 90%
points on the leading edge of a pulse. Use the cal-
brated time base and the 5X expand on the PS163 to
make this measurement.
1. Connect the PS163 to observe the desired pulse,
and adjust the PS163 controls to obtain at least one
cycle on the scope screen.
2. Adjust the PS163 PPV PER DIV switch, and the
center knob of the PPV PER DIV switch so that the
waveform is exactly 5 divisions high on the scope
screen.
22
3. Use the TRACE POSITION and HORIZONTAL
POSITION controls to center the waveform on theo
major vertical grid line, so that it is exactly 2% diy-
isions above and below the major horizontal grid line,
as shown in Fig. 18.
Fig. 18. Centering waveform for rise time
measurement.
4. Pull the HORIZONTAL POSITION knob to act-
ivate the 5X expand, and adjust the HORIZONTAL
POSITION control, so that the 10% point of the lead-
ing edge crosses at the intersection of the horizontal
and vertical grid lines as shown in Fig. 19.
10%
Rise Time.
Fig. 19. Measuring rise time.
5. Measure the horizontal distance in graph screen
divisions between the 10% and 90% points on the
leading edge.
6. The rise time equals the distance measured in step
5, divided by 5 for the 5X expand, and multiplied by
the setting of the TIME BASE - FREQUENCY switch.
In
Fig.
19
the
distance
is
1.8
divisions,
and
the
set-
ting of the TIME BASE - FREQUENCY switch is
20uSec/DIV.
1.8 DIV
Rise
Time
=
X
20uSec/DIV
5
= 17.2uSec