Philips 6652C - Page 36

Look-out for th e impedance of the count er !
The input impedance of t he cou nt er is reduced to
fr om 1 Mohm to 500 kohm when:
a)
COMMON i npu ts have been selected .
b)
P -WIDTH, RISE / FALL TIME or DUTY FACTOR mea-
surements are sele c ted.
This means that a probe compensated with a
1
Mohm load from the counter is un-compen-
sated i n the above cases (5 00 kohm).
In conc
lu s ion , a ca librated probe should only be
used
when se pa rate A and B inputs have been se-
lected otherwise the probe will no longer be
com p ensated.
EXAMPLE: Wh en e.g. rise time should be measured
using probes, a Time Int erva l measure-
ment
with separate inputs must be se-
lected . The input signal mu st ex
ternally be fed to both input s via two
sepa ra te (c alibrated ) pr ob es . Thu s the
'COM'-function should NOT be used.
The levels must be set manually to 10%
(A-c hannel) and 90% (B-cha nn el) r esp.
Probe compensation in PM 6652 and PM 6654
1) Set coun ter i nputs to ATTx1, 1 Mohm termi-
n ation and SE PARAT E inputs (N OT ' COM') .
2)
Se l ec t Vpp-fun cti o n.
3) Connect probe to input A.
4) Connect a generator, v ia the probe, and set
square wave signal, 500 Hz and 20 Vpp•
5) Tur n Probe tr immer until a sta bl e •i~
volt age reading is obtained.
6) Turn probe trimmer slowly un til the voltage
reading i n creases sha rpl y.
7) Turn
probe trimmer backwards (very s l o wl y)
until minimum voltage plus 10 mV is obtained.
8) Th e probe should now be correctly co mp e n-
sated , see the "knee" in the trimm er curve
b
el ow.
Undercon>pt!n.$Qfed
Co,..recl/'1
cornp en ~olecl
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