Philips SCOPEMETER PM93 Service Manual page 51

CIRCUIT DESCRIPTIONS
input
coupling
input
protection
A1
P.C.B.
,----------------,
I
C2203
I
I
HF
I
I
attenuator
I
I I
L ________________
~
HF-path
LF-path
impedance
converter
,----------------,
I I
I
LF LF
I
I
attenuator calibration inverter
I
I I
L_ ____ _ ____
~
OFFSET
OFFSET
DAC
LF
calibr.
from
IlP
D-ASIC
position
Figure 3.9 Schematic diagram attenuator section
- Detailed circuit description
See figure 3.7 and circuit diagram A2a (figure 10.4).
Input coupling
emitter
follower
: FEEDBACK :
: LOOP
I
L. _ _ _ _ _ _ _ • _ _ _ .I
3 - 15
to
A-ASIC
The incoming signal first passes the AC/DC coupling section (C2202). When relay K2201 is opened,
the signal is AC coupled via C2202.
H.F. (high frequency) path
After the coupling section, the L.F. part of the signal is blocked by capacitor C2203. Only the H.F. part
of the input signal enters the H.F. attenuator. This is a triple capacitive divider, consisting of a 1 to
100, a 1 to 10, and a 1 to 1.48 divider.
The 1 to 1.48 divider section is switched on when relay switches K2202 and K2203 are in the "upper"
position (as shown on circuit diagram A2a, figure 10.5).
The 1 to 1.48 divider consists of C2203 and C2209 in parallel with some parasitic capacitors. The
attenuation of 1.48 times in this straight-on path is compensated for later in the circuitry.
The separate sections are switched in the signal path, depending on the attenuation required:
Table 3.2 Sections used in various attenuator settings
Attenuator Settings
Sections Used Attenuation
5 mV/div 100 mV/div 1.48x 1.48 times
200 mV/div 1 V/div 1.48x, 10x 14.8 times
2 V/div 10 V/div 1.48x, 100x
148 times
20 V/div 100 V/div 1 .48x, 10x, 100x 1480 times