Fluke PM3082 Service Manual page 57

5.1 -2
5.1.1.2 Input attenuators and calibrator
Diagram 1, 2, 3, 4
The attenuators of channels 1,2, 3 and 4 are identical, therefore only channel 1 is explained. Every
attenuator basically consists of four sections. These sections are:
- The input circuit with 50ft termination resistor and AC/DC input coupling circuit.
- The high impedance attenuator with divide by 1, divide by 10 and divide by 100 sections.
- The impedance converter. This is the active stage.
- The low impedance attenuator that can give an additional divide by 2.5.
The attenuator sections are switched by voltage pulse operated relays. This reduces power
dissipation. Every relay has two change-over contacts. After a pulse the contacts stay in the selected
position. One side of each relay coil is connected to the common potential AT12REP;
the other side to a voltage that differs per relay.
Setting a relay in the position as drawn in the diagram (rest position) is achieved by a positive 10 ms
pulse on common line AT12REP while the other control line stays at 0 V level. The other side of the
relay coils that must not switch are applied to the already mentioned 10 ms positive pulse. Switching
a relay to the position opposite to the one in the diagram (activated position) is achieved by a 10 ms
positive pulse at the other side of the relay while AT12REP is kept at 0 V. This principle is
demonstrated in figure 5.1 where the /1 relay is switched to the rest position and the /10 relay is
activated.
RELAY /001
TO REST POSITION TO ACTIVE POSITION
AT12REI
AT1/001
AT1/010
Figure 5.10 Switching pulses for relay
Input circuit
The 50ft termination resistor is formed by the resistors R1006/R1007. It is switched by relay contact
K1001. The NTC resistor R1011 is placed between R1006 and R1007 and measures the temperature
of these resistors. This information is routed to the microprocessor on unit A3. If necessary the 50ft
position is switched off.
The DC blocking capacitor is C1001. In DC position the capacitor is not used and discharged via
R1002. The signal path is via K1002.
High impedance attenuator
The divide by one section (/1) is switched via the relay contacts K1003. It switches the input signal
through without further attenuation.
The divide by ten section (/10) is present between the relay contacts K1004. The /10 is accomplished
by R1016, R1027, R1031 and R1032.
The divide by hundred section (/100) is present between the relay contacts K1006 and K1004/6,4.
The /100 is accomplished by R1021 and R1023.
LF square wave calibration is achieved via the dual varicap diode V1002. Influence of the signal on
the capacitance value is eliminated by two diodes with opposite polarization. The compensation
factor and consequently the voltages AT1LFCOR1 and AT1LFCORO are depending on the selected
attenuator position.
RELAY /010
ST«2M
UNIT DESCRIPTIONS