Fluke PM6690, Service Manual, Page: 39 / 200

A semi-protected AC/DC module is placed on spacers over
the main circuit board. It delivers three regulated DC voltages
to the counter, + 15 V, + 5 V and –15 V. When the counter is
connected to line power, these voltages are always present.
There is a + 5 V trimmer potentiometer on the AC/DC mod-
ule.
The ON/OFF circuit is active as soon as the counter is con-
nected to line power. The instrument automatically powers up
when line power is applied. Only the processor can switch the
circuit to OFF state (standby mode). Then only the power but-
ton on the front panel can switch the instrument to ON. Once
connected to line power and in OFF state (standby mode),
supply v12 VU is distributed to the oven oscillator to
keep it warm. In standby mode a red LED is lit on the front
panel. To keep the AC/DC module stable in this mode, a
dummy load is connected to draw a quiescent current of 0.1 A
at +5 V.
The three main voltages are +15 V, +5 V and –15 V from the
AC/DC module. All other supply voltages are generated from
them and are, except for +12 VU to the oven oscillator,
switched off in standby mode.
Linear regulators are used for generating stable voltages with
low noise and without spikes. Some of them have a resistor
net at the input to dissipate part of the heat generated. Thus the
regulator itself will run cooler. The following voltages are
generated in this way:
+3.3 VA from +5 VU
-5.2 VA from -15 VU
-5.2 VI from -12 VA
-2.1 V from -12 VA
-12 VA from -15 VU
+12 VA from +15 VU
+12 VU from + 15 VU, to oven oscillator, not switched off in
standby mode
For digital and general use supply voltages are generated by
DC/DC converters:
+3.3 VD from +5 VU
+5 VD from +5 VU via a switch IC
+15 VD from +15 VU via a switch transistor
+1.8 V from +5 VU
Communication Interfaces
Both a GPIB and a USB interface are available for communi-
cation with external bus controllers. The connectors are lo-
cated on the rear panel.
The GPIB consists of an IC. Only 8 bits of the 32-bit micro-
processor bus is used. Due to different supply voltages for the
processor (+3.3 V) and the GPIB circuit (+5 V), a buffer IC is
used for isolating the +5 V GPIB IC from the processor bus. A
40 MHz crystal is used for generating a local clock.
The USB consists of an IC. It communicates with the proces-
sor over the 32-bit microprocessor bus, but only 16 bits are
used. A 6 MHz crystal is used for generating a local clock.
Rear Panel Unit
The rear panel is made of aluminum. A number of connectors
mounted on this unit are accessible to the user, see Figure 4-9.
I/O:
– GPIB communication connector.
– USB communication connector
INPUTS:
– External reference (BNC)
– External arming (BNC)
– Power supply inlet including EMI filter
– Optional main inputs replacing corresponding front
panel inputs
Hardware Functional Description 4-17
!
!
!
!
!
!
10 MHz OUT
1Vrms IN 50 W
EXT REF
FREQ INPUT
EXT ARM
INPUT E
INPUT A INPUT B INPUT C
GPIB IEEE 488
USB
191125
Figure 4-9
Rear panel, PM6690.