Fluke PM6690, Service Manual, Page: 36 / 200

Hardware Functional Description
Front Unit
The front unit consists of a front piece in molded aluminum, a
silicon rubber keypad with conducting contact surfaces, a
graphic LCD with LED backlight, and a PCB having etched
gold-plated keyboard contacts and a dedicated LCD power
supply.
Display
The display is a 320*97 pixels graphical LCD with LED
backlight. The LCD controller is part of the processor, and it
sends data and control signals to the drivers in the LCD mod-
ule. Display ON is controlled via the I
2
C bus and the keyboard
IC. The LCD voltages are generated by a DC/DC converter.
Five different voltages are generated. A control signal from
the processor switches the converter ON/OFF and also sets
the contrast of the display.
The 4 LEDs on the display board are controlled from the mea-
suring logic and the ON/OFF circuit.
Keyboard
The PCB covers the back of the front unit. The keys are laid
out as a crosspoint switch matrix consisting of 25 regular push
buttons plus a special power button. When one of the buttons
is depressed, the dedicated keyboard IC U3 responds by send-
ing an interrupt to the processor. The processor scans the key-
board over the I
2
C bus to find out which button calls for atten-
tion. The power button differs from the others by having a
dual function. In
Standby Mode it turns on the ON/OFF circuit
directly, but when the counter is ON, it is read as any other
button. Then the processor turns off the counter. This arrange-
ment is necessary since the processor is inactive in standby
mode.
Main Board
Input Amplifiers
Input amplifiers A and B are identical with >300 MHz band-
width. They are controlled by the processor with relays etc.
The analog input signal is transformed to a digital signal. The
output signals are LVPECL (+2.4 V and +1.2 V approxi-
mately) and fed to the measuring logic.
For a block diagram of the input amplifiers, see Figure 4-8.
n
Impedance Selector
This stage selects 1 M
W
or 50
W
input impedance with a relay.
In 50
W
mode 11 resistors are used for dissipating the input
power, up to a maximum of 2.9 W (12 V
rms
).
n
Attenuator
This stage has two parts, a fixed preattenuator (approximate
attenuation
2.5´
) and a cascaded relay-operated
1´
/
10´
step
attenuator. The variable capacitors are used for adjusting the
frequency response of the attenuators.
n
AC/DC
This stage selects AC or DC coupling with a relay.
n
Limiter
The voltage limiter protects the impedance converter against
overvoltage. The
±
5 V applied to the input BNC is divided to
approximately
±
2.1 V by the attenuator. The limiter clamps
the voltage to approximately
±
2.8 V.
n
Impedance Converter
Split-band technique is used for achieving good frequency re-
sponse over a wide range. The HF signal is fed via an
AC-coupled FET stage. The LF signal (bandwidth DC to ap-
proximately 10 kHz) goes via an operational amplifier. The
signals are added together at the source of the FET. The output
signal from the buffer stage (see below) is fed back to the op-
erational amplifier. A trimmer potentiometer is used for
equalizing the gain in the two signal paths (approximately
´
0.9).
n
Filter
A lowpass RC filter with an approximate cutoff frequency of
100 kHz can be switched in via a transistor.
n
Buffer
Before the signal is fed to the Crossover Switch , it passes a
current-amplifying buffer stage that can drive the following
low impedance stages.
4-14 Hardware Functional Description