HP 8672S Operating And Service Manual page 96

Models 8672S/86720A
Service
SERVICE SHEET 1 (cont'd)
This leveling is accomplished by digitally sam¬
pling and holding the output of the integrator.
When the FEU is switched to pulse modulation,
the Analog-to-Digital Converter (ADC) control
enables the ADC to accept 80 kHz clock pulses.
The analog output of the integrator is converted to
its digital equivalent. The output of the ADC is
then clocked into Digital-to-Analog Converter
(DAC). The ADC control now disables the ADC
and the digital sample-and-hold is complete. The
analog output of the DAC is the same as the output
of the integrator when it was sampled. This value
is retained until the FEU is cycled out of and then
back into pulse modulation.
The CW/Pulse control logic determines which
FET switch is active. During CW operation, the
continuously updated analog signal is applied to
the exponential amplifier. During pulse operation,
the stored analog output from the DAC is used.
The dBm meter and the LVL UNCAL LED are
used only in low band CW operation. If the ALC
loop is unable to level the RF power, the unleveled
detector lights the LVL UNCAL LED and discon¬
nects the dBm meter. During pulse or high band
operation the meter is disconnected and the LVL
UNCAL LED is disabled.
Pulse Modulation
The FEU provides pulse modulated output in both
bands. However, only the low band pulse output is
leveled. In both cases, an external pulse input to
the pulse modulator drive circuits is required.
The modulator drive circuits provide the signals
necessary to switch the selected pulse modulator
circuit on and off. To achieve rapid rise and fall
time on the RF output pulses a series/shunt type
modulator is used. The pulse on/off logic and
driver circuits provide the series drive to the bias
tee. The pulse modulation driver circuit provides
the shunt drive to the pulse modulator.
There are separate bias tees and pulse modulators
for the high and low bands. The drive signals are
routed to the selected pulse modulation path by a
relay controlled by the low band signal.
Power Supplies
The FEU contains the following three power
supplies:
a. The main power supply that supplies +24
Vdc and +5 Vdc throughout the instrument. It is
located on the A6 Motherboard Assembly.
b. The pulse driver power supply that supplies
+12 Vdc, -12 Vdc and +5.2 Vdc to the pulse driver
board. It is located on the Al Pulse - Driver
Assembly.
c. The ALC power supply that supplies +20
Vdc and -10 Vdc to the ALC Amplifier Assembly
and some of the RF Path circuits. It is located on
the A9 ALC Power Supply Assembly.
CAUTION :
DO NOT remove or insert any board
assembly with the instrument LINE
switch turned on. Always turn the
switch to OFF prior to removing or
inserting a board. There are some com¬
ponents that could be damaged by tran¬
sients generated when power is on.
If the instrument is operating correctly in the local
mode but does not operate correctly during HP-IB
operation, begin troubleshooting by performing
the procedures on Service Sheet 2.
If the instrument is not operating correctly in the
local mode, begin troubleshooting by performing
the preliminary check and power on procedure
below.
Preliminary Check and Power On Procedure
1. Verify the Signal Generator and the FEU are
correctly interconnected. Refer to Section 2.
2. Apply power to the Signal Generator. Refer to
Section 3 of the HP 8672A Operating and Ser¬
vice Manual. If power cannot be applied to the
Signal Generator, troubleshoot it as directed in
the manual.
3. Set the FEU LINE switch to ON. Verify the CW
LED lights. If power cannot be correctly ap¬
plied to the FEU, perform the power on trouble¬
shooting procedure below.
If power can be correctly applied to the system,
perform the Signal Generator Operational Verifi¬
cation Test in Section 4.
8-15