HP 8648 A/B/C/D - Changpuak.ch

A2 Power Supply The power supply is a switching power supply producing 4 voltages;
+ 5 V, + 15 V, - 15 V, and + 38 V. The switching supply will only
regulate when connected to a load. The power supply receives mains
(line) voltage through the power switch on the front panel and the
line module on the rear panel. Post regulation on the A3 assembly
produces f12 V, filtered +5 V, +5 V, +24 V and +8 V. The +24 V
and +8 V are used only in the HP 8648B/C/D.
A3 Motherboard The motherboard contains four functional blocks: modulation
distribution, controller, post regulation, and diagnostic latching,
The modulation distribution block produces two level-calibrated
modulation frequencies, 1 kHz and 400 Hz. The frequencies are
derived by dividing a 200 kHz signal from the A4 reference module
by 200 and 500. The internal signals are leveled by an amplifier and a
12-bit DAC. When FM is enabled, it is routed to the A5 sig gen synth
module. When AM is enabled, it is routed to the A6 output module
when the RF output frequency is _< 1000 MHz. When the RF output is
> 1000 MHz, the AM signal is routed to the A10 frequency extension
module.
When internal modulation is turned on, the signal is also routed to
the front panel MOD INPUT/OUTPUT connector. External signals
are also routed through the modulation distribution block. External
signals can be either ac- or dc-coupled. The modulation signals can be
checked at A3531-4 for the FM signal, while the AM signal is available
at A3J31-1 (RF _< 1000 MHz) or A3531-3 (RF > 1000 MHz).
The controller contains the microprocessor, memory, serial I/O for the
other assemblies, HP-IB, front panel control, and serial interface for
the external control options. Memory is broken into four sections;
EEPROMs, ROM, volatile SRAM and non-volatile battery-backed
SRAM. The ROMs are contained on a separate memory board, A3Al.
The power supply provides f15 V, + 5 V, and + 38 V to the
motherboard. The post regulation circuitry contains regulators for
+ 12 V, -12 V, + 5 V (shown as + 5 V(Fl), +24 V, and +8 V. There is
additional circuitry to derive a filtered +5 V from the power supply
for the digital circuitry. The + 12 V, + 8 V, and + 5 V regulators
are driven by the + 15 V supply from the A2 assembly. The - 12 V
regulator is driven by the - 15 V supply and the + 24 V regulator
is driven by the + 38 V supply. There are indication LEDs on the
motherboard for the +38 V, 312 V, + 5 V, and +5 V(F1) power
supplies.
The diagnostic latches latch signals from around the instrument that
are read by the controller during power-up.
The motherboard contains calibration EEPROM that is specific to this
instrument. If the A3 assembly is replaced, information specific to
that HP 8648 must be downloaded to the calibration EEPROM and
then the calibration data must be regenerated. Refer to “Motherboard
Repair Utility” in Chapter 7 for information on the using the HP
Service Support Software to download the instrument information to
the EEPROM. See Chapter 7 to regenerate the calibration data using
the adjustments.
Theory of Operation 5a-5