5a-8
Theory of Operation
A3 Motherboard
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 A3J31-4 for the FM signal, while the AM signal is available at
A3J31-1 (RF ≤1000 MHz) or A3J31-3 (RF > 1000 MHz).
The controller contains the microprocessor, memory, serial I/O for the other assemblies,
GPIB, 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, A3A1.
The power supply provides ±15 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(F1), +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,
≤12 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 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 Agilent Service
Support Software to download the instrument information to the EEPROM. See
Chapter 7 to regenerate the calibration data using the adjustments.
To Next Page
Loading...
Need help?
General
