Anritsu 37 D Series, Руководство по обслуживанию, Страница: 55 / 190

A7 PCB, LO3 The A7, Third Local Oscillator (LO3) Assembly, provides a fixed
10 MHz Reference Timebase and a 2.42 MHz Local Oscillator signal
that is used on the A3, A4, and A6 PCBs to down-convert the 2.5 MHz
IF signals to 80 kHz. It also provides an 80 kHz standard signal for
the IF Section Calibration process that occurs automatically
approximately every six minutes. This automatic IF Section
Calibration is one of the VNA features that ensures rated
measurement accuracy. Automatic IF Calibration can be turned off
and/or invoked at any time during measurement sweeps.
A5 A/D Converter PCB The A5 A/D Converter PCB contains a six-channel, two stage,
switched-filter sample-and-hold circuit and a 20 bit A/D converter.
Each of the six DC signals from the A3, A4, and A6 PCBs are input to
a separate channel of the PCB. The first stage of each channel is a
low-pass filter with four selectable cutoff frequencies of 10 kHz, 1 kHz,
100 Hz, and 10 Hz. The second stage of each channel is a sam-
ple-and-hold amplifier that stores the signals during the A/D conver-
sion process. Each channel is sequentially selected for input to the
20 bit A/D converter.
The A5 A/D Converter PCB also derives the 109.89 kHz Power Supply
Synchronization Signal and the 80 kHz IF Synchronization Signal
from the 10 MHz Reference Timebase. Additional functions of the A5,
A/D Converter Assembly include:
q
Measurement of power supply voltages and other internal nodes
of the 37XXXD for diagnostic purposes.
q
Measurement of an externally applied analog input signal. This
function is used for service purposes only.
q
External Trigger Input signal processing (from rear panel)
q
External Analog Output signal generation (to rear panel)
The A/D converter circuitry located on the A/5 PCB is used as a DVM
to measure various internal system analog monitor points on the
A1 to A8 and A21A1/ A21A2 PCBs. It is also used to monitor power
supply voltages and other critical points throughout the VNA, which
can be readout via the Diagnostics Menus. DVM readings are also re-
corded in the service log for certain system failures.
NOTE
If this PCB is replaced, switch settings on the new PCB must
be changed to match settings of the old PCB.
THEORY OF OPERATION ANALOG SUBSYSTEM ASSEMBLIES
37XXXD MM 3-19