Keysight NFA Series Noise Figure Analyzers Service Guide 261
Analog/Digital IF Troubleshooting
25 MHz BW IF Section
A3 Digital I.F. Assembly Theory of Operation
Data Acquisition
The 22.5 MHz IF comes from the A2 Analog IF assembly. The input level to
the A3 Digital IF assembly is −25 dBm. The IF input has a 25 MHz
bandwidth centered at 22.5 MHz. The analog circuitry leading to the ADC
converts the singled ended signal from the Analog IF to differential required
by the ADC. In addition, it is part of a filter, part of which is on the AIF,
which improves distortion. Finally, it couples in the dither signal. The ADC is
a 14 bit device sampling continuously at 90 Ms/Second.
Rear Panel Triggers
The board has two trigger inputs and two trigger outputs all used via a BNC
connector. The trigger inputs are used when an external device has a
trigger signal and the user wants to use that external trigger to trigger the
signal analyzer. The trigger outputs are used to synchronize other pieces of
test equipment to the analyzer. These outputs are configurable through the
Input/Output menu via the front panel of the instrument.
The trigger inputs each allow trigger levels to be set from −5 to +5 volts
using the control DAC. The circuits have relatively high input impedance.
The trigger outputs have 50 Ω source impedance with TTL drive levels into
no load.
Control DAC
The control DAC is used to set trigger levels and the gain of the
reconstruction system. All three outputs can be adjusted from −2.5 to +2.5
volts.
Sample Rate Generator
The 10 MHz reference signal comes from the A16 Reference Assembly. This
signal is fairly high power at +10 dBm. The signal goes through a 10 MHz to
30 MHz tripler. A 0 to 5V 10 MHz square wave is generated. Capacitors
form a single-pole band pass filter to select the 3rd harmonic, 30 MHz.
The signal then passes through a 30 MHz to 90 MHz tripler. A 0 to 5V
30 MHz square wave is generated. Capacitors form a single-pole band pass
filter to select the 3rd harmonic, 90 MHz.
Refer to Chapter 12, “Block Diagrams.”.
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