238 N9030B PXA Signal Analyzer Service Guide
Analog/Digital IF Troubleshooting
A3 Digital IF Assembly Theory of Operation
A3 Digital IF 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 when observing the 50 MHz
calibrator signal. 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 16 bit device sampling
continuously at 100 Ms/second.
The 250 MHz IF comes from the A15 Front End Control assembly. The input
level is approximately -25 dBm when observing the 50 MHz calibrator
signal. The IF input has a 40 MHz bandwidth centered at 250 MHz. The
analog circuitry leading to the ADC converts the single-ended signal from
the Front End Control to differential required by the ADC. Finally, it couples
in the dither signal. The ADC is a 12-bit device sampling continuously at
200 Ms/second.
Rear Panel Triggers
The board has two trigger inputs and two trigger outputs all used via BNC
connectors on the rear panel. 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 PXA. 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.
Refer to Chapter 13, “Block Diagrams.”.
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