238 N9030B PXA Signal Analyzer Service GuideAnalog/Digital IF TroubleshootingA3 Digital IF Assembly Theory of OperationA3 Digital IF Assembly Theory of OperationData AcquisitionThe 22.5 MHz IF comes from the A2 Analog IF assembly. The input level tothe A3 Digital IF assembly is −25 dBm when observing the 50 MHzcalibrator signal. The IF input has a 25 MHz bandwidth centered at 22.5MHz. The analog circuitry leading to the ADC converts the singled endedsignal from the Analog IF to differential required by the ADC. In addition, itis 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 samplingcontinuously at 100 Ms/second.The 250 MHz IF comes from the A15 Front End Control assembly. The inputlevel is approximately -25 dBm when observing the 50 MHz calibratorsignal. The IF input has a 40 MHz bandwidth centered at 250 MHz. Theanalog circuitry leading to the ADC converts the single-ended signal fromthe Front End Control to differential required by the ADC. Finally, it couplesin the dither signal. The ADC is a 12-bit device sampling continuously at200 Ms/second.Rear Panel TriggersThe board has two trigger inputs and two trigger outputs all used via BNCconnectors on the rear panel. The trigger inputs are used when an externaldevice has a trigger signal and the user wants to use that external trigger totrigger the signal analyzer. The trigger outputs are used to synchronizeother pieces of test equipment to the PXA. These outputs are configurablethrough 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 voltsusing the control DAC. The circuits have relatively high input impedance.The trigger outputs have 50 Ω source impedance with TTL drive levels intono load.Control DACThe control DAC is used to set trigger levels and the gain of thereconstruction system. All three outputs can be adjusted from −2.5 to +2.5volts.Sample Rate GeneratorThe 10 MHz reference signal comes from the A16 Reference Assembly. Thissignal is fairly high power at +10 dBm. The signal goes through a 10 MHz to30 MHz tripler. A 0 to 5V, 10 MHz square wave is generated. Capacitorsform a single-pole band pass filter to select the 3rd harmonic, 30 MHz.Refer to Chapter 13, “Block Diagrams.”.
