This video documents the diagnosis and repair of an Agilent N5182A MXG Vector Signal Generator exhibiting a "fractional-N PLL unlocked" error across multiple frequency bands. The diagnostic process begins by characterizing the failure using a spectrum analyzer to map the working and non-working frequency ranges. Guided by the instrument's block diagram, a systematic investigation is conducted into the core synthesizer's PLL loop. Key components, including the VCO, switchable filter banks, and the custom fractional-N divider, are tested using an active probe, a vector network analyzer, and open-loop voltage injection techniques, ultimately proving them to be functional. Suspicion then falls on the custom gallium arsenide (GaAs) phase detector. After this component is replaced with one from a donor board, the synthesizer's functionality is fully restored across its entire 6 GHz range. A final phase noise measurement confirms that the instrument's performance is successfully restored to near-datasheet specifications.
Diagnosing and Repairing a Faulty Agilent Signal Generator
00:00:09 The Problem: The video tackles the repair of an Agilent N5182A signal generator that fails to produce an output in specific frequency ranges, displaying a "fractional nll been unlocked" error.
00:01:55 Visualizing the Fault: Using a spectrum analyzer's max hold feature while sweeping the generator's frequency, the fault is visualized as distinct bands where the signal is present and where it disappears. This pattern provides a clear map of the failure.
03:18 Analyzing the Block Diagram: The instrument's block diagram reveals that the core synthesizer relies on a Voltage-Controlled Oscillator (VCO) operating from 750 MHz to 1.5 GHz. Its signal is then multiplied, divided, and filtered to cover the full range. The fault is traced to this main fractional-N PLL loop.
11:46 On-Board Investigation: The main RF board is examined to identify the physical components of the PLL: the discrete VCO, doublers, switchable filter banks, a custom fractional-N divider IC, and a phase detector.
16:50 Live Signal Probing: With the generator operating in a working frequency range (2 GHz), an active probe is used to trace the signal path. The VCO output (1 GHz), the doubled signal (2 GHz), and the filtered signal are all verified to be correct, confirming the basic signal path is intact.
21:47 Ruling Out the Filter Bank: The PLL unlock occurs near a filter bank transition point. To investigate, the filter bank section is isolated and its S-parameters are measured with a Vector Network Analyzer (VNA). The filters and switches are found to be working correctly, ruling them out as the cause.
26:24 Testing the VCO: The PLL loop is manually broken by injecting a DC voltage to control the VCO's tuning line. Probing its output reveals the VCO can be tuned across its entire required frequency range (750 MHz - 1.5 GHz), proving the VCO itself is not faulty.
30:33 Testing the Fractional-N Divider: In another open-loop test, a fixed frequency is fed into the custom fractional-N divider while the instrument is programmed for that same frequency. The divider correctly produces the required 50 MHz output for the phase detector, indicating it is also functional.
33:11 The Final Suspect: With all other major components in the loop verified, the final suspect is the custom Agilent gallium arsenide (GaAs) phase detector. The hypothesis is that its gain characteristic has degraded over time, causing loop instability in certain frequency ranges.
35:20 The Fix and Verification: The phase detector IC is carefully removed and replaced with a component from a donor board. Upon powering on, the signal generator immediately works correctly across its entire frequency range, confirming the diagnosis.
38:03 Final Performance Check: A phase noise measurement is performed to ensure the repair did not compromise performance. The results are compared to the datasheet and found to be very close, with only minor deviations, confirming a successful and complete repair.