Anritsu MT8801 Communication Analyser
Problem
Radio communications analyser performs several functions, including acting as a radio receiver and spectrum analyser. The spectrum analyser function was displaying the frequency spectrum shifted by a random and inconsistent frequency offset.
The spectrum analyser has a heterodyne structure with two stages of mixing. The first local oscillator is swept for tuning, provided by a phase locked loop (PLL) with a YIG oscillator.
The second mixing stage shifts the signal to a low intermediate frequency, using a phase locked loop with a dielectric resonance oscillator (DRO).
Both phase locked loops were becoming unlocked causing the frequency shifts that are seen on the spectrum analyser display.
Diagnosis
Checking the many electrolytic power smoothing capacitors showed many were beginning to fail, the equivalent series resistance was higher than it should be.
All these needed to be replaced to prevent further issues appearing in future, and to reduce power supply noise.
Using a spectrum analyser and frequency counter to monitor the local oscillator behaviour, it was possible to check the stability of the signals while operating the analyser across different frequency ranges.
By examining the feedback to the phase detector in the PLL it became clear that an intermittent loss of signal was causing the unlocked condition. This led us to look for a poor connection.
Solution
Replacing the many capacitors that were beginning to fail improved the stability of the PLLs but did not completely resolve the issues.
Adjusting the DRO plunger of the second local oscillator brought the startup frequency closer to the centre of the PLL capture range and allowed the PLL to remain locked correctly as the unit warmed up.
Part of the swept local oscillator PLL feedback loop was failing and the signal was disappearing. The cause was traced to a broken solder joint, losing bias current for a single transistor amplifier. With this fixed, the signal to the phase detector was reliable and the unit operates correctly.
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