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This is a common scenario when troubleshooting a Cassini system. During Cal, a validate fails, you troubleshoot the problem to a bad switch, you replace the switch, you run diagnostics, and now diagnostics fail.



What’s wrong? (Hint: It's probably a Source)
The source (a.k.a. synthesizer) contain as much circuitry as everything else in the test system combined. The fact that they are complex and that there are 4 or 5 of them in a system means the synthesizers are the most common failure item in a system. We don’t expect you to become experts in the design of microwave synthesizers. However, a basic understanding of how the sources work and how they are used in the system will help you troubleshoot source problems and perhaps even repair sources on-site. A synthesizer simultaneously achieves high resolution and low noise through a complex arrangement of phase lock loops. There are 4 loops in a synthesizer, as shown.

The YIG (Yttrium Iron Garnet) oscillator produces the actual output. It is able to be tuned from 2 GHz to 20 GHz. For frequencies below 2 GHz, the YIG oscillator is mixed with a fixed frequency oscillator in the Down-Converter to produce 10 MHz to 2 GHz.

Common Source Failures

A3 Reference loop
    • Certain vintages
    • Time base ‘shifts’ after 1-3 years
Down Converter
    • Dead
    • Phase lock (frequency accuracy) problems
    • ALC (power control) problems

Source Basics

Startup Source Tests
    • Check all loops
    • Check ALC range
    • Single frequency test
Self Test (Control Panel)
    • Check all loops
    • Check ALC range
    • Multiple frequency test

There are many self-tests that help to diagnose source problems. The nature of synthesized sources is such that it is relatively easy to confirm all the loops are properly locked.

The software gives the sources a quick test whenever a system startup is performed. A more comprehensive test can be performed by running the self test in the control panel.

To run the self test, open the control panel, highlight the appropriate source, and select ‘Instrument’ ‘Self Test’. The test system will run the self test and show the results in the programmer’s message window.

Source Troubleshooting Hints

    • Problem only below 2 GHz, likely a down converter
    • Different averages can help diagnose lock problems
    • Power meter can eliminate confusion from receiver issues (remember Murphy’s corollary)
    • Power correct can be turned off (but not all corrections are removed)

With the internal self-tests and your knowledge of sources, source problems are usually straight-forward to find.

One exception can be subtle phase lock problems, where the self test can’t detect the problem.

Averaging Can Find Phase Lock Errors

Changing averages can find source phase lock errors. This is because the average is a vector average. If the source is not phase locked, its phase will be randomly changing with respect to the receiver. When averaged, the result will be a smaller amplitude.

In the diagnostics for each source, there is a phase lock test that performs this function.


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