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Why does my analyzer work OK sometimes, but give me phase lock errors when I test certain devices?

If the analyzer appears to work normally under some conditions, but under different conditions it starts producing phase lock errors, the problem is probably inadequate power to the Reference channel. The R channel requires a power level between 0 and -30 dBm, with -10 dBm being the optimum value.

Certain instrument setups are particularly prone to this problem, most notably high power testing with external components such as amplifiers and couplers. A common problem is a band limited device somewhere in the R channel signal path.

The problem arises because of an internal analyzer algorithm called phase lock calibration. In a typical scenario, the user has installed an external coupler in the R channel path. The coupler has a frequency bandwidth from 500 Mhz to 5000 Mhz. The user knows this, and sets his start and stop frequency within that band.

But the analyzer constantly monitors the amount of voltage required to achieve phase lock. When this voltage exceeds a set amount, the analyzer executes the phase lock calibration algorithm. This consists of lowering the source to a known frequency, and resetting some constants stored in memory so that accurate phase lock is achieved.

The frequency used for this algorithm is 100 Mhz. Since the low end of the example couplerAA's range is 500 Mhz, it blocks all signal from reaching the R channel receiver, and phase lock is lost.

There recommended workaround is to go into the [System] {Service Menu}{Service Modes} and set {PLL Auto} to OFF.

This is little impact to accuracy provided you recalibrate every day, and the analyzer does not experience temperature changes of more than +/- 3 degrees between calibrations.