Lab.gruppen PLM 20000Q - Manual (Page 30)

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Operation and Performance
PLM Series Operation Manual rev 1.2.3
VHF protection is dependent on a combination of output power level and frequency. Figure 5-2 shows a
decreasing power threshold, from approximately 10 kHz upwards, which illustrates increasing sensitivity of
the protection system with frequency. When continuous output power above the threshold line is detected,
VHF protection becomes active.
Figure 5-2: VHF Protection Frequency Sensitivity
The attack time of the VHF protection circuitry also changes with frequency, becoming shorter at higher
frequencies. This is shown in Figure 5-3.
Figure 5-3: VHF Protection Attack Time Variations
The VHF protection circuit is NOT a limiter and does not alter the PLM’s frequency response. It is imple-
mented solely to detect continuous VHF content. HF content of normal music or speech signals at peak
levels will be passed in full.
Operation of the VHF protection circuits is indicated by one (or more) of the output channel LEDs (in the
right-hand soft function buttons) showing steady red. The adjacent fault message will show VHF FAULT. It is
also reported as a fault via the control network.