User's Manual - Crown Broadcast

4.5 Metering CircuitThe ALC and metering circuitry is on the motherboard (see Illustration 6–6). This circuitprocesses information for the RF and DC metering, and produces ALC (RF level-control)bias. It also provides reference and input voltages for the digital panel meter, voltages forremote metering, fan control, and drive for the front-panel fault indicators.Illustration 6–6 and accompanying schematics complement this discussion.PA voltage and current come from a metering shunt on the power regulator board. The PAIinput is a current proportional to PA current; R153 converts the current to voltage used formetering and control. A voltage divider from the PAV line is used for DC voltage metering.Metering SectionIllustration 4–3 Metering CircuitU23A, U23B, and U24A, with their respective diodes, are diode linearity correction circuits.Their DC inputs come from diode detectors in the RF reflectometer in the RF low-pass filtercompartment.U24B, U24C, are components of a DC squaring circuit. Since the DC output voltage ofU24C is proportional to RF voltage squared, it is also proportional to RF power.U22C, U22A, U20A, and U22D are level sensors for RF power, reflected RF power, PAtemperature, and external PA current, respectively. When either of these parameters exceedsthe limits, the output of U22B will be forced low, reducing the ALC (RF level control)voltage, which, in turn, reduces the PA supply voltage.The DC voltage set point for U22A (reflected RF voltage) is one-fifth that of U22C (forwardRF voltage). This ratio corresponds to an SWR of 1.5:1 [(1+.2)/(1– .2)=1.5]. The U25 invertersdrive the front panel fault indicators.Principles of Operation4-7