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Additional resources for CROWN FM500 Broadcast Transmitter
15 Receiver Circuit Board Option This option allows the transmitter to be used as a translator. The receiver board receives terrestrially fed RF signal and converts it to composite audio which is then fed into the exciter board. Microprocessor controlled phase lock loop technology ensures the received frequency will not drift, and multiple IF stages ensure high adjacent channel rejection. Refer to illustrations 4–6, 6–16 and its schematic for the following discussion. The square shaped metal can located on the left side of the receiver board is the tuner module.
Q5 is a precision-matched transistor pair. Q5 and U13B form a log converter, so that a given voltage change produces a given change in gain control dB of U5 (U6). The log conversion ensures uniform level-processing characteristics well beyond the 20 dB control range. The log conversion has an additional benefit; it allows a display of gain control on a linear scale with even distribution of dB. Q1 (Q2) is a recover/expansion gate with a threshold about 18 dB below the normal program level. The amount of short-term expansion and gain reduction is controlled by R650, located on the front panel display board.
Inductors L1121 and L1122 provide power line filtering. The amplifiers are surrounded by a 50 Ω impedance, input/output combiner board which takes the 15 watts input and divides it equally to each power amp. Then the output from each amplifier is combined to form a single output. 12 Chassis The AC power supply components, as well as the bridge rectifier and main filter capacitor are mounted on the chassis. 2 for switching and fuse information. The board also includes MOV voltage-surge suppressors and in-rush current limiters as well as a 12 volt power supply for the PA assembly cooling fans.