User's Manual

IV-4
Filter
The passive bandpass filter rejects spurious and harmonic output products and passes the VHF
channel RF output. The cavity resonator uses aperture coupling and is a linear resonator design.
Typical insertion loss is 0.6 dB to 1.0 dB depending on channel frequency. Average roll off is –33
dBc at a point 4.5 MHz below the peak visual carrier frequency and –30 dBc 9.0 MHz above the
peak visual carrier frequency. The filter is DC grounded on both the input and output for additional
lightning protection.
Directional Coupler
The Technalogix dual directional couplers provide DC voltages proportional to forward and reflected
RF power monitoring. These analog voltages are converted for processing using analog-to-digital
converters and provide the control system with valuable data for monitoring purposes. The
directional couplers installed in the power amplifier and filter enclosures have peak detection
circuits on the forward RF power side of the coupler and average detection circuits on the reflected
RF power side of the coupler. This is to allow the end user to set power in a manner that is more
independent of modulation and closer to a true tip-of-sync meter. Hence the readings of the
displays in the power amplifier system are peak for forward and average for reflected. Output
power should be set following the operating procedure found elsewhere in this manual. The
directional coupler has a typical insertion loss of 0.5dB and its Type N connectors can handle 1,500
watts peak.
Isolator
The power amplifier pallets are protected in part by the isolator located in the filter enclosure. It is
actually made up of a circulator and 50-ohm dump resistor. The circulators’ specifications include
an insertion loss of less than 0.2dB with an isolation rating better than 20dB. Any reflected power
gets dumped into the flanged power resistor. Even though the flanged power resistor is rated for
only 150-watts, and there could potentially be 500-watts being reflected back into the circulator, the
software will recognize quickly that reflected power is present and turn the carrier off. This way,
there is instantaneous protection due to the isolator setup and long term protection due to the
software.
TAV-250