A Primer on Diplexer/Triplexer Adjustment Procedures – Technical Note 118

An increasing number of Medium Wave (AM) diplexers and triplexers are being placed in service. The use of these devices to feed multiple transmitters to common antenna systems may be a cost effective way to minimize the substantial physical plant requirements of MW broadcast stations.

The design and adjustment of such systems is usually quite complicated and should be handled by experienced manufacturers and field engineers. However, it is recognized that maintenance and commissioning personnel may need a general guide to the process, particularly when experienced personnel are not available.

The guide below is not offered as an all-inclusive "cookbook" for multiplex system adjustment. In many ways, this may be closer to art than science. Thus, it should be viewed as an educational tool. Be warned, if a multiplex system is improperly adjusted, restoration of proper functioning may be time consuming and expensive!

• Adjust ATU’s for approximate match to the design impedance. This is likely to be different from the final impedance, so a fine match is not needed. For this step, disconnect the filter chain at the output of the AM antenna tuning units (ATU's)  section and attach a dummy load representing the calculated impedance seen by the output of the ATU. Use the RF bridge to get approximately 50 ohms at the ATU input.

• Adjust pass/reject filters. Initial adjustments may be with doors open or access panels off. Proceed by isolating the filter under test. Ground one end. Separate the pass and reject legs. Bridge the "pass" leg and adjust to zero impedance. Bridge the "reject" leg and adjust for design impedance. Combine the two legs and adjust for maximum impedance at the reject frequency.

An alternate method of adjusting the reject trap is to connect the pass and reject legs of the filter in series, isolated from any other connections. One end is grounded, and the other is measured at the reject frequency. When adjusted for maximum reject, the impedance to ground is zero ohms.

Leaving the legs combined, install the access door(s) and optimize both "pass" and "reject" tuning.

• Measure impedance at ATU. Isolate and terminate ATU/filter chain output in its appropriate antenna impedance. Connect filter chain elements and secure filter door(s). Disconnect ATU connection. Measure impedance of load at input to filter chain. Record. Make engineering evaluation.

Note: Shunt capacitance may substantially alter connected antenna impedances as measured at the ATU. This may require installation of compensators, ATU readjustment or other steps.

• Measure the filter shunt impedance. Terminate each filter chain into it’s ATU. Terminate each ATU with 50 ohms. Measure the impedance looking into each filter chain output at the reject frequencies for that cabinet independently. Record. Make engineering evaluation.

• Validate circuit parameters. Using measurement data, review impact of distributed capacitance on working impedances. If significantly different from design, adjust component setting and/or characteristics to yield operation with desired safety margins.

• Final isolation setup. All filters doors should be closed up, after filters are fully adjusted individually, and the several antenna outputs connected together. For each reject (undesired) test frequency, attach an appropriate load impedance corresponding to the undesired frequency antenna impedance. Using a calibrated signal generator, inject appropriate undesired signal across the antenna load with no other loads attached. Read Dbm power value and adjust for matching loss. Use selective voltmeter at each ATU 50-ohm input and read Dbm at undesired frequency. Record. If rejection is inadequate, enter required filter and make incremental adjustment, close and retest as required to reach desired rejection.

• Final ATU setup. After all modifications are made and system is fully connected, measure appropriate impedances at each ATU, using test loads simulating antenna impedances, one at a time. Using final impedance values, adjust each ATU and measure carrier and +/- 10KHz impedance at ATU input. Record.

• Final evaluation. Make engineering review of all tests and determine that operation will be adequate. Check and record all component values and settings. Validate that working voltage and current margins are adequate in all components. Revise drawings as needed. Photo-document all aspects of the system hardware.