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If you've installed an FM band HD Radio transmitter then you likely relied upon the unit's power meter (calibrated at the factory) to set the digital power level. You may very well want another means, though, to monitor digital power. And since the in-line wattmeters we've all used over the years are not adequate for measuring the power out of an HD Radio transmitter (because wattmeters associated with these transmission line sections are calibrated to read average power, and don't work well when there is a high peak-to-average ratio as there is with the IBOC technology we are using) you'll probably want to consider the purchase of a new wattmeter.
Coaxial Dynamics offers the 91090 Digital Wattchman monitor and alarm system. This unit is similar in function to its more familiar 81090 series, but Digital Wattchman makes use of two digital plug-in elements so it can monitor forward and reflected power simultaneously. (Using the digital plug-in elements allows the user to measure the power of complex modulations schemes such as those used in DAB and DTV.) The 3RU display unit includes two 4.5" mirrored analog meter movements. Accuracy of the power measurement is specified as ±5 percent; alarm response time for the unit (including the time for necessary for the relay contacts to move) is specified as 15ms. Amplified dc outputs are also included so your remote control can be connected directly.
Bird Electronics offers the BPM-E (Broadcast Power Monitor-Enhanced) for the measurement of complex RF waveforms such as those encountered in the transmission of HD Radio. The BPM-E comes mounted to a Bird Thru-line section, and has the simultaneous forward and reflected monitoring capability expected today. It also includes an integral RF test port that allows for convenient testing of the spectrum of the RF mask. Both RS-232 and Ethernet communications means are included for remote monitoring via a Web interface. The unit interfaces with a Bird 3129 display unit (1RU) so power can be read locally at the unit.
After installing and commissioning an HD Radio transmitter, a proof of performance needs to be run. A spectrum analyzer is needed for this, and perhaps you'll be lucky enough to have one at the ready. Fortunately there are several relatively inexpensive analyzers out there that fill the bill.
First is the Boonton 9102, which is a hand-held device that measures from 100kHz up to 4.0GHz (optionally up to 7.5GHz). The display features Resolution Bandwidth from 100Hz to 1MHz, and video bandwidth from 10Hz to 1MHz; and one of its power measurement capabilities is known as channel power — probably the most accurate way of measuring the HD Radio power spectrum. The 9102 also has an Ethernet interface that allows TCP/IP communication via a LAN or WAN.
Another instrument manufacturer with a real legacy in broadcast engineering is Agilent Technologies (formerly known as HP). One of its newest offerings is the N1996A CSA spectrum analyzer. This unit covers from 100kHz to 3GHz (optionally to 6GHz) with RBW in 10 percent adjustable steps between 10Hz and 200kHz, and then in steps of 250kHz, 300kHz, 1MHz, 3MHz and 5MHz. VBW can be set between 1Hz and 8MHz. This unit can also perform the channel power type of measurement, and features Ethernet connectivity as well as USB connectivity.
LP Technologies offers the LPT-3000 spectrum analyzer. This device covers from 9kHz through 3GHz, measures channel power, has the appropriate resolution and video bandwidth settings, and even has pre-programmed Ibiquity-specified RF masks. It also has storage space for up to 900 waveforms and it can convert its measured data to an Excel file format. Communication with the unit can be made via serial or USB, with Ethernet as an option. LP Technologies also makes a rackmount version of the same analyzer (LPT-3000R) that is specifically designed to live at a remote site. This unit has an optional four-input RF switcher that gives it the ability to multi-task. It is also priced at less than half the cost of most other analyzers.
HD Radio is a modern technology that requires the station engineer to learn about new techniques in RF communications. Let's face it: AM and FM technology date from the 1920s and 1930s, and share very little technologically with digital radio. The inclusion of HD Radio necessitates new test and monitoring capabilities; fortunately the proliferation of digital transmission has attracted many manufacturers to build the necessary equipment, thus keeping the prices reasonable. That benefits us as broadcasters, and by extension, our listeners as well.
Irwin is the chief engineer of WKTU-FM, New York City.
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