The Linear Industries AT8001 digital television exciter
When asked if I’d like to review a digital television exciter, I had flashbacks to an old Radio Corp. of America tube-type low-band VHF exciter from yesteryear. I remember more than once showing up for my transmitter shift (some of you younger readers may not even know what that means), and the guy I was relieving advising me “Good luck, the exciter is acting funny today…” However, that’s what you were dealing with back then, especially when that particular exciter had been placed into service the very same year that I was born. But I digress.
In flashing forward to the present, I promptly reminded myself that we are in the modern era, life is grand and so are modern exciters. I quickly accepted— with enthusiasm—the chance to review a slick new exciter product from Linear Industries, an Elgin Ill.-based provider of TV transmission equipment. The product was their AT8001 ATSC exciter.
The AT8001 is a mobile DTV-ready ATSC exciter, packaged in a 1RU design. It’s A/153 compliant, includes linear and nonlinear pre-correction (with memory effects compensation and crest factor reduction). Applications include DTV, LPTV, and SFN service. The exciter’s power output is +20 dBm max, and there’s linear correction of up to 4 microseconds peak-peak group delay capability. On-board measurements include amplitude modulus, AM/AM and AM/PM, CCDM, constellation (with contour), an eye diagram, frequency response and group delay, power spectrum density, symbols, spectrogram (with contour), and time domain.
The AT8001, as supplied for this review, included an optional scheduler application to run corrections and make measurements, as well as an optional internal GPS receiver and internal OCXO with ±50 ppb frequency stability and 1 Hz frequency step. Further, the user has five options to lock the oscillator based on their hierarchical needs.
For connectivity, the AT8001 is equipped with front and rear USB ports for measurement and precorrection, a front USB port for firmware upgrades, a front panel Ethernet port, two SMPTE 310/ASI inputs, and two ASI outputs from the TS rate adaption and PCR re-stamping circuits.
In addition to the precorrection scheduler, and internal GPS receiver and crest factor reduction already noted, exciter options include dynamic nonlinear correction, and single frequency network over the ATSC A/153 M/H framework, and mobile DTV operation.
A lighted menu display area is available on the front of the AT8001. Just beside the display is a slick up-down-left-right, “enter” button for menu navigation/action, along with a separate (and comforting) “ESC” button. There are four status LED’s between the display and control surface: Power On, Sync Loss, Current Alarms, and Past Alarms.
The rear of the AT8001 reveals most of its signal connectivity, including a power connector/switch and an Ethernet and the previously mentioned USB ports. Rear BNC connectors include two ASI/310 inputs and two ASI outputs, a 1 PPS in and out, and a 10 MHz reference in and out. RF input connections include GPS Antenna, Tuner in, Before Filter, After Filter, Forward Power and Reflected Power. For control I/O, there’s a 20 pin connector on the rear, and a substantial ground lug as well. Outputs include RF Sample and RF Out.
Being old school, the first thing I connected to the AT8001 after racking it was a Bird 50 ohm, 50 Watt Termaline load. After that, I hooked up the AC power, and ran a SMPTE 310 feed into my test area from our station’s encoder. I then powered the unit up, and was rewarded with the setup menu appearing on the display. The “Power On” LED lit up, along with the “Sync Loss” and “Current Alarms”. I connected the 310 feed to one of the rear inputs, and this extinguished the “Sync Loss” and “Current Alarms” LEDs. The “Past Alarms” LED lit at this time.
I had already decided that until I got the unit figured out, I wasn’t going to hook up my Agilent E4407B Spectrum Analyzer and decided that the next order of business was to select a channel on which to operate.
After scrolling the menu to “Frequency Setup”, I found the “Channel” menu and selected Channel 32 (578-584 MHz). Next, I scrolled to the “Power Setup” menu and found it very simple to raise the output power up to +20 dBm. Now I was transmitting, but the dummy load was still cool. I then adjusted the output to −10 dBm and connected the spectrum analyzer to the AT8001 via the “RF Sample” output.
The spectrum display was dead on with the pilot frequency falling right in at 0.309 MHz above the lower frequency limit (578.309 MHz), and I was looking at some nice, low shoulders. I figured it would be a good time to let the unit “cook” a bit, so I powered up my old laptop and tried out IP connectivity to the AT8001. After learning the IP address of the unit, I connected via Firefox. The site does require a username and password, but it’s very straightforward.
The Linear connection application provided a very friendly overview of many of the menu items. There are five pages (or tabs) that deal with including alarms, setup (for power levels), measurements (with a system overview), and remote and user housekeeping information.
For in-depth measurements, the optional advanced measurement GUI Matlab is required, which runs from a command line on your PC. Due mostly to my slow PC, this required a call to Linear to get me up to speed. However, in very short order, I had the Matlab application up and running, and found it very easy to make most common measurements. Matlab enables you to make most exciter measurements, including constellation, constellation contour, eye diagram, power spectrum density, group delay, spectrogram, spectrogram contour, and more.
I kept the AT8001 “on the air,” working into the dummy load for several weeks. Occasionally, as a test of its resilience to power interruptions I’d walk by the unit and pull the power plug for a few seconds and then re-attach it. Each and every time, the unit powered back up and never skipped a beat. The RF signal always looked picture perfect when I checked it. Of course, not having a transmitter connected made the test a bit easier to pass, but, assuming that the corrective abilities of the AT8001 are as solid as the rest of the unit, a bit of fine correction should be a piece of cake. With products like the Linear AT8001, it’s no wonder that we don’t have to “babysit” transmitters anymore.
Joey Gill is chief engineer at WPSD-TV in Paducah, Ky. and has been with the station for 30 years. He has worked in television since 1977. He may be contacted firstname.lastname@example.org.
DTV and mobile DTV exciter, SFN or translator use
One-click automatic linear and nonlinear correction
MSRP $14,000 to $27,000, depending on options
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