CAN'T FIX WHAT YOU CAN'T SEE

Oct 1, 2001 12:00 PM, PETER H. PUTMAN, CTS

The transition to an all-digital television system in this country has created all kinds of headaches for both commercial and residential systems integrators, not least of which has been trying to receive over-the-air digital TV signals. In a world that has largely migrated away from outside antennas to cable TV, DSS/DBS or both, the idea of putting up a TV antenna seems quite outdated.

But that's the way the game is played if your clients want to watch digital television and its widescreen subset, high-definition television. With few exceptions, the majority of broadcast DTV signals are only available by using an antenna (inside or outside) and a set-top receiver. True, you can get HBO's and Showtime's HDTV movie offerings, plus CBS' HDTV programming via DSS in selected areas. But you can't get NYPD Blue, The Practice, The Tonight Show, or other HD offerings unless you put up an antenna.

This, of course, presupposes you have digital television broadcasts available in your area. If you're located in one of the top 30 markets, chances are you do have DTV broadcasts available locally. The catch is to figure out the right antenna with which to receive them.

Since writing several articles for S&VC on reception of DTV signals, I have received calls from dealers, installers and end-users asking me to help them with tricky DTV reception problems in New York, Philadelphia and Los Angeles. (Use the search engine at the top of this screen to view these articles in my archives.) In most cases, an existing UHF antenna — and sometimes a preamp — was all it took to fix the problem. But I can say that the process would have taken much longer had it not been for a very useful tool: the spectrum analyzer.

Think of a spectrum analyzer as a sophisticated receiver/scanner, capable of tuning in signals across a wide range of radio frequencies. By using special circuitry inside the receiver's intermediate frequency (IF) and detector/demodulator stages, we can actually see the presence of a signal or signals. We can tell how strong they are and whether they are amplitude- or frequency-modulated.

We can also tell if interfering signals are present and see the effects of multipath. With an analyzer, we can detect sources of RF interference such as power lines, computers and other electronic gizmos. Filters to tune out or remove such interference can be tuned up quickly with an analyzer. (Yes, we can even find wireless microphone bugs and hidden transmitters!)

BACK IN THE DAY…

In the old days (meaning any time before around 1992), the purchase of a spectrum analyzer was an expensive proposition. As a ham radio operator, there was nothing I coveted more than a Hewlett-Packard analyzer with its companion tracking generator. Unfortunately, its $50,000 price tag left me a little cold, so I depended on the goodwill of some employees at a nearby HP facility to let me come in after hours and use their box (plus their way-cool anechoic chamber for antenna tests).

It wasn't until recently that spectrum analyzers became available at reasonable prices. Mind you, many of these models are stripped-down versions that don't provide all of the functionality of an HP or Tektronix box. But in many cases, you don't need a laboratory-grade model; you just need something to keep you from stumbling around in the dark.

IN THE SPOTLIGHT

Two such models have crossed my test bench lately. The first, suggested to me by Jim Burns of Burns Digital Consulting, is made by B&K Instruments of Placentia, California, and goes by model number 2625 (it's one of four such models). The other, recommended by John Ramsay of Ramsay Electronics in Rochester, New York, is the Avcom PSA-65C. Avcom, based in Richmond, Virginia, has been making small, lower-cost SAs for some time and recently merged its operations with Ramsay.

Both models are functional enough to be a part of your tool kit, particularly if you are installing DSS/DBS dishes and setting up DTV antennas. You may recall from my previous columns that ATSC DTV signals have a distinct shape when they are received properly. If multipath is present, the waveform will be degraded and signal dropout may occur. If multipath is severe, there may be no reception at all.

The best way to take the guesswork out of a DTV antenna install — inside or outside — is to use the power of the spectrum analyzer to position and aim your antenna. The goal with DTV reception is to get the strongest and cleanest signal possible. You may not be able to achieve both, but the best combination of the two (as well as a current-generation, set-top receiver) will usually do the trick. Figure 1 shows a good 8VSB signal and one with severe multipath.

YOUR ANALYZER AND YOU

By careful adjustment of the settings on the analyzer, you can tell a lot about the quality of the signals you are receiving. This goes for cable TV, DBS/DSS, FM broadcasts, and even cordless phones and “rabbits.” Let's take a quick tour of the functions you'll use:

Tuning

As simple as it sounds, this control sets the center frequency on the display. Depending on the analyzer, you can have it read out to 100, 10 or even 1 kHz (on more expensive models).

Span

This control determines how much of a view you'll have or how wide a range of frequencies the analyzer will be scanning (sweeping) and displaying. Selections are usually in pre-determined ranges, like 100, 50, 20, 10, 5, 2, 1 and 0.5 MHz.

Resolution Bandwidth

This determines how much detail you can see in a given signal or signals. The RBW generally goes down as you scan a wider range of frequencies. Consequently, it can go up higher as you look at a much narrower span of frequencies. On the less-expensive analyzers, the RBW is usually preset.

Reference Level

You can increase the level (amplitude) of a displayed signal by selecting a lower reference frequency. This is useful for analyzing weak signals (like that DTV station over 50 miles away that your client wants to watch). Selecting higher reference levels reduces the amplitude of the received signals on the display.

Attenuators

Spectrum analyzers are sensitive pieces of electronic equipment. If too strong a signal is fed into them, their front-end RF stages can be damaged (not cheap to fix). All analyzers come with selectable, stepped attenuators to prevent overload. These usually come in 10dB steps.

Sweep Rate

How fast will the analyzer sweep through the preset span of frequencies? If you make the analyzer sweep faster, your resolution will go down and the analyzer will be out of calibration. You can sweep a smaller span of frequencies to maintain calibration or select a slower rate in this case.

Market Generator

If your analyzer has one, you can use this control to precisely determine the frequency along any part of a displayed carrier wave.

There are other functions and accessories for analyzers, but for straight-on measurements and troubleshooting, these basic functions are the ones you'll use most often. In addition, some of the inexpensive analyzers provide a calibrated front-panel graticule, or etched grid lines. This lets you make a quick and accurate measurements of signal strength by comparing RF carriers to the graticule divisions. Figure 2 shows an example of a digital phone carrier and a range of FM broadcast signals.

THE B&K 2625

I've owned the B&K 2625 for several months now, and although it lacks some higher-end features, it does a great job analyzing RF signals. I've used it to troubleshoot several DTV systems, as well as to determine whether a digital cordless phone was operating correctly. I also used it to help build a notch filter to reduce receiver overload from a local FM broadcast station. Figure 3 shows a B&K 2625 in operation.

The B&K 2625 has 10 selectable scan widths from 100 MHz down to 100 kHz, and also includes a Zero-Scan feature that allows you to tune in a small part of any displayed carrier. It has four 10dB attenuators available, a video filter (to improve resolution on cable, DSS and over-the-air signals), and selectable resolution bandwidths of 400 kHz or 20 kHz.

There are coarse and fine frequency controls, a digital readout using LED numerals and a marker generator. Additional controls allow adjustment of the intensity and focus of the display as well as its calibration. The useable frequency range is 150 kHz to 1.05 GHz, which takes in all of the VHF, FM, UHF, and cellular frequencies as well as amateur radio, commercial two-way radio, and airport channels.

My biggest complaint about the 2625 is the lack of any engraved calibration on the front-panel graticule. You need to know something about the strength of the incoming signal to determine any calibration, which is a bit of a nuisance. However, if you are using it to determine the best antenna position or correct signal strength, this isn't as much of a problem.

Model 2625 does have a nice CRT display that can be photographed. Not only that, it's a fairly flat CRT display, and the sharpness of the IF filters in the 2625 result in accurate waveform display, no matter what frequency span selection you make. It's fairly lightweight at 15 pounds, but you'll need to have an AC receptacle or power inverter handy to run it.

THE AVCOM PSA-65C

Avcom's PSA-65C is a true portable spectrum analyzer. You can equip it with a battery and go anywhere in the field to make RF measurements. The frequency range is a bit wider than the B&K box, spanning 1 to 1250 MHz (with the 10kHz RBW option), and there's also a built-in demodulator so you can actually listen to analog AM or FM signals in the zero-span mode (this includes TV and FM broadcasts).

The PSA-65C adds a vertical sensitivity control to its reference level adjustment. You can set the display to read out 2 dB or 10 dB per vertical division for really detailed signal analysis. The front-panel frequency display is a monochrome LCD type, which is a little harder to read in darkrooms or in the shade. There are six selectable span widths: 125, 50, 10, 5, 1 and 0.2 MHz.

The resolution bandwidth is set for you automatically as you change the span, and it ranges from 3 MHz at 125 MHz/horizontal division down to 75 kHz at 0.2 MHz/horizontal division. The optional 10kHz add-on drops the RBW down to 10 kHz in the 0.2 MHz/horizontal division mode. For the majority of installation applications, these ranges are sufficient.

The PSA-65C's CRT has a noticeable curve to it, which is somewhat distracting when you don't view the CRT head-on. Also, I found that the shape of the displayed waveform changed drastically as I expanded or decreased the span control. In fact, the IF filters used in the PSA-65C may not have tight enough skirts.

One big advantage is the PSA-65C's ability to connect to a PC and let you store screen captures as bitmap (.bmp) files for future reference. An ADA-10A interface is required, along with some software. What you get is a detailed readout with many of the signal parameters already entered for you. But the software isn't 100% interactive. You have to type in data such as the center frequency and span width. Figure 4 shows a sample screen grab with analog and digital TV signals.

For heavy-duty DSS work, Avcom also makes a spectrum analyzer that scans up from 10 to 1750 MHz and 3.5 to 4.2 GHz in five bands. You can add that functionality to the PSA-65C with the optional outboard frequency extender series. They also cover up to 4.2 GHz for DSS/DBS installation and troubleshooting.

IN SUM

Both B&K and Avcom provide excellent documentation for their spectrum analyzers. The Avcom documentation also includes several sample waveforms of satellite signals so you'll be better equipped to identify what you're looking at and to find what you're looking for. Other examples of waveforms are scattered throughout this article, and most of them were captured bitmap files from the PSA-65C.

Either of these products will probably save you a lot of time and money in setting up any kind of digital TV or satellite system. You'll spend less time in trial-and-error problem-solving if you use an analyzer to help locate the source of the problem. They work with digital cable, too.

If you do decide to buy one, spend some time looking at good DTV and DSS patterns so you can recognize when you've got a bad one. With practice, you'll be able to aim antennas and find and troubleshoot problems in short order — and get on to the next job.

Peter H. Putman owns PHP Communications, Doylestown, Pennsylvania. He is the author of The Toastmasters Guide to Audio/Visual Presentations.