Can't fix it...

DIGITAL TELEVISION

CAN'T FIX IT, IF YOU CAN'T SEE IT
The spectrum analyzer and its place in the systems installation world

by 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 the least of which has been trying to receive over-the-air (OTA) digital TV signals. In a world that has largely migrated away from outside antennas to cable TV, DSS/DBS, or both, the concept of putting up a TV antenna seems quite outdated.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 TV and its widescreen subset, high-definition television (HDTV). 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 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 of some sort.

This, of course, presupposes you have digital TV 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, and actually snagging those broadcasts.

Since I wrote several articles for S&VC on reception of DTV signals, I have received several calls from end-users and dealers/installers asking me to help them with tricky DTV reception problems in New York, Philadelphia, and Los Angeles. In most cases, an existing UHF antenna - and sometimes a preamp - was all that 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, as well as see the effects of signal reflections known as 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, too!)

WHEN I WAS YOUR AGE...

In the old days (that means anytime up to about the early 1990s), 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 in reasonable prices. Mid you; many of these models are stripped-down versions and don't provide all of the functionality of an H-P 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, CA and goes by the model number 2625 (it's one of four such models). The other, recommended by John Ramsay of Ramsay Electronics in Rochester, NY is the Avcom PSA-65C. Avcom, based in Richmond, VA, has been making small, lower-cost SAs for some time now, 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 also setting up DTV antennas. If you recall from my previous columns, ATSC DTV signals have a distinct shape when they are received properly. If multipath is present, the waveform will be degraded and signal drop-out 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 two-fold: (1) Get the strongest signal possible, and (2) get the 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.

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 likely run across (and need):

TUNING - as simple as it sounds. This control sets the center frequency on the display, and 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 MHz, 50 MHz, 20 MHz, 10 MHz, 5 MHz, 2 MHz, 1 MHz, and .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 50+ miles away 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 (and that's not cheap to fix!). All analyzers come with selectable, stepped attenuators to prevent overload. These usually come in 10 dB steps.

SWEEP RATE - How fast the analyzer sweeps 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.

MARKER 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, the cluster of functions shown above 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 measurement of signal strength by comparing RF carriers to the graticule divisions.

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 with analyzing RF signals. I've used it to troubleshoot several DTV systems, as well as determine whether a digital cordless phone was operating correctly and also build a notch filter to reduce receiver overload from a local FM broadcast station.

The B&K 2625 has 10 selectable scan widths from 100 MHz down to 100 kHz, and also includes a "Zero Scan" feature - basically, you are tuning in a small part of any displayed carrier. It has four 10 dB attenuators available, a video filter (to improve resolution on cable, DSS, and OTA 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 Model 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 best antenna position or correct signal strength, this isn't as much of a problem.

The Model 2625 does have a nice CRT display that is very easy to photograph for reference. 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.

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 MHz to 1250 MHz (with the 10 kHz 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 would include 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 darkened rooms or in the shade. There are 6 selectable span widths - 125 MHz, 50 MHz, 10 MHz, 5 MHz, 1 MHz, and 200 kHz.

The resolution bandwidth is automatically set for you as you change the span, and ranges from 3 MHz @ 125 MHz/horizontal division down to 75 kHz @ .2 MHz/horizontal division. The optional 10 kHz add-on drops the RBW down to 10 KHz in the .2 MHz/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 the shape of the displayed waveform changed quite drastically as you expanded or decreased the span control. In fact, the IF filters used in the 65C may not have tight enough skirts.

One big advantage of the PSA-65C is its ability to connect to a PC and let you store screen captures as bitmap (.BMP) files for future reference. An optional interface (ADA-10A) 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'll have to keystroke in data such as the center frequency and span width.

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 5 bands. You can also add that functionality to the PSA-65C with the optional and outboard frequency extender series - they also cover up to 4.2 GHz for DSS/DBS installation and troubleshooting.

WRAPPING UP

Both companies provide excellent documentation for their spectrum analyzers. The Avcom products also include several sample waveforms of satellite signals so you'll know what you are looking for. Other examples of waveforms are scattered throughout this article, and most of 'em were captured bitmap files from the PSA-65C. I use a Kodak DC-290 digital camera to shoot screen captures off the B&K 2625, and once you have the focus and framing set, it works quite well.

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. (Yes, they work with digital cable, too!) If you 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 troubleshoot any problems in short order - and get on to the next job.

Copyright ©2001 Peter H. Putman / Primedia Business Publications.
This article appears in the September 2001 issue of Sound and Video Contractor.


						DIGITAL TELEVISION
					CAN'T FIX IT, IF YOU CAN'T SEE IT The spectrum analyzer and its place in the systems installation world


					by 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 the least of which has been trying to receive over-the-air (OTA) digital TV signals. In a world that has largely migrated away from outside antennas to cable TV, DSS/DBS, or both, the concept 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 TV and its widescreen subset, high-definition television (HDTV). 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 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 of some sort. This, of course, presupposes you have digital TV 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, and actually snagging those broadcasts. Since I wrote several articles for S&VC on reception of DTV signals, I have received several calls from end-users and dealers/installers asking me to help them with tricky DTV reception problems in New York, Philadelphia, and Los Angeles. In most cases, an existing UHF antenna - and sometimes a preamp - was all that 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, as well as see the effects of signal reflections known as 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, too!) WHEN I WAS YOUR AGE... In the old days (that means anytime up to about the early 1990s), 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 in reasonable prices. Mid you; many of these models are stripped-down versions and don't provide all of the functionality of an H-P 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, CA and goes by the model number 2625 (it's one of four such models). The other, recommended by John Ramsay of Ramsay Electronics in Rochester, NY is the Avcom PSA-65C. Avcom, based in Richmond, VA, has been making small, lower-cost SAs for some time now, 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 also setting up DTV antennas. If you recall from my previous columns, ATSC DTV signals have a distinct shape when they are received properly. If multipath is present, the waveform will be degraded and signal drop-out 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 two-fold: (1) Get the strongest signal possible, and (2) get the 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. 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 likely run across (and need): TUNING - as simple as it sounds. This control sets the center frequency on the display, and 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 MHz, 50 MHz, 20 MHz, 10 MHz, 5 MHz, 2 MHz, 1 MHz, and .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 50+ miles away 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 (and that's not cheap to fix!). All analyzers come with selectable, stepped attenuators to prevent overload. These usually come in 10 dB steps. SWEEP RATE - How fast the analyzer sweeps 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. MARKER 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, the cluster of functions shown above 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 measurement of signal strength by comparing RF carriers to the graticule divisions. 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 with analyzing RF signals. I've used it to troubleshoot several DTV systems, as well as determine whether a digital cordless phone was operating correctly and also build a notch filter to reduce receiver overload from a local FM broadcast station. The B&K 2625 has 10 selectable scan widths from 100 MHz down to 100 kHz, and also includes a "Zero Scan" feature - basically, you are tuning in a small part of any displayed carrier. It has four 10 dB attenuators available, a video filter (to improve resolution on cable, DSS, and OTA 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 Model 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 best antenna position or correct signal strength, this isn't as much of a problem. The Model 2625 does have a nice CRT display that is very easy to photograph for reference. 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. 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 MHz to 1250 MHz (with the 10 kHz 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 would include 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 darkened rooms or in the shade. There are 6 selectable span widths - 125 MHz, 50 MHz, 10 MHz, 5 MHz, 1 MHz, and 200 kHz. The resolution bandwidth is automatically set for you as you change the span, and ranges from 3 MHz @ 125 MHz/horizontal division down to 75 kHz @ .2 MHz/horizontal division. The optional 10 kHz add-on drops the RBW down to 10 KHz in the .2 MHz/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 the shape of the displayed waveform changed quite drastically as you expanded or decreased the span control. In fact, the IF filters used in the 65C may not have tight enough skirts. One big advantage of the PSA-65C is its ability to connect to a PC and let you store screen captures as bitmap (.BMP) files for future reference. An optional interface (ADA-10A) 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'll have to keystroke in data such as the center frequency and span width. 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 5 bands. You can also add that functionality to the PSA-65C with the optional and outboard frequency extender series - they also cover up to 4.2 GHz for DSS/DBS installation and troubleshooting. WRAPPING UP Both companies provide excellent documentation for their spectrum analyzers. The Avcom products also include several sample waveforms of satellite signals so you'll know what you are looking for. Other examples of waveforms are scattered throughout this article, and most of 'em were captured bitmap files from the PSA-65C. I use a Kodak DC-290 digital camera to shoot screen captures off the B&K 2625, and once you have the focus and framing set, it works quite well. 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. (Yes, they work with digital cable, too!) If you 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 troubleshoot any problems in short order - and get on to the next job. Copyright ©2001 Peter H. Putman / Primedia Business Publications. This article appears in the September 2001 issue of Sound and Video Contractor.