Digital RF signals require new tools

Nothing deflates a holiday gathering faster than watching youngsters excitedly unwrap much-wanted presents only to discover they can’t play with them because the right kind of batteries aren’t in the house. Public-safety organizations risk the same sort of letdown if they are not careful when structuring bids for new, multimillion-dollar digital wireless networks. All of the robust features are quickly forgotten when coverage is spotty — something that easily can happen when the transition from an analog system to a digital network doesn’t include a corresponding upgrade of the tools used to measure the power of RF signals.

“It’s a problem,” said Tim Holt, director of application engineering for Bird Electronic. “What we have seen is folks trying to use conventional power meters for digital applications, and it just doesn’t work.”

In analog systems, there is little or no difference between peak and average signal strength, so traditional analog power meters work fine. However, the pulsing characteristics associated with digital signals often generate erroneous readings in analog power meters, resulting in an indication that the signal is stronger than it really is, Holt said. Such misreadings can result in coverage-area dead spots (see diagram).

“And no one wants to read the headline, ‘Expensive new system doesn’t work,’” said Clay Bilby, product marketing manager for signal analysis at Agilent Technologies.

Tools to measure digital RF signals are more expensive than equipment for analog systems, but even the priciest six-figure spectrum analyzers are a pittance compared to the overall cost of a Project 25-compliant network. The problem occurs when such expenditures are considered after the fact. Asking for more money after paying for a multimillion-dollar system is never fun, according to Jack Daniel, president of Jack Daniel Co.

“When you’re spending $25 million-35 million for a large system, spending $100,000 on test equipment isn’t that much,” Daniel said. “But, if you’re on a tight budget and it wasn’t expected, it can be a problem.”

And test equipment is an easy item to overlook when preparing bid specifications. Many public-safety officials are not aware of this need, because the digital experience is relatively new to the sector, Holt said. Additionally, contractors often will not include test equipment in a bid proposal because of fears that doing so would artificially raise the final price, Daniel said.

Others question whether expensive spectrum analyzers are necessary. M/A-Com’s OpenSky digital network base stations yield “fairly accurate measurements … within a few tenths of a dB,” with traditional power meters, according to John Struhar, principal engineer.

Radios in the field yield false power readings when active, but M/A-Com developed a special test mode so analog power meters still can be used. “We created [it] so our customers don’t have to buy a bunch of expensive equipment,” Struhar said.

Holt said TDMA solutions tend to be more friendly to analog power meters than other technologies, but even the difference at the base station cited by Struhar could lead to coverage gaps. The differences are greater when dealing with the FDMA technology used in Motorola’s digital systems and OFDM platforms in Project 25-Phase II systems, Holt said.

Though traditional power meters may accurately measure RF outputs at the base station, they are not as effective away from the tower, because they do not discriminate between signals from the desired base station and those from a cellular company, Bilby said.

In contrast, modern digital spectrum analyzers specify signal strength in a given channel from a specific base station, which determines the effectiveness.

“What’s important is whether that guy in the field has a radio in his hand that’s working,” Bilby said.