Figure 1. Carrier-to-noise = 16dB. Click here to see an enlarged diagram.
It's breaking news. Your ENG truck pulls up, and the camera man jumps out to capture the action. The technician raises the mast and lines up the shot. The link is established, and you're feeding live signal. The competition powers up their transmitter, and your feed to the station drops off. Unlikely?
With the move to digital COFDM transmissions in ENG, this scenario is all too likely. While COFDM is designed to work well in multipath environments, interference can affect signal levels.
COFDM signals are susceptible to the cliff effect and to oversaturation; both result in the failure of the receive chain to produce a useable signal. The cliff effect refers to the drop off in reception resulting from marginal signal levels. When analog signals degrade, the picture quality degrades with an increase of sparkles or snow. However, digital signals lack the corresponding degradation of picture quality. When the signal drops below the minimum threshold, the digital picture simply and abruptly disappears. Conversely, oversaturation occurs when the signal level exceeds the maximum threshold. This causes excessive intermodulation or side band re-growth, preventing the receive chain from decoding the signal.
What about BER?
In either situation, the picture quality does not give any indication of the proximity of the signal to the relative threshold until it's too late.
Figure 2. Averaged, with markers Click here to see an enlarged diagram.
Antenna alignment is critical, and traditional alignment methods are no longer practical. In the analog world, technicians using a TV monitor and waveform monitor easily align, transmit and receive antennas. In the digital world, the picture is either perfect or simply not present. Using an analog signal to establish the link and then switching to digital can make antenna alignment faster, but it does not address how close the signal is to the threshold of the receiver. Is it right at the edge, ready to drop off? And what do you do when your ENG truck is completely digital?
Bit error rate (BER) and modulation error ratio (MER) can indicate a signal's proximity to the threshold. BER refers to the number of bits that must be corrected by the receiver over a period of time. BER significantly increases when a signal approaches the threshold. Unfortunately, BER only identifies the problem when the threshold is near.
Proactive monitoring and quantifiable measurement
MER is a ratio computed to anticipate system performance. MER increases in proportion to signal degradation or oversaturation, providing warning that the threshold is approaching. BER and MER are relative measures based on already established links. They do not provide useful information when trying to establish a link or when trying to identify the cause of a problem.
Figure 3. Sideband re-growth. Click here to see an enlarged diagram.
The key to effectively managing COFDM signals lies in the use of a spectrum analyzer at the central receive site. This allows measurement of the signal's power level and the carrier-to-noise ratio, ensuring optimal antenna alignment and reception power. When measuring carrier-to-noise, use of the averaging function will provide an accurate, quantifiable figure. Set a marker at the peak of the signal, and set another marker in the noise. The delta between the two markers is displayed at the bottom of the spectrum analyzer screen. Figure 1 shows a signal with a 16dB carrier-to-noise ratio. Figure 2 shows that same signal, averaged, with the markers clearly indicated.
Using a spectrum analyzer to view saturation
Saturation of the receiver is also clearly visible on a spectrum analyzer. Figure 3 shows a COFDM signal with sideband regrowth. Note the increased noise level at the edges of the carrier, resulting in a sloped rather than a flat noise floor. Monitoring a signal's carrier-to-noise and power level provides a technician with conclusive information to assess antenna alignment.
Figure 4. 65MHz center frequency. Click here to see an enlarged diagram.
The spectrum analyzer can also be used to identify interference. Prior to establishing a link, the analyzer will show the presence of any signals that may cause interference. If the interference occurs after the link has been established, the technician should see changes in the signal, which vary depending on the source of interference. Proactive monitoring gives a technician the opportunity to take corrective action before the interference causes the signal to drop.
The use of offsets is a common practice in ENG transmissions. Say a field team setting up a link is unaware of the previous use of an offset. Monitoring the receiver's IF signal with a spectrum analyzer will reveal the offset. Illustrated in Figures 4 and 5 are two spectral traces displaying an offset of 5MHz. The spectrum analyzer clearly displays the center of the signal at 65MHz (Figure 4) instead of 70MHz (Figure 5), indicating the use of a 5MHz offset.
Figure 5. 70MHz center frequency. Click here to see an enlarged diagram.
A spectrum analyzer is the only reliable tool for quickly diagnosing and eliminating digital RF problems. However, traditional spectrum analyzers are expensive and are designed for use in manned receive sites. Because many central receive sites are in isolated locations, the use of a spectrum analyzer would seem impractical. Even with an inexpensive, scaled-down model, a spectrum analyzer in this application would necessitate the ability to access and control it remotely.
Fortunately, Morrow Technologies has developed the VC70BMS, a real-time, remote access spectrum analysis tool designed specifically for use in ENG operations. This analyzer is physically small in size and is engineered for more rugged environments. It has full functionality, and it can tap into the 70MHz IF output of the central receiver. The analyzer is accessed and controlled remotely from the station's transmission center or other network operations center. (See Figure 6.)
Figure 6. Configuration for remote monitoring of the central receive site. Click here to see an enlarged diagram.
Remote connectivity is the key
While traditional spectrum analyzers are designed to cover a broad range of frequencies, the VC70BMS has a focused 60MHz to 80MHz frequency range, thus eliminating additional circuitry and inherent cost.
The analyzer features Virtual Front Panel software that allows any authorized PC to access the remote unit. The software has all the control and display functionality of a traditional spectrum analyzer such as resolution bandwidth, video bandwidth and markers. The software can simultaneously display spectral traces from multiple units, allowing monitoring of multiple receive antennas or multiple receive sites. Multiple users can simultaneously view the same receive site from PCs in a station's transmission area, the engineering offices and anywhere else the software is installed. An engineer from home can assist a technician troubleshooting a problem. When installed on a PC in the ENG truck, users can gain remote access with a cell phone.
The analyzer is a self-contained unit with a built-in PC, eliminating the need for additional equipment, software and systems integration. It can be accessed by a variety of communication methods, including dial-up, LAN, Internet and wireless — all easily configured via the VFP. Simply install the analyzer by connecting it to the receiver's 70MHz output, power and communications outlet. The analyzer is available in a compact 10in × 8.75in × 2.312in model weighing under 5lb or a 1RU 19in rack-mount model, making remote deployment a breeze.
The transition to digital ENG necessitates changes not only to transmit and receive hardware but also to the tools and methods used to monitor and maintain signal quality. Historically, the TV monitor approach has been the de facto industry standard for analog transmissions. The remote spectrum analyzer will become the new standard for monitoring and control in the digital domain.
John Morrow is CEO and Debbie Mucciolo is project manager of Morrow Technologies.
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