Broad Signals

Ten years ago, it was unusual to find TV stations on adjacent channels in the same market; the FCC rules prohibited it. Now it is not unusual to tune across the dial in medium and large TV markets and find many stations operating on adjacent channels. Microwave channels are being squeezed as well, with the bandwidth of 2-GHz ENG channels dropping from 17 MHz (18 MHz for Channel 1) to 12 MHz. Microwave Radio's TwinStream squeezes a 16QAM digital signal and an analog FM video signal with audio subcarriers in one 25 MHz broadcast auxiliary channel.

Increasing frequency congestion has made it necessary for broadcasters to pay attention to the bandwidth and spurious signals from all their transmitters if they want to be good neighbors and avoid a citation from the FCC. Broad signals are not appreciated! This month I'll look at the FCC rules regarding out of channel emissions, ways to check them and ways to reduce them.

ANALOG INTERFERENCE

Section 73.687 of the FCC rules states that for analog TV transmitters, "spurious emissions, including radio frequency harmonics, shall be maintained at as low a level as the state of the art permits." The only specification, however, for out of channel emissions is that they have to be attenuated no less than 60 dB at frequencies in excess of 3 MHz above or below the respective channel edge. The rule notes, however, that the 60 dB attenuation may be increased at a later date and states, "In the event of interference caused to any service, greater attenuation will be required."

If you are a ham operator or shortwave listener, you have probably noticed harmonics in the lower portion of the HF bands generated by the 15.734 KHz horizontal sweep circuitry in nearby TV sets. These harmonics can be present in improperly adjusted pulsed klystron UHF transmitters as well. If the harmonics extend below the channel edge, they could interfere with weak (or low power) low adjacent DTV channel signals. From my experience, the amplitude of these signals falls off rapidly outside the TV channel, so interference to co-located medium or full power DTV stations should be a problem.

Intermodulation products can create broad signals from analog transmitters. When an IOT or klystron tube is having trouble making power, it is tempting to crank up the sync level using an outboard proc amp to boost sync without adjusting linearity precorrection at IF in the exciter to correct for non-linearity resulting from an over-driven tube. Unfortunately, this simple fix often increases the amplitude of the lower sideband of the analog TV signal.

A common problem in analog transmitters (low and high power) using common amplification for visual and aural carriers is intermodulation between the 4.5 MHz sound carrier and the visual carrier, resulting in spurious signals 4.5 MHz below visual carrier and 9 MHz above it. Both are well inside the adjacent channels and could cause interference. I've seen more problems with these spurious signals on lower power transmitters, which often use notch filters instead of a bandpass filter to remove specific spurious products. When these sharp notch filters drift (depending on the mounting, gravity can contribute to this), they no longer provide much attenuation to the spurious signals.

DTV BANDWIDTH LIMITS

The FCC places stricter limits on spurious emissions from DTV transmitters. Section 73.622(h)1 states: "The power level of emissions on frequencies outside the authorized channel of operation must be attenuated no less than the following amounts below the average transmitted power within the authorized channel. In the first 500 kHz from the channel edge the emissions must be attenuated no less than 47 dB. More than 6 MHz from the channel edge, emissions must be attenuated no less than 110 dB. At any frequency between 0.5 and 6 MHz from the channel edge, emissions must be attenuated no less than the value determined by the following formula:

Attenuation in dB = -11.5(?f + 3.6);

Where: ?f = frequency difference in MHz from the edge of the channel."

Therefore, emissions at the output of a DTV transmitter must be attenuated at least 75.9 dB at a frequency 3 MHz above or below its channel edge, almost 16 dB more than is required for analog transmitters. Section 73.622(h)2 of the FCC rules specify how measurements are to be made:

"(2) This attenuation is based on a measurement bandwidth of 500 kHz. Other measurement bandwidths may be used as long as appropriate correction factors are applied. Measurements need not be made any closer to the band edge than one half of the resolution bandwidth of the measuring instrument. Emissions include sidebands, spurious emissions and radio frequency harmonics. Attenuation is to be measured at the output terminals of the transmitter (including any filters that may be employed). In the event of interference caused to any service, greater attenuation may be required."

Modern DTV transmitters use a combination of linearity precorrection in the DTV modulator/exciter and an external bandpass filter to meet these requirements. The usual cause of excessive out-of-channel emissions is uncorrected non-linearity in the high power IOT or solid-state amplifier portion of the transmitter. DTV filter manufacturers have done an excellent job making their bandpass filters stable over a reasonable range of operating temperatures. However, an air conditioner duct blowing directly on a filter or, alternatively, overheating due to insufficient airflow around the filter or a bad fan (if used) can cause filter drift.

Measuring out of band products at the output of the DTV mask filter is difficult, as few spectrum analyzers have the ability to handle signals over a 110 dB dynamic range. As a result, out of band emissions are often measured at the input to the DTV mask filter and the attenuation from the filter added to that measurement to determine if the station is in compliance with FCC rules. Using a spectrum analyzer to monitor the output of the DTV transmitter before the filter will allow you to see changes or growth in the "shoulders" of the 8-VSB signal that may not be as obvious after the filter. Harris' Real Time Adaptive Correction (RTAC) system in its new APEX exciter samples the transmitter output before and after the mask filter to comply with FCC DTV emission limits in the 500 kHz regions close to the channel edge that aren't attenuated by a standard mask filter.

To avoid broad DTV signals, check non-linear precorrection regularly as the IOT ages or if any adjustment is made to the IOT operating parameters. Some DTV exciters can adjust linearity precorrection adaptively, but even these should be checked regularly to ensure they are operating correctly and that the precorrection is within the limits of the exciter and the tube.

MODULATION BANDWIDTH

With the bandwidth of 2 GHz broadcast auxiliary system (BAS) channels being reduced to 12 MHz, bandwidth requirements for ENG and fixed link microwaves have been a hot topic.

Section 74.637(a) of the FCC Rules specifies the out of channel emission limits for BAS microwave transmitters. For analog FM microwaves, the limits are simple:

74.63(a)1: When using frequency modulation:

i. On any frequency removed from the assigned (center) frequency by more than 50 percent up to and including 100 percent of the authorized bandwidth: At least 25 dB in any 100 kHz reference bandwidth (BREF);

On any frequency removed from the assigned (center) frequency by more than 100 percent up to and including 250 percent of the authorized bandwidth: At least 35 dB in any 100 kHz reference bandwidth;

On any frequency removed from the assigned (center) frequency by more than 250 percent of the authorized bandwidth: At least 43+10 log10 (PMEAN in watts) dB, or 80 dB, whichever is the lesser attenuation, in any 100 kHz reference bandwidth.

For example, in a 25 MHz authorized bandwidth, emissions more than 12.5 MHz and up to 25 MHz (the center of the adjacent channel if the channels are 25 MHz wide) from the center frequency have to be attenuated by at least 25 dB. In the same authorized bandwidth, emissions up to 75 MHz away from the center frequency have to be attenuated by at least 35 dB.

The emission limits for transmitters using digital modulation are more complex:

74.63(a)2: When using transmissions employing digital modulation techniques:

i. For operating frequencies below 15 GHz, in any 4 kHz reference bandwidth (BREF), the center frequency of which is removed from the assigned frequency by more than 50 percent up to and including 250 percent of the authorized bandwidth: As specified by the following equation but in no event less than 50 decibels:

A = 35 + 0.8 (G - 50) + 10 log10 B.

(Attenuation greater than 80 decibels is not required.)

Where:

A = Attenuation (in decibels) below the mean output power level.

G = Percent removed from the carrier frequency.

B = Authorized bandwidth in megahertz.

Using our same 25 MHz bandwidth example, emissions outside the allowed bandwidth would have to be attenuated at least 50 dB. At a frequency 25 MHz removed from the center frequency (100 percent of the bandwidth), the minimum attenuation would be 80 dB.

As you can see, the emission limits for digital modulation are much stricter than those for analog modulation. This points out the complexities frequency coordinators face when coordinating a mixture of analog and digital microwave transmitters.

Many stations do not have spectrum analyzers that work into the microwave range and those that do may find it difficult to find a point in the transmitter where the microwave signal can be sampled. Fortunately, it isn't necessary to measure the bandwidth at the microwave carrier frequency. It can be easily checked at the IF frequency, either at the transmitter if a sample port is available or at the receiver, most of which include an IF output. If a program video stream is being monitored on an analog microwave, the emissions may change depending on the program material. Use the peak hold feature available on most digital spectrum analyzers to plot the signal over time to verify it meets the FCC rules.

Assuming the microwave transmitter is operating correctly, there is little that can go wrong to increase the occupied bandwidth. My experience has been that most analog FM bandwidth errors (low or high) occur when the microwave transmitter modulation is adjusted to correct for improper adjustment of the baseband gain in the microwave receiver at the other end of the path. Another common problem is setting subcarrier injection levels too high. Subcarrier modulators can also generate harmonics if not correctly adjusted. For example, the second and third harmonics of a 6.8 MHz subcarrier can show up 13.6 and 20.4 MHz from the carrier.

Note than in a dual channel microwave, such as MRC's TwinStream radio, the output power cannot be set at the same level used for analog FM. Power must be reduced to move the operating point of the power amplifier into a linear region. Nonlinear operation creates intermodulation between the digital and analog carriers, increasing spurious emissions. Digital modulation also requires keeping the microwave power amplifier linear. You will notice that the output power for a particular microwave transmitter using digital modulation is a few dB less than that when the same transmitter is used for FM.

I hope this introduction to bandwidth consideration for TV and microwave transmitters has been helpful. If you would like to see a future article with more information on bandwidth measurement, or any other RF related, topic, send me a note at dlung@transmitter.com

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.