With the gradual adoption of digital terrestrial television worldwide, many broadcasters are planning ahead and budgeting for the future. In most instances, the analog service must continue for several years, while viewers slowly migrate toward new digital receivers or set top boxes. While the exact termination date for analog transmission may not yet be determined, it is possible that in many cases, it will be only a few years away. Broadcasters who must replace an old and aging analog transmitter must make a business decision as to the viability of what may well be a high capital expense.

Several options could be considered:

• Continue to operate the old transmitter, hoping that no serious problems develop
• Replace the old analog transmitter with a new one
• Purchase a new transmitter that has been optimized for both digital and analog operation

The first option is probably the most risky. If the transmitter fails or needs service, replacement parts may be hard to find, even unavailable. This can often result in significant down time. In addition, knowledgeable and trained service personnel are becoming harder to find for old equipment. The second option is a much safer alternative. If, however, the newly purchased transmitter is simply a standard analog design, it may prove difficult to adapt it for digital operation. At best, it may be expensive to convert or its digital performance characteristics may be compromised. This paper focuses on the third option. A properly designed and optimized transmitter system can be used for current analog service and later, for digital operation. By minimizing the required changes to the system, the upgrade to digital can be fast and very cost effective. This is likely to be the ideal solution for the broadcaster who is interested in maximizing his investment and minimizing problems when the time comes to switch to digital.

Analog and Digital Differences Analog and digital modulation standards are different in their characteristics. Although analog TV and digital TV systems both use amplitude modulated RF, there are some important practical differences. Analog systems have two distinct RF signals - an FM aural signal and an AM visual signal. Since the aural RF carrier is frequency modulated, the power is invariant and does not require linear amplifiers. Most CCIR standards use negative amplitude vision modulation and transmitters are typically rated on their peak visual power output. With analog television systems, the average RF power level will vary significantly as picture level varies table below.

The wide variation of average visual power in analog transmitters caused by picture level variations results in several design challenges. The DC power supplies for the RF power amplifiers must include load regulation for the varying current drawn by class AB stages. The RF power amplifiers must be designed to accommodate the thermal changes caused by varying RF power. If not properly compensated for, unacceptable picture distortions can occur, including overshoots, line tilt and field tilt. Typical compensation techniques involve fast acting AGC circuitry or dynamic bias control to cancel unwanted device gain changes.

All current digital formats, both COFDM and 8-VSB, result in a constant average RF power regardless of program content, and transmitters are normally rated by their average power output.

Another key difference is in the peak-to-average power ratio (PAR). In digital systems, the PAR remains a steady figure, when measured over a long enough time period. The DVB-T signal can be statistically modeled as a two-dimensional Gaussian process. For DVB-T, the PAR figure is somewhat independent of filtering. With the 8-VSB ATSC signal, the PAR is set by the roll-off factor of the spectrum-shaping filter table below.

The relatively high peak-to-average ratios of current digital modulation schemes, results in relatively high peak power handling requirements for transmitter RF stages. Amplifier non-linearity and pre-correction are important design aspects if 
good performance is to be achieved. In practice, the types of precorrection needed for both analog and digital systems are the same. Both phase and amplitude corrections are required over the bandwidth and the modulation range. For optimal performance, however, the adjustment of such correction will be different for analog and digital systems. For example, analog visual modulation does not approach RF carrier pinch-off, as does digital. Digital pre-correction techniques have evolved in recent years, replacing the ever-present manual trimmers of older analog designs with modern PC based set-up techniques. In newer systems, the correction parameters are completely software addressable and can be stored in memory (look-up-table) and selected as desired depending upon frequency and linearity requirements.

An uncommon approach to amplification

Most (if not all) manufacturers of medium-to-high power analog TV transmitters use what is commonly termed "separate amplification" or "externally diplexed" operation. Such systems employ completely independent paths of amplification for the visual RF signal and the aural RF signal. Externally diplexed systems are simple, easy to implement and can provide very good performance. By using separate amplification, amplifiers can be optimized properly for visual or aural operation, resulting in the highest system power efficiency.

A less often used approach for analog transmitters is "common amplification." This technique is frequently employed for lower power (up to about 2kW) solid state transmitters, as it is less costly to implement. Historically, most manufacturers have not even considered common amplification at higher power levels. There have been several drawbacks to this approach - efficiency and performance being the major ones - until recently. However, today nearly all current high power IOT transmitter designs use common amplification for analog operation. Modern pre-correctiontechniques allow very good intermodulation performance and freedom from visual/aural cross modulation.

It is possible to optimize each amplifier section for maximum performance and efficiency (for instance, the use of class C amplifiers for the aural PA stage). In practice, most designers opt for identical visual and aural amplifiers to allow interchangeability and simplify spares requirements.

Note that the notch diplexer has been replaced with a filter. The Intermodulation Distortion (IMD) filter is used to reduce any unwanted 3rd order intermodulation products created by non-linearities in the class AB power amplifier stages. The in-channel IMD levels can be corrected to acceptable levels using digital pre-correction techniques. Typical figures of -60dB can be achieved, well below the threshold of visible picture impairment. There are many advantages to this approach. For example, all of the RF power amplifiers can be identical and in parallel. Since they operate in parallel, a high degree of redundancy is provided. Should an amplifier need to be removed for service, the remaining amplifiers can operate at a reduced total output power. By comparison, in an externally diplexed system there are very few aural amplifiers because the aural power is typically only 10% of the peak visual power. In a 5kW or 10kW transmitter, for example, there may sometimes be only a single aural amplifier. A second aural amplifier may be an option, but at additional hardware cost.

Common amplification generally provides lower DC power to RF power efficiency. This can be attributed to the additional peak power handling headroom required, plus any back-off needed for linear operation to keep IMD products under control. By careful design, these limitations can almost be avoided. In fact, the difference in amplifier efficiency can be very small.

The figure later summarizes the DC to RF power efficiency for a 10kW amplifier in both externally diplexed and common amplification modes. In this example, the externally diplexed system uses seven visual PA's and one aural PA, while the common amplification system uses 8 PA's operating in parallel.

The digital upgrade - keeping it simple

While there may be a very small penalty to pay for efficiency, common amplification provides an extremely attractive alternative to external diplexing when it comes time to upgrade for digital operation. A typical externally diplexed transmitter is depicted in a simplified form in figure 3.

Conversion to digital will require replacement of the exciter/modulator and RF system. In addition, the aural RF path will need to be disabled or removed, since it makes most sense to use the visual RF path for the digital signal. If power output needs to be maximized, the aural amplifiers may be used; however, the RF power dividers and combers will require replacement. Another drawback is the fact that the notch diplexer will be of no use; it must be replaced with a suitable mask filter. Other items such as power metering, peak detectors, etc., may also require replacement or adjustment. Upgrading a common amplification transmitter for digital service is depicted in the figure.

In this case, the upgrade path to digital is extremely straightforward. The exciter/modulator and possibly RF detectors and metering must be modified (or replaced) but the rest of the system requires no significant changes. The same RF path can be re-used; even the output filter may be used as the DTV mask filter, providing it was properly specified for both applications in the first place. It is clear that the common amplification approach offers a very elegant solution for a digital upgrade in the future. There will be far more re-use of the original system and because of this, the cost of conversion will be significantly lower.

An exciter for Analog and Digital operation

In both the transmitter conversion scenarios considered above - externally diplexed systems or common amplification transmitters - exciter replacement was mandatory. But what if there were an integrated exciter/modulator that could be used for either analog or digital operation? In fact, this concept has already been proven in a current product offering. The design is based on a field-proven DVB-T exciter. A digital modulator capable of processing analog video and audio signals, together with NICAM, dual carrier or BTSC stereo is used. The example shown can be used as a PAL or NTSC exciter initially and later upgraded for DVB-T operation, with minimal hardware changes.

The block diagram for the analog transmitter configuration, includes the digital signal modulator, which processes the video and audio analog baseband signals to an I/Q serial signal at 364MB/s. The following stage contains the digital I/Q pre-correction stage and D/A converter. The signal is then handled in a conventional manner, using upconversion to UHF and linear amplification.

The only significant hardware change required is the replacement of the digital signal modulator with the COFDM encoder. All other items remain the same as for the analog case. The system is re-configured for optimum digital performance by adding DVB-T firmware and new software, which is loaded from an external PC.

Power supply considerations

The power supplies used in an analog TV transmitter must be able to handle the variation in DC current that results from the use of efficient class AB power amplifier stages. The current draw can vary significantly between black and white picture levels. Lead resistance, inductance and filter capacitance, play a large part in determining the performance of the system since transitions from black to white can cause sudden voltage swings resulting in poor line time and field time distortion performance. The power supply implementation approach used by Harris is to integrate the power supply into the PA module assembly, placing the regulation as close to the amplifier as possible. This same power supply will work properly without modification for the digital signal, where load regulation, filter capacitance, and lead inductance are of less concern.

The "smart" power amplifier

The RF Power Amplifier must also be able to handle the variation of average power levels of an analog signal as well as providing good efficiency and linearity. Testing has shown that an amplifier optimized for good digital performance will operate very well with an analog signal. In fact, using combined amplification reduces the variation of RF power as the modulation changes. This is due to the constant aural RF power being part of the combined visual/aural signal. This reduces thermal gain related changes in the RF power devices. In addition, the RF carrier never approaches zero, even with visual overmodulation. For excellent linearity, LDMOS power devices were chosen. The design is fully broadband across the entire UHF TV band (band IV/V), allowing simple and quick channel change capability. An onboard controller provides complete protection and control via a serial bus connection from the system main controller. Optimization of performance in both analog and digital modes is also handled automatically by the controller as well as compensating for minor gain variations across the UHF band. This is accomplished via a look up table pre-programmed at the factory.

Analog and digital performance

Good performance in both modes is provided using the digital I/Q pre-corrector built into the exciter. The spectrum analyzer display shows that excellent in-channel IMD performance is possible, (typically -60dB). All video and audio parameters are well within standard acceptable levels for analog transmitters.

The exciter used for this example has a DVB-T encoder module. By simply replacing the analog module with the DVB-T encoder module, then installing the correct firmware and software, the transmitter can be converted into a DVB transmitter in a matter of a few minutes. The overall amplitude/frequency and shoulder level performance is shown in the figure. Out-of band adjacent channel RF performance can be tailored to suit standard or critical mask requirements by selection of a suitable filter.

Measured data shows that all critical DVB-T performance parameters are fully met, with a Modulation Error Ratio (MER) figure exceeding 33dB.

Conclusion

The typical life span of a TV transmitter is 18 years. With many countries transitioning to terrestrial digital systems, a new analog transmitter with simplified digital upgradability makes economic sense. Re-use of all major components saves considerable future investment. Even if the digital transition plan involves simulcast period requiring both digital and analog transmission, the analog system can be re-purposed and used at a different location or as a backup. Furthermore, by using a fully broadband, frequency agile design, channel changes can be made easily, in the event that the digital frequency is not the same as the analog frequency.