1. Modern Broadcast Transmitter Architecture Power Amplifier and Power Supply Considerations
Alexander Kurz
GatesAIr
Good morning / afternoon everyone
Since this will be a short presentation, I will focus today on only two important aspects of a broadcast transmitter design – The Power Amplifiers and the Power Supplies that provide DC power to the final Power Amplifiers.

2. Important Features in a Broadcast Transmitter
Core Technologies
Serviceability
Reliability
Performance
In the overall transmitter system, our design engineers focus on several aspects of the overall system that are important. Some of these are critical to the broadcaster.
A modern TV or Radio transmitter needs to be capable of operating under harsh conditions, often in very remote locations. It is essential that it utilizes the latest technologies and also is designed for the utmost reliability, to minimize service calls and interruptions to the broadcast signal.
Therefore, it goes without saying that the transmitter must be very reliable and also incorporate a modular approach that allows quick diagnostics and fast easy servicing should this be required.
Along with the reliability and serviceability aspects, the transmitter of course has to provide a high quality signal, as demanded by top broadcasters, like you.

3. Power Amplifier Considerations
One very important component of any over the air broadcast transmitter is the final RF Power Amplifier, or PA module. While most manufacturers use similar technologies and offer similar performance characteristics, there are a few aspects of the PA design that I’d like to discuss today.

4. RF Power Amplifier Techniques
Class A (PA Efficiency typically 15%)
Very linear, low distortion but very inefficient
Used for low level circuits <1 Watt
Class AB (PA Efficiency typically 30%)
Less linear but easy to pre-correction, more efficiency than class A
Used for many years for final output stages in Solid State transmitters
Envelope Tracking (PA Efficiency typically 60%)
Device drain voltage is modulated with amplitude variations of the RF carrier
Very efficient but also complex, more expensive and hard to pre-correct
Doherty – (PA Efficiency typically 52-56%)
Invented in Divides the signal into two (carrier and peak) and amplifies them, separately for higher efficiency
Limited bandwidth, hard to pre-correct but simple and easy to implement
There are several classes of amplification that can be used for the PA design.
Class A amplification is the least efficient, because the RF devices are always biased on. This provides the cleanest and most linear amplification requiring almost no pre-correction. However the low efficiency is in itself the reason why no manufacturer uses class A amplification for power levels above about 1 Watt.
Class AB amplifiers are very linear abut do require additional pre-correction to overcome non-linear distortions that are inherent in this technique. These pre-corrections can be easily handled by todays techniques. Class AB is still commonly used in broadcast transmitters, but mostly for lower power levels below 250W.
A technique called “envelope trackin” has been demonstrated by GatesAir and at least one other transmitter manufacturer. The idea to vary the drain voltage to the RF device such that it is highest during the signal peaks and reduced during the lower parts of the envelope. This is difficult to achieve and it becomes quite costly and complex. As far as I know, no transmitter manufacturer has released a commercial product based on envelope tracking PA’s.
The concept for the Doherty power amplifier configuration was invented by William H. Doherty of Bell Telephone Laboratories in 1936 using vacuum tubes (valves). In its simplest form it uses two amplifiers, one handling the peak signal levels and the other carrying the lower levels of the envelope, the main amplifier. Doherty amplifiers have been implemented by all leading broadcast transmitter manufacturers as they are relatively easy to implement, provide excellent efficient and are reliable.

5. Large vs. Small PA Modules
Large PA modules
Av. power: 1.2kW-1.5kW per PA
RF pallets 6 to 8
Typical Weight: 28kg
Small PA modules
Av. power: 500W-600W per PA
RF pallets: 3 or 4
Weight: < 12kg
One aspect that GatesAir looked at closely was the size and power level for PA modules.
Certainly using a large high power amplifier can reduce the complexity and number of ports for the RF power dividers and combiners as well as minimizing the number of interconnections for RF and liquid inside the rack. This can result in lower cost per Watt but there are a few drawbacks.
This slide shows the typical power level (on the left) of a high power PA module that uses either 6 or 8 parallel pallets (or boards) combined to provide the final power level.
On the right side, the GatesAir approach is shown. We have elected to use 3 pallets combined in parallel for a power output in the 600W range. This is about ½ the power of a typical 6 pallet PA module which will produce approximately 1200W at its output port.
One obvious difference will the weight of the PA module. The GatesAir module weighs less than 12kg, while leading competitor PA modules weigh around 28kg.

6. PA Module Size & Weight 28kg 11.3kg
Easy to remove light weight PA module
28kg
The GatesAir liquid-cooled PA module is light enough to be lifted with one finger. A 28kg PA module requires 2 people to safely remove or replace it from a transmitter rack.
Very heavy PA module

7. Transmitter During PA Servicing
How much will the RF system level power drop when a PA is removed?
Depends on several factors:
Total number of PA modules in the transmitter
Combiner type (most use hybrid or Wilkinson)
For an example, using a 2.4kW transmitter:
Transmitter A uses 4 x 600W PA modules combined
Transmitter B uses 2 x 1200W PA modules combined
Assuming standard hybrid combining of the PA modules
What is the effect of removal of a PA module for servicing?
One of the most compelling reasons to use a lower power module is that more of them will be needed to produce a given transmitter power output….
For example – for a 2.4kW average power digital transmitter, only 2 PA’s would be needed, if each were the large size that can product 1200 Watts per PA.
A similar power level transmitter using the 600W PA modules would of course need 4 PA’s.
You may be wondering why using more PA’s is better?

8. 2.4kW Transmitter – Normal Operation
Large PA’s, 1200W each:
Small PA’s, 600W each: PA
600W
1200W
2400W PA
1200W 0W
2400W PA
600W PA
1200W 0W PA
600W
1200W 0W 0W PA
600W
Let’s look at the RF block diagram of the output stage of a 2.4kW transmitter.
The diagram on the left shows a system using two 1200W PA modules combined for a total of 2.4kW at the transmitter output. A standard 3 dB hybrid combiner is used to combine the RF output from each PA module. In a perfect world with no losses, equal power levels and correct phasing, there will be no power lost to the reject load.
The diagram on the right shows a 2,4kW transmitter using four 600W PA modules combined with a tw- stage hybrid combiner to provide the total system level power. Again if there are no losses and everything is phased and gain matched, all of the power is presented at the output and nothing to the reject loads.

9. 2.4kW Transmitter – 1 PA removed
Large PA’s, 1200W each:
Small PA’s, 600W each: PA
600W
1200W
1350W PA
1200W 0W
600W PA
600W PA 0W PA
600W
300W
600W
150W
300W PA 0W
Power drops to 25% (600W)
Power drops to 56.25% (1350W)
If a PA module is removed or is shut off, the transmitter power is affected due to two factors:
There are less PA modules in service e
Comber losses
In the example using the 1200W PA’s, removal of one leaves only 1 PA still in service. Does the transmitter drop to ½ power? No. The 3dB hybrid combiner now becomes a 3dB splitter. ½ of the remaining PA module power is seen at the transmitter output and ½ of the power is dissipated in the reject load as heat. So in this case the system level power drops to 25% of the original transmitter power.
On the right, the 4 PA transmitter using 600W PA’s fares better. In this case, when a single PA is removed, there are three left in service. I won’t go into the maths due to limited time, but with standard 3dB cascaded hybrid combining, the total system power drops to 56.25% of the original power.
Advantage goes to the smaller PA modules.

10. Power Output Comparison
Transmitter Power (Average)
1200W PA's
600W PA's
Max Power with 1 PA removed (kW)
Tx Power (W)
# PA's
Power (W) %
1200 1 2 0%
300
25%
2400 4
600
1350
56%
3600 3 6
1600
44%
2500
69%
4800 8
2700
3675
77%
Note large difference in available power
To see the difference between transmitters using 1200W PA modules and one using 600W PA modules for different power level transmitters, the table here can be used.
For smaller transmitters the difference is greater and as the transmitter power level is increased the difference is less. However, in all cases (an above the 4.8kW power shown), the transmitter using lower power PA modules always has the advantage.
Every broadcaster wants to stay as close as possible to their licensed operating power.

11. Power Supply Considerations
The power supplies that convert the AC power to DC for the PA modules is also a critical part of any broadcast transmitter. Let’s look at some considerations that are important.

12. PA Power Supplies
Power Supply design is critical for reliability and serviceability as well as providing excellent efficiency
In liquid-cooled TV and Radio transmitter designs - should the power supplies also be liquid-cooled and integrated into the PA assembly?
Single unit
Separate units
Power Amplifiers
Chiller plate PS
Power Amplifiers
Power Supply
Chiller plate
Integrated Liquid-cooled Power Supply
Separate Air-cooled Power Supply
The DC power source for the final RF Power Amplifiers is also an area that the transmitter design team must take avery close look at.
Modern switch-mode power supply technology has progressed quickly over the past several years. Not too long ago an AC to DC conversion efficiency of 85% to 90% was considered outstanding. The newer generation of power supplies now exceed 96% efficiency. This dramatically reduces the waste heat dissipated by each power supply.
Older generation liquid-cooled transmitter PA modules used less efficient switch-mode devices in the power supply, which led to the liquid-cooled power supply being a requirement. Placing the power supply internally to the PA creates a large and heavy single assembly.
A newer approach pioneered by GatesAir uses separate power supplies that are not integrated into the PA module. This technique has now become the industry standard in new transmitter designs.

13. PA Power Supplies
Modern air-cooled COTS power supplies are now a very viable option for a liquid-cooled transmitter architecture:
High efficiency – over 96% - Minimizes heat load to the building
Much lighter and smaller in size – easy to replace, 2.2kg versus 28kg
Less costly
Easier and faster to replace
In the GatesAir TV and Radio/DAB+ transmitters, standard COTS power supplies are used. These are air-cooled but are 96.3% efficient which minimizes any waste heat to the building.
Because each power supply is separate and not integrated into a large PA module, it is small and light, weighing about 2.kg each.
Since it is a widely available power supply, used in several industries, it is relatively inexpensive to replace, especially when compared to custom integrated power supplies that are only available from a transmitter manufacturer.
An obvious advantage is that if the power supply should ever need to be replaced, it is hot-swappable and can be removed and replaced in just a few seconds.

14. Key Summary Points
Separate PA Modules and Power Supplies provide several benefits:
Lower replacement cost (Separate units versus a single integrated unit)
Lighter and easily replaced quickly by a single person (A large 28kg unit requires 2 persons for health & safety)
Easier spare parts handling and shipping
Modern COTS air-cooled power supplies are extremely efficient (>96%), minimizing unwanted heat load to the building
Smaller PA modules with lower output power provide excellent parallel redundancy and far less effect on the system output power when one is removed for servicing
To summarize the key points that I have made today:
The idea of having separate assemblies for the PA module and its corresponding Power means that each unit will have a lower cost to replace.
The units are also lighter in weight and therefore much easier to service. A single person can replace either the power supply or the PA module, whereas the heavier single unit would require to two persons to execute s similar replacement.
The modern Commercial Off The Shelf (COTS) air cooled power supply is very efficient minimizing any unwanted heat load to the room.
As noted, a big advantage to using smaller lower power PA modules is that the effect on the total transmitter power output on removal of one PA for servicing is far less than with the larger higher power AP module.

15. Mississipi River, Quincy, Illinois, USA
Questions?
Thank you for listening to my short overview on our architecture for the PA modules and Power Supplies. Any questions?
Mississipi River, Quincy, Illinois, USA