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Land Mobile/Public Safety Radio PA Revolution

Parent Category: 2014 HFE

By Raymond A. Baker, Walter H. Nagy, David W. Runton, Robert A. Sadler

Public Safety Radio – A Brief History

Mobile wireless communication for public safety was first realized in the 1920s with vacuum tube radios that consumed the entire back seat of a vehicle. Over time, the technology became more practical with improvements in size, cost, and reliability. Transistors brought reduced size and higher efficiency enabling even smaller portable, handheld, radio platforms. At the same time, operational frequencies steadily increased, from HF through VHF, UHF, and up to 800 MHz. New allocations at 700 MHz and access to public networks at 900 MHz are further expanding this communication solution.

In the United States, public safety frequency allocations include the following specific bands (see Figure 1):

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Figure 1 • Public safety frequency allocations.

These bands are often collated into the following bands: 

  • 25 – 50 MHz
  • 136 – 174 MHz
  • 220 – 222 MHz
  • 380 –  520 MHz
  • 763 – 870 MHz

Although FM (frequency modulation) was the primary protocol for trunking radios throughout the 1980s, digital modes were added in the 1990s allowing for voice, data, and encryption using M-FSK or GMSK modulation. North American users generally use the APCO P25 standard, while in Europe the standard is TETRA. Eventually systems will migrate towards the LTE (Long Term Evolution) standard for the 700 MHz band enabling wideband data and additional interoperability with public networks at 900 MHz.

Despite radio standard evolution, interoperability with older modulation formats/services remains a requirement, and the incorporation of these frequencies and standards into a single radio system poses a serious design challenge. The most simplistic solution is to stack multiple single narrowband radios, but this compromises size and cost. A more ideal solution would be to implement a single broadband radio, but this places several restrictions on the designer requiring multi-band, multi-mode operation without compromising cost, size, or efficiency, which is a significant challenge.

Because the RF power amplifier dominates overall radio power consumption, the amplifier is often the key to achieving the design goals. The advent of Gallium Nitride (GaN) technology opens a practical path to next generation, flexible, future-proof, frequency agile architectures for both handheld and mobile radios.

Wideband Architecture and LPFs

Power amplifier design for land mobile applications begins with a few fundamental requirements.

  • Output Power
  • Modulation Format and Linearity
  • Efficiency
  • Bandwidth
  • Harmonic Suppression

Several basic block diagrams are shown below outlining design options to address these requirements.  

Considering only the transmit portion, the simplest solution is the conventional narrow or single band PA (power amplifier) chain of Figure 2.

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Figure 2 • Single band power amplifier chain.

To optimize power amplifier efficiency, RF PA’s are typically designed to operate at or near saturation. Harmonic content represents a significant challenge for the PA design since regulations prohibit out of band emissions. Second and third harmonic levels may rise, reaching -15 to -10dBc and placing a difficult burden on the low pass filter. This filter is typically designed with five or more poles to reduce output harmonic levels below -60dBc (or more) to meet emission specifications.

Operation across two or more bands has historically required separate amplifiers and an assembly of low pass filters and an n-plexer. Figure 3 shows a multiband implementation with conventional narrowband amplifiers using silicon or LDMOS devices.

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Figure 3 • A multiband implementation with conventional narrowband amplifiers using silicon or LDMOS devices.

This approach necessarily requires wide gaps between adjacent bands to relax filter requirements that drive cost and insertion loss. In addition to harmonic attenuation, the filter-multiplexer must isolate each power amplifier filter chain from those of every other band. This configuration works well for two band radios such as 150 MHz/450 MHz or 450 MHz/800 MHz where band edges are separated by many multiples of the passband, keeping filter orders reasonable. Figure 3 shows an implementation with low-pass filters, but in practice some of these may be band-pass, with or without stopband zeros. Filter design for this application presents a host of choices and tradeoffs. 

Conceptually, a single wideband amplifier and a low loss switch could consolidate the multiple amplifiers of Figure 3 into the concept shown in Figure 4. A switch, whether FET based or PIN, offers lower cost than a complete band specific amplifier chain and consumes less circuit board area. On the other hand, the amplifier must offer higher power and efficiency to offset increased insertion loss of the switch. Figure 4 also requires guard bands, i.e. gaps in the frequency coverage to ensure deselected filters don’t load the operating path and interfere with PA performance.

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Figure 4 • A single wideband amplifier and a low loss switch could consolidate the multiple amplifiers of Figure 3 into the concept shown.

Figure 5 shows a single amplifier solution that offers continuous frequency operation.  Switches at both the filter bank input and output isolate all but the desired channel. This allows for filter design to be less rigorous since pass-bands may have a slight overlap.  Compared to Figure 4, however, this configuration doubles the switch insertion loss.  This may be, at least, partially overcome by the elimination of the multiplexer requirement and lower overall filter insertion loss.  

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Figure 5 • A single amplifier solution that offers continuous frequency operation. 

On the output side, harmonic content from the switch must remain below regulatory levels, under all operating conditions. In the past, this function required mechanical or a conservatively designed PIN diode matrix. New SOI based FET switches can now meet this requirement for handheld radio power levels (up to 10W), and with continued advancements, the technology may eventually support 50W vehicle based power levels as well. [1]

Because Land Mobile Radio (LMR) frequency allocations have large gaps between the lower bands, the multi-band architecture shown in Figure 4 is a realistic approach. Figure 5 is the best solution for mobile military applications or other products that require continuous coverage.

PA Refresher

The wideband power amplifiers suggested in Figures 4 and 5 is a significant design challenge for conventional low voltage silicon and LDMOS amplifiers. Consider the operating conditions in a typical radio. Whether handheld or mobile, the amplifier typically draws power directly from the handheld or vehicle battery.

The theoretical natural load impedance for a Class AB power amplifier, ignoring saturation or knee effects, is given by the familiar maximum power load-line equation:

1404 HFE powerAmps eq01

A typical handheld radio designed for 5W from a 7.2V battery yields a load resistance of 5.1 ohms.  A higher power mobile radio designed for 50W operating from a typical 13.6V supply requires a 1.8 ohm load.  Both of these are very low impedances requiring high Q networks to achieve a 50 ohm match. 

Generally, the ability to match a transistor over a desired bandwidth is limited by two fundamental quantities.

  • The transformation ratio, i.e. the ratio of the load R from 50 ohms
  • The device Q, related to the transistor’s input and output resistance and capacitance

Higher operating voltage is the only way to reduce the transformation ratio in these examples. As the battery voltages are fixed by practical considerations, in order to increase the operating voltage of the transistor the addition of a high efficiency DC-DC boost converter is required.  

The DC-DC boost converter eases the design challenge. Using the same equations, 5W from a 22V supply yields a load R of 48 ohms for the handheld PA, and 50W from a 50V supply yields a load R of 25 ohms for the mobile case (exactly 2:1 from 50 ohms). At these frequencies, ferrite loaded broadband transformers can implement low integer impedance ratios like 2:1 or 4:1. Higher operating voltage and broadband transformers can be used to mitigate the transformation ratio limitation and enable wide operating bandwidth.

GaN: Solving Problems

Despite the above simplification, RF power transistors also have parasitic reactances, such as shunt capacitances like CGS and CDS, which can’t be ignored. With these additions, Q becomes a limiting factor as frequencies approach 1 GHz. GaN solves both problems as it offers high power density devices operating to 48V with intrinsic capacitances that are a fraction of comparable power LDMOS devices.

Although the usual high efficiency PA architectures of Class-D, E, F, etc. offer very high efficiency, these techniques are usually narrowband and highly non-linear. A class-AB GaN based amplifier can routinely attain 60% to 70% CW efficiency while retaining some linearity.  Public safety networks use a variety of waveforms but most are constant or near constant envelope like FM, MSK, PM, or PSK. Future requirements for LTE or other high peak-to-average ratio (PAR), high linearity wireless waveforms, can be satisfied by pairing a reasonably linear Class-AB amplifier with baseband digital pre-distortion or other linearity correction technique [2].

Collecting these arguments, the ideal wideband LMR amplifier would have the following characteristics:

  • High efficiency with correctable linearity 
  • Constant gain and power across the band
  • High voltage operation – 28V to 48V to simplify the wideband match
  • Low intrinsic capacitance for low Q at the highest operating frequency
  • Rugged and low cost

The Handheld Mobile Radio

  • Consider a typical handheld radio PA design requirement:
  • Battery packs of 4.5-12V, usually around 2Amp-Hours 
  • Moderate output power, 1W to 10W at the PA, 1-8W at the antenna
  • RF power control for battery conservation, -6 dB to -10 dB reduction typical
  • ALC to maintain constant output power
  • High PA efficiency to extend operating time, reduce battery size and weight
  • 50 ohm load impedance but must tolerate moderate VSWR from broadband antennas
  • Small area and volume, low cost

With a boost converter the PA supply voltage becomes somewhat arbitrary, but what is the optimum voltage? As shown earlier, 22V was ideal for a 5W power level, confirmed by the Figure 6 plot of output power versus load resistance. The 5W and 50 ohm intersection occurs around 22V.

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Figure 6 • Plot of output power versus load resistance. 

For the highest power handheld radios the 28V curve intersects 50 ohms at 8W. Adding headroom in the boost converter to 32V, a nominal 28V design can deliver up to 10W into 50 ohms. Tracing the 50 ohm termination line, the output ranges from 1W to 10W as the supply voltage varies from 10V to 32V. This approximate 3:1 voltage ratio delivers a 10:1 power ratio as expected.

As a contrast, Figure 6 also shows the load impedance versus output power using a typical 7.2V battery pack. The target impedance is very low, providing the justification for classic handheld power amplifier modules being inherently narrow/single band.

The Handheld Solution – the NPA1006

The NPA1006 is a new integrated GaN power amplifier that offers 10W minimum from a 28V supply continuously from 20MHz to 1GHz. The package is a low profile overmolded plastic 6 x 5 x 1mm surface mount (SMT) package well suited to the space limitations of a handheld product. External support circuitry is minimal requiring only a few passive lumped elements on the output to improve the high frequency performance and proper depletion-mode GaN transistor biasing [3].

The NPA1006 includes an internal input matching network providing a near 50 ohm input impedance. The output of the PA is unmatched, but even so a simple matching network provides a broadband match to 50 ohms. Combining high operating voltage and low intrinsic capacitance, the transistor works well natively from low to mid band for both large and small signal conditions. At high frequencies the simple external match of the applications circuit maintains good power and efficiency through 1GHz. The external output match can also be optimized for narrower band performance if desired (such as VHF/UHF or UHF/800 MHz only).

In theory, these external chip components could be integrated into the NPA1006 but the losses, particularly of the inductor, would be higher and it would also increase the module cost.  Figure 7 shows the broadband performance of the NPA1006 at 41dBm (12.5W) output power with power added efficiency (PAE) ranging from 50 to 85 percent.  

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Figure 7 • Broadband performance of the NPA1006 at 41dBm (12.5W) output power with power added efficiency (PAE) ranging from 50 to 85 percent.

Figure 8 shows both gain and PAE versus output power. These plots show that although the NPA1006 can operate at 12.5W output power across the band of interest, the actual saturated power and therefore peak efficiency varies across that band.  

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Figure 8 • Gain and PAE versus output power.

Targeting peak efficiency will provide the longest battery life for a portable radio.  Unlike the battery only configuration, the battery plus DC-DC converter configuration can throttle the amplifier voltage up or down as needed. For every combination of frequency, desired output power, and load VSWR there is an optimum supply voltage for best efficiency. The efficiency difference between the conventional case, using power control by reducing input drive, and the proposed configuration, which adjusts both the input drive and the supply voltage, can be significant. Figure 9 contrasts these two approaches with the NPA1006 operating at 100 MHz.  

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Figure 9 • This figure contrasts these two approaches with the NPA1006 operating at 100 MHz. 

By reducing both the drive and the supply voltage the efficiency remains well above 60% from under 2W to more than 10W as seen in the upper curve. Using drive reduction alone the lower red curve reveals that the efficiency falls rapidly at low power levels. A variable supply voltage, intelligently controlled, can reduce or counter the effect of these factors while maintaining maximum performance.

Thermally, the NPA1006 is an optimized device with a low 4.6°C/W thermal resistance (RJC).  Proper control of the thermal path from the backside of the device through the underlying PCB via array to the heat sink requires careful design for optimal thermal transfer [4].

The Vehicle Mobile Radio

Vehicle mounted radios face a different set of challenges.

  • 13.6VDC nominal vehicle supply, subject to large swings and transients
  • Efficiency critical to reduce heat dissipation, not conserve energy
  • Higher power levels: >50W output at VHF/UHF, to 35W at 700-900 MHz typical
  • Wide operating temperature range, -30C to +50C typical
  • Nominal 50 ohm load impedance, but potentially poor antenna VSWR

Unlike the handheld case, higher power levels and reduced size restrictions favor a discrete transistor PA solution. As with the case of the handheld, however, we must also consider the optimum operating voltage. To deliver 50W at the antenna port with the functional diagram of Figure 3, the amplifier must provide an additional 1 - 2 dB to compensate for the switch and filter losses, or nominally about 80W. The ideal voltage for 80W into 50 ohms is 90V, well above the current RF GaN technology limit of 50V.  But 80W and a 48V supply yields a load R of 14.4 ohms, nearly a 1:4 from 50 ohms.  With broadband ferrite loaded transformers this is a realizable solution.

NPT2022 LMR Mobile Power Amplifier Reference Design

Figure 10 shows the layout of a broadband LMR mobile PA reference design that operates from 100MHz through 1GHz. The design uses the NPT2022, a new 100W, 48V GaN HEMT in the TO272-2 plastic package. This is the largest CW capable plastic packaged GaN device on the market today. The plastic package technology drops the pricing of a 100W GaN on Si device by about half and provides a cost reduction roadmap that can challenge LDMOS pricing in high volumes. Thermally, the JEDEC standard outline, TO272-2, offers better performance than the same die in a conventional, and expensive, air cavity metal flange. Thermal resistance (RJC) is 1.3 C/W.

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Figure 10. The layout of a broadband LMR mobile PA reference design that operates from 100 MHz through 1 GHz. 

With a nominal 48V supply, the design delivers more than 80W across the entire band with well-behaved gain and good input and output return loss.  A parallel R-C network at the input adds low frequency loss that both reduces gain and improves stability.

The three key components are the two 4:1 ferrite loaded transformers at the input and output and the 100W, 48V plastic GaN HEMT. The board shown uses a custom SMT transformer, but conventional coax plus binocular core construction works equally well.

Figure 11 shows the performance of this design over 100 MHz to just below 1 GHz.  

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Figure 11. Performance of this design over 100 MHz to just below 1 GHz.  

Output power is at or above 80W with a worst-case efficiency just below 50%.

Regulatory limits set lower maximum power levels for the upper UHF bands, requiring power backoff at the higher frequencies. Analogous to the earlier handheld discussion, power control using a combination of reduced drive and supply voltage will maintain high efficiency over the operating envelope. This lightens the thermal load at 800 MHz where the ferrite transformers tend to become lossier. The backoff from a typical 50W to 35W output is 1.5 dB, requiring a DC supply range of 40V to 48V.  

Summary

The next generation of LMR radios must support both legacy and LTE modulation and frequency bands. While it is possible to continue to expand capability through stacked system blocks, the more efficient method takes advantage of software defined radio advances combined with broadband GaN based power amplifiers. These new systems provide ultimate flexibility supported by simplistic transceiver architectures. Radios based on these design platforms are both cost effective and configurable for land mobile radio standards deployed worldwide today while being adaptable for the standards of tomorrow.

For more information on these reference designs, please contact Nitronex Applications, applications@nitronex.com.

About the Authors:

Raymond A. Baker, Walter H. Nagy, David W. Runton and Robert A. Sadler are with Nitronex, LLC.

References:

[1] “AN18 – RF Switch Performance Advantages of UltraCMOS™ Technology over GaAs Technology,” www.psemi.com.

[2]  Runton D, Zavosh, F, Thron, C, “Digital Predistortion Linearizes RF PA’s”, Microwaves & RF, August 2000.

[3] “GaN Essentials™: AN-009: Bias Sequencing and Temperature Compensation for GaN HEMTs,” www.nitronex.com.

[4] “GaN Essentials™: AN-012: Thermal Considerations for GaN Technology,” www.nitronex.com.

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