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Changing the Operating Frequency of an RF Power Amplifier Circuit

Parent Category: 2015 HFE

By Donna Vigneri

Standard practice when changing the operating frequency range of an existing RF power amplifier circuit dictates that an entirely new design, extensive simulation, and a new circuit board are required, assuming of course that the new frequency range is within the capabilities of the RF power transistor. However, this is actually not always necessary if the design is single-ended and has no complicated matching sections, and the new frequency range is not too distant from the original. In fact, it can be accomplished using only Smith chart and transmission line calculation software and by replacing a few inexpensive components on the board. The benefits are significant, especially when there is neither the time nor budget for wholesale changes, as it can allow a manufacturer to use a single design for multiple products. This article shows how to change the operating frequency using an actual example that took only one day to complete.

 

The Challenge

The evaluation board used in this discussion (Figures 1a and b) was created using Freescale Semiconductor’s 7-W AFT09MS007N Airfast LDMOS RF power transistor. The unmatched device delivers 7 W CW with 120 mW of drive with efficiency up to 70% over its 136 to 941 MHz frequency range. It is designed for use in battery-operated, handheld radios that have a supply voltage of about 7.5 VDC. The device can operate into a VSWR greater than 65:1 at all phase angles without degradation, even when driven at twice its rated drive power and 30% overvoltage. It includes an internal electrostatic discharge protection circuit that protects the MOSFET gate structure from short duration, high-voltage discharge that may be encountered during assembly. Documentation and tools for the AFT09MS007N are available from Freescale1.

1501 HFE powerAmp 011501 HFE powerAmp 02

Figures 1a and 1b • Original (a) and revised circuit in evaluation board (b).

It’s important to note that the steps described below can be applied to other RF power transistors, RF output powers, frequency range conversions, and bandwidth extensions.

The original design required that the amplifier cover a 70 MHz bandwidth between 450 and 520 MHz. However, a subsequent specification required that the device cover a 120-MHz frequency range of 350 to 470 MHz while maintaining the same gain, RF output power, and efficiency of the original design (Figure 2). As there was no time for extensive additional simulations or a new PC board, it was obvious that a different approach was required to effect the change.

1501 HFE powerAmp 03

Figure 2 • Power gain, RF output power, and drain efficiency of original 450 to 520 MHz circuit versus frequency and a constant RF input power (7.5 VDC).

Rather than resorting to the use of a “full-up” design suite (which anyway was not available on short notice), the Iowa Hills Smith Chart program from Iowa Hills Software2 was used instead to facilitate the frequency shift. This software is one of many free Smith chart programs available on the Web. The transmission line parameter calculator from Clemson University3 was used to estimate the degrees of separation and impedance of the transmission line. Other calculators are also available, including the Freescale Engineering Tools 3 app for Android and iPhone4, Transmission Line Calc for iPhone from Black Cat Systems5, and TX-LINE, a Windows-based transmission line calculator from AWR6.

Experimental tuning was first employed in an attempt to shift the frequency directly from the test bench and was based on tuning notes compiled during the 450 to 520 MHz design. This allowed a 70-MHz bandwidth from 400 to 470 MHz to be achieved, and more tuning resulted in a bandwidth of 85 MHz from 380 to 465 MHz. However, the desired 120-MHz bandwidth from 350 to 470 MHz remained elusive. Performance at the high end of the frequency range was acceptable but at lower frequencies around 350 MHz the total impedance was probably too low to also allow matching. The apparent bandwidth might have been the result of a high Q factor. To test if this was the cause, the match was revised using the Smith Chart.

The Solution

To achieve the 120-MHz bandwidth, the current PC board, its microstrip transmission lines, and the position of the lumped element were retained to save time and money. The transmission line impedances were calculated in the transmission line program based on the material properties of the board. The number of matching sections used in the original design was maintained and component values were optimized using the Smith Chart. An adjustment of the transmission line characteristics between these lumped components would have required a new PC board because applying copper tape bridges and cutting PC board traces would not be practical for large-scale manufacture.

The Iowa Hills Smith Chart program uses a combination of fixed and optimized values to achieve the desired impedance transformation. The starting position in a 50-ohm system at the RF input or output is assumed to be 50 + j0 ohms, so the Z0 of the Smith chart is then 50 ohms. For the AFT09MS007N, the gate and drain impedances were each expected to be around 2 to 3 ohms based on interpolation between known narrowband impedances at 136 MHz and broadband impedance from 450 to 520 MHz (the bandwidth of the original circuit). The impedance of LDMOS transistors typically increases as frequency decreases because the reactive part of the impedance is more capacitive, which causes the impedance to be inversely proportional to frequency. The capacitive reactance XC=1/(2π x f0 x C).

As can be seen from the Smith chart (Figure 3), the transformation plot follows a tighter Q circle than the actual measurements portray, probably because ideal components and approximations are used in the plotting program. The impedance was transformed with three matching sections of shunt capacitor plus series inductor, following a reasonable Q circle across the Smith chart (Figure 4). The Q was determined via the classic method, Q=center frequency/bandwidth.

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Figure 3. Smith chart for 350 to 470 MHz.

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Figure 4 • Series equivalent series circuit source and load impedance (350 to 470 MHz).

Optimization

The process of optimization in the Smith Chart program consisted of checking multiple frequencies across the band of interest, making sure that the resulting impedances for 350 and 470 MHz were not too distant from each other, which is a hallmark of broadband design. Once an optimal output match was determined in the Smith Chart program, the components were changed, and focus moved to the input-side transformation. The same process was followed: The components on the RF input were modified and the new circuit design was taken to the test bench.

Using a vector network analyzer, S 11 was checked and a few components were adjusted on the input match to improve the input return loss so that high gain could be achieved and the frequency response could be centered. It became obvious from this exercise that the new match was superior to the old one, which was saved for comparison. The new match improved the return loss by twice the original value and over a wider frequency range. 

At this point, large-signal testing was performed and although efficiency was initially low the desired output power was achieved across the entire 120-MHz bandwidth and actually beyond, to 300 MHz. After about a half hour of tuning, the bandwidth was tightened to the 350 to 470 MHz specification, the efficiency met the required minimum of at least 60% across the band, gain was reasonably flat, and output power with a fixed input power of 250 mW was at least 7 W. In short, the frequency shift was accomplished in less than one day. A reference design for this circuit is available7.

The original board required 25 passive components (19 unique values), and the new board required 27 (with 21 unique values) so the cost of increasing the bandwidth from 70 MHz to 120 MHz was negligible. The gain, RF output power, and efficiency of the final circuit are shown in Figure 5.

1501 HFE powerAmp 06

Figure 5 • Power gain, drain efficiency, and RF output power versus frequency at a constant RF input power of the revised design covering 350 to 470 MHz.

Summary

The process of shifting the frequency of the original design was achieved using only Smith chart and transmission line calculation software, illustrating that simple tuning and review with a Smith chart can eliminate the need for an extensive, time-consuming, costly redesign.

As stated earlier, frequency conversion can be achieved in this manner only when the original and revised frequency ranges are reasonably close to each other. So if the original design was centered at 20 MHz (for example) and the new one is centered on 500 MHz, it’s unlikely that the conversion can be achieved. In addition, if the conversion is to a higher frequency, component values get smaller, but transmission lines may have to be widened to reach the desired impedance. If the conversion is to a lower frequency, transmission line widths can probably remain the same as in the original design, but component values get larger. Finally, if the change also requires increasing the RF output power, all new components, the board, and the microstrip line must be able to handle the increased power.

About the Author

Donna Vigneri is an RF applications engineer in the RF Group at Freescale Semiconductor, where she designs many types of power amplifiers and provides global applications support for industrial, scientific, medical, and avionics applications. She received her BSEE from Florida Institute of Technology, is an active radio amateur (KF7SJF), president of the Freescale Amateur Radio Society (club station W7FSL), and supports educational outreach with the Society of Women Engineers and Arizona Science Outreach with the goal of drawing more young people into the field of engineering. 

Additional Reading

Chris Bowick, RF Design, 2nd Edition, 2008, pp. 69–102 (for Q and Smith chart impedance matching).

David M. Pozar, Microwave Engineering, 3rd Edition, 2005, p. 272, Table 6.1 (for common equations used in RF circuit design).

References

1. AFT09MS007N documentation and tools. http://freescale.com/AFT09MS007N

2. Iowa Hills Software (Smith chart program).

http://www.iowahills.com/9SmithChartPage.html

3. Clemson University transmission line parameter 

Calculator. http://www.cvel.clemson.edu/Emc/calculators/TL_Calculator/

4. Freescale RF Engineering Tools App V1.0 (Transmission line calculator and other features). http://www.freescale.com/RFENGTOOLS

5. Transmission Line Calc for iPhone from Black Cat Systems. http://www.blackcatsystems.com/iphone/transmission_line_calc.html

6. AWR Corp. TX-LINE (Transmission line calculator), http://www.awrcorp.com/products/optional-products/tx-line-transmission-line-calculator

7. UHF Power Amplifier for Professional Mobile Radio Reference Design, Freescale Semiconductor Technical Data, http://cache.freescale.com/files/rf_if/doc/support_info/RDAFT09MS007N_UHF_Radio.pdf

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