Application Note (AN-60-033) Enhanced Linearity in the HELA-10 Power Amplifier 1.0 Introduction To better satisfy the high linearity requirements of multiple-carrier wideband communication systems such as CATV when using Mini-Circuits model HELA-10, a combination of techniques has been found advantageous: • Biasing at a higher DC current for better second- and third-order intermodulation suppression. • Improving symmetry in the application circuit for better second-harmonic cancellation and further second-order intermodulation suppression. Application note AN-60-009 included a description of how 2nd order and 3rd order intermodulation intercepts (IP2 and IP3) are improved by operating at higher current in a 50-ohm system, and explained how the balanced amplifier configuration suppresses even-order harmonics and even-order intermodulation products. This new application note extends the work of AN-60-009 by showing how intermodulation performance is improved using specially designed transformers optimized for symmetry, both at normal current and at the higher current. It presents typical performance in a 75-ohm system as used in the CATV industry. 2.0 Application Circuit A schematic diagram of the application circuit used to obtain the results reported herein is given in Figure 1. It is designed for use in a 75-ohm system. The components are surface-mount. Bias resistors R2 and R3, which increase the DC current typically from 380mA to 520mA with 12V supply, are the same values as used in Section 4.2 of AN-60-009 for that purpose. Figure 1 – Schematic Diagram with Bias Set for Higher Current AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 1 of 8 3.0 Tests Performed Linearity testing was done with 2 tones 1MHz apart, swept over the frequency range 250 to 350MHz. Second- and third-order intermodulation products as well as second-harmonic power were measured. The significant 2nd order intermodulation product is the sum frequency 500 to 700MHz which, together with the 2nd harmonic, falls within the 50 – 850MHz CATV band. The output power at each tone was 14.5dBm. In addition, the following characteristics were measured over the ranges 10 – 50MHz, 50 – 850MHz, and 850 – 1400MHz: Gain, Isolation, Input and Output VSWR, and Output Power at 0.5dB and 1.0dB compression. All of these tests were done with 12V supply, utilizing the circuit in Figure 1 and 75-ohm instrumentation. 4.0 Results Figure 2 shows the typical power in each second harmonic as well as the typical power in the sum-frequency 2nd intermodulation product, relative to the power in each output tone. Up to 2.5dB advantage is obtained using the higher current. Note also that there is a 6dB difference between the 2nd harmonic and the 2nd order IM product. This is explained by the following analysis. Figure 2 Second Harmonic and Second-order IM, with Output Power = 14.5dBm HELA-10 at Normal & Higher Currents 2nd Harmonic & 2nd-order IM, dBc -70 -75 -80 -85 2nd harm. Normal Current -90 2nd harm. Higher Current IM2 Normal Current -95 IM2 Higher Current -100 250 260 270 280 290 300 310 320 330 340 350 Frequency, MHz AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 2 of 8 Consider a signal VIN consisting of 2 equal-amplitude cosine waves cos ω1t and cos ω2t that is inputted to a device having a nonlinear response: VOUT = a1VIN + a2VIN2 + a3VIN3 + ⋅ ⋅ ⋅ Ignoring 4th order and higher terms, the response is: VOUT = a1 (cos ω1t + cos ω2t) + a2 (cos ω1t + cos ω2t)2 + a3 (cos ω1t + cos ω2t)3 Expanding the squared term: a2 (cos ω1t + cos ω2t)2 = a2 cos2ω1t + a2 cos2ω2t + 2a2 cos ω1t cos ω2t = a2 [1 + ½ cos 2ω1t + ½ cos 2ω2t + cos (ω1 + ω2)t + cos (ω1 – ω2)t] Note that the second-harmonic terms (for frequencies 2ω1 and 2ω1) inside the brackets of this equation have coefficient ½, while the second-order intermodulation terms (for the sum and difference frequencies ω1 + ω2 and ω1 – ω2) have coefficient 1. This shows that each of the second-order intermodulation products (IM2) is 6dB greater than each second-harmonic component in the output spectrum. When the two test tones have frequencies close together, 300 and 301MHz for example (representing adjacent channels in a multi-carrier signal), the output spectrum in the second-harmonic region will contain the sum frequency 601MHz surrounded by the two second-harmonic components 600 and 602MHz, which are each 6dB below the 601 MHz component. Figure 3 shows second-order intermodulation intercept, IP2, calculated from the intermodulation data. AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 3 of 8 Figure 3 Output IP2, HELA-10 at Normal & Higher Currents 100 98 Higher Current 96 Normal Current IP2, dBm 94 92 90 88 86 84 250 260 270 280 290 300 310 320 330 340 350 Frequency, MHz Table 2 compares the above IP2 results with AN-60-009 (the normal current in Figure 18 and the higher current in Figure 43, of AN-60-009). The present work shows typically 4 to 13dB better performance. The present work AN-60-009 At normal current At higher current 87.5 to 95.5dBm 90 to 98dBm 82dBm 86dBm Table 2 – IP2 Comparison at 250 – 350MHz The advantage of up to 2.5 dB afforded by the higher current is on top of the advantage gained by the more symmetrical application circuit. AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 4 of 8 Typical third-order intercept is shown in Figure 4. Operating at the higher current yields 4dB better performance. Figure 4 Output IP3, HELA-10 at Normal & Higher Currents 60 58 56 IP3, dBm 54 52 50 48 Higher Current 46 Normal Current 44 250 260 270 280 290 300 310 320 330 340 350 Frequency, MHz Gain and reverse isolation are shown Figure 5. Gain is essentially flat from 50 to 1000MHz. Directivity, which is the dB-difference between isolation and gain, is typically 7dB. HELA-10 in its high-symmetry application circuit ensures excellent match to 75 ohms, as shown in Figure 6. Mid-band VSWR is typically 1.05:1. Over the range 50 – 850MHz it rises typically to 1.3:1 at the input and 1.2:1 at the output. To complete the picture, power output at 1-dB compression is shown in Figure 7. In mid-band typical P1dB is 31dBm, and over 50 – 850MHz it remains above 29.5dBm. 5.0 Conclusion Substantially improved performance has been demonstrated using an application circuit having enhanced symmetry that provides superior cancellation of second-order distortion. This is in addition to improvement in both second-order and third-order distortion performance achieved by biasing at a higher operating current. AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 5 of 8 Figure 5 Gain and Isolation, HELA-10 at Higher Current 20.0 19.0 18.0 Gain and Isolation, dB 17.0 16.0 15.0 14.0 Isolation 13.0 Gain 12.0 11.0 10.0 9.0 8.0 0 100 200 300 400 500 600 700 800 900 1000 Frequency, MHz VSWR, HELA-10 at Higher Current Figure 6 1.60 1.50 VSWR ( :1) 1.40 Input 1.30 Output 1.20 1.10 1.00 0 100 200 300 400 500 600 700 800 900 1000 Frequency, MHz AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 6 of 8 Power Output at 1-dB Compression HELA-10 at Higher Current Figure 7 31.5 31.0 P1dB, dBm 30.5 30.0 29.5 29.0 28.5 0 100 200 300 400 500 600 700 800 900 1000 Frequency, MHz AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 7 of 8 IMPORTANT NOTICE © 2015 Mini-Circuits This document is provided as an accommodation to Mini-Circuits customers in connection with Mini-Circuits parts only. In that regard, this document is for informational and guideline purposes only. Mini-Circuits assumes no responsibility for errors or omissions in this document or for any information contained herein. Mini-Circuits may change this document or the Mini-Circuits parts referenced herein (collectively, the “Materials”) from time to time, without notice. Mini-Circuits makes no commitment to update or correct any of the Materials, and Mini-Circuits shall have no responsibility whatsoever on account of any updates or corrections to the Materials or Mini-Circuits’ failure to do so. Mini-Circuits customers are solely responsible for the products, systems, and applications in which Mini-Circuits parts are incorporated or used. In that regard, customers are responsible for consulting with their own engineers and other appropriate professionals who are familiar with the specific products and systems into which Mini-Circuits’ parts are to be incorporated or used so that the proper selection, installation/integration, use and safeguards are made. Accordingly, Mini-Circuits assumes no liability therefor. In addition, your use of this document and the information contained herein is subject to Mini-Circuits’ standard terms of use, which are available at Mini-Circuits’ website at www.minicircuits.com/homepage/terms_of_use.html. Mini-Circuits and the Mini-Circuits logo are registered trademarks of Scientific Components Corporation d/b/a Mini-Circuits. All other third-party trademarks are the property of their respective owners. A reference to any third-party trademark does not constitute or imply any endorsement, affiliation, sponsorship, or recommendation: (i) by Mini-Circuits of such third-party’s products, services, processes, or other information; or (ii) by any such third-party of Mini-Circuits or its products, services, processes, or other information. AN-60-033 Rev.: A M150261 (04/14/15) File: AN60033.doc This document and its contents are the property of Mini-Circuits. Page 8 of 8

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