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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

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

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:

Ignoring 4th order and higher terms, the response is:

Expanding the squared term:

Note that the second-harmonic terms (for frequencies 2ω1 and 2ω1) inside the brackets of this equation have coefficient 1/2, 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.

Figure 3

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.

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.

Typical third-order intercept is shown in Figure 4. Operating at the higher current yields 4dB better performance.

Figure 4

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.

Figure 5
Figure 6
Figure 7