AN60-033 - Mini Circuits

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
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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
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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.
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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.
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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
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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
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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
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