Tuning Doherty Combiners
Rev A
DOHERTY COMBINER
Introduction
Anaren Doherty combiners are designed for symmetrical two‐way Doherty power amplifier architectures that have even power slitting ratio between main and peak power amplifiers. A typical Doherty combining circuit consisting of two quarter‐wave‐length transmission lines of 50 Ohm and 35 Ohm are packaged in the combiner as shown in Figure 1. Pin 1
Pin 4
90°
90°
50 Ohm
90°
Pin 2
Pin 3
35 Ohm
Figure 1: Doherty combiner transmission line impedances A transmission line of fixed physical length is of quarter‐wave length only at a single frequency, this type of Doherty combiners are considered narrow band. Typically +/‐ 5% of bandwidth are achieved in this type of Doherty amplifiers. With several models, the Anaren Doherty combiner product line covers major communication bands ranging from 700 MHz to 2700 MHz. Each model is optimized for the center frequency of targeted frequency band. However, as new spectrum is being released worldwide to meet urgent capacity demands, the pre‐designed standard models can be tuned in customer applications to meet performance in these nearby frequency bands This document provides guidelines to tune these standard models for off‐band applications. Available on Tape
and Reel for Pick and
Place Manufacturing.
USA/Canada:
Toll Free:
Europe:
(315) 432-8909
(800) 411-6596
+44 2392-232392
Model X3DCXXXXS
Rev A
Analysis
Figure 1 schematic serves two functions for required Doherty operation. At maximum mode when the main and peak amplifiers deliver equal amount of output power, the combining circuit functions as a 1:1 combiner. The 50 Ohm quarter‐wave‐length transmission line provides required phase delay and amplitude division. The 35 Ohm quarter‐wave‐length transmission line severs as an impedance transformer transforming 25 Ohm to 50 Ohm. At backoff mode or low power condition where the peak amplifier is turned off and presents high output impedance, the combining circuit functions as a 1:2 impedance transformer, providing the required load pulling‐up for the main amplifier. The 35 Ohm quarter‐wave‐length transmission line still serves as a 1:2 impedance transformer transforming 25 Ohm to 50 Ohm. The 50 Ohm quarter‐wave‐length transmission line severs as a 1:4 impedance transformer transforming 25 Ohm to 100 Ohm. The bandwidth of the Doherty combiner is limited to +/‐ 5% by this 1:4 impedance transforming function of the 50 Ohm line, while as a 1:2 impedance transformer, the 35 Ohm line provides much wider bandwidth and it typically can cover more than +/‐ 15% bandwidth. Ideally if a pre‐designed standard model is to be tuned outside its designated frequency band, both the 50 Ohm line and 35 Ohm line need to be adjusted to the center of the new band. However, if the application band is not too far away from the original targeted band center, it is possible that the 35 Ohm line without length adjustment can still cover the new band and provide good enough impedance transforming. In this case, only the 50 Ohm line needs its length adjusted. And even more conveniently, the increase or reduction of the 50 Ohm line can be easily absorbed in the offset phasing line of Doherty amplifier design. Once it is tuned to new center frequency, the impedance transforming function as well as the phase delay requirement can be re‐satisfied in the new band. Amplifier designers are encouraged to take the s‐
parameter of Anaren Doherty combiner into the amplifier simulation to verify the performance. Example
This example demonstrates how X3DC08E2 can be used outside its designated frequency. X3DC08E2S is originally optimized at the 878 MHz targeting LTE band 5 of 862 – 894 MHz. By increasing the offset line on the main amplifier path, as shown in Figure 2, X3DC08E2S can support several new bands at lower frequencies. The required line length adjustment and corresponding electrical specifications are shown in table 1. And typical RF performances with tuning are also shown in the plots below. Similarly, using the same principle, the X3DC08E2S can be tuned for higher frequencies by subtracting the offset line on the main amplifier path. USA/Canada:
Toll Free:
Europe:
(315) 432-8909
(800) 411-6596
+44 2392-232392
Available on Tape and
Reel for Pick and Place
Manufacturing.
Tuning Doherty Combiners
Rev A
Frequency
Phase Adjustment at
main amplifier path
Return Loss
(Max Power Condition)
Insertion Loss
(Max Power Condition)
MHz
Degrees @ 878MHz
dB Min
dB Max
862 – 894
793 – 803
783 – 793
773 – 783
Amplitude
Imbalance
(Max Power
Condition)
0
+9.5
+11
+12
20
20
20
20
0.15
0.15
0.15
0.15
Phase
Imbalance
(Max Power Condition)
Return Loss
(Low Power Condition)
Insertion Loss
(Low Power Condition)
dB Max
Degrees
dB Min
dB Max
±0.12
±0.12
90 ± 3.0
90 ± 3.0
20
20
0.25
0.25
±0.12
90 ± 3.0
20
0.25
±0.12
90 ± 3.0
20
0.25
Table 1: Electrical Specification of X3DC08E2 with Offset Line Tuning Offset line
RF input
RF output
0°
Main Amp
90°
50Ω
90°
NC
35Ω
90°
Peak Amp
Anaren Doherty Combiner
50Ω
Figure 2: Adjusting Offset Line to Tune the Doherty Combiner Conclusion
Anaren Doherty Combiners are versatile for today’s amplifier architectures. For more information on tuning Anaren Doherty Combiners, please contact Anaren Microwave Inc or Richardson RFPD. Available on Tape
and Reel for Pick and
Place Manufacturing.
USA/Canada:
Toll Free:
Europe:
(315) 432-8909
(800) 411-6596
+44 2392-232392
Model X3DCXXXXS
Rev A
Typical Performance : 750-950 MHz
Return Loss for X3DC08E2S (Max Power Condition)
Insertion Loss for X3DC08E2S (Max Power Condition)
0
0
-5
-15
-0.05
Tuning offset line along the main amp
path does not affect the max power
return loss
-0.1
Insertion Loss(Max) (dB)
Return Loss(Max) (dB)
-10
-20
-25
-30
-35
-0.15
-0.2
-0.25
-0.3
-0.35
-40
-0.4
-45
-0.45
-50
750
800
850
Frequency (Mhz)
900
-0.5
750
950
Amplitude Balance for X3DC08E2S (Max Power Condition)
-70
0.15
-75
0
-0.05
-105
-110
750
950
-5
-0.05
-10
-0.1
-15
-0.15
-20
-25
-30
-35
-45
-50
750
USA/Canada:
Toll Free:
Europe:
0deg
9.5deg
11deg
12deg
800
800
850
Frequency (Mhz)
900
950
Insertion Loss for X3DC08E2S (Low Power Condition)
0
Insertion Loss(Low) (dB)
Return Loss(Low) (dB)
Return Loss for X3DC08E2S (Low Power Condition)
0
-40
0deg
9.5deg
11deg
12deg
-95
-0.15
900
950
-90
-100
850
Frequency (Mhz)
900
-85
-0.1
800
850
Frequency (Mhz)
-80
Phase Balance(dB)
Amplitude Balance (dB)
Tuning offset line along the main amp
path does not affect the amplitude
balance
0.05
-0.2
750
800
Phase Balance for X3DC08E2S (Max Power Condition)
0.2
0.1
Tuning offset line along the main amp
path does not affect the max power
insertion loss
0deg
9.5deg
11deg
12deg
-0.2
-0.25
-0.3
-0.35
-0.4
-0.45
850
Frequency (Mhz)
(315) 432-8909
(800) 411-6596
+44 2392-232392
900
950
Available on Tape and
Reel for Pick and Place
Manufacturing.
-0.5
750
800
850
Frequency (Mhz)
900
950